Cross and direct fertilization. Scientists: Diet prolongs life only in hermaphrodites. A unique object for studying the role of males

What is the benefit? How does random mutation transform underdogs into thriving winners? What is more important for evolution - war or cooperation?

The book by Alexander Markov and Elena Naimark talks about the latest research by molecular geneticists and the findings of paleontologists that provide answers to these and many other questions about modifications in nature. Thousands of discoveries made since the time of Darwin confirm the guesses of the founders of the theory of evolution; new data does not destroy the fundamentals at all evolutionary theory, but on the contrary, they only strengthen them.

Alexander Markov, head of the Department of Biological Evolution, Faculty of Biology, Moscow State University, and Elena Naimark, presenter Researcher Paleontological Institute named after. A. A. Borisyak, are famous scientists and popularizers of science. The two-volume book “Human Evolution” (2011), co-written by them, has become a reference book not only for students and biologists, but also for many people outside the professional community.

Let's consider an example that shows that the gains from sex in dioecious organisms still outweigh the double losses in the number of offspring. Such an example must be selected especially carefully, because you need a good control. In this case, we need two groups of organisms (two populations), identical in everything except one - the ability to have sex. And biologists managed to create such populations.

Biologists from the University of Oregon ( Morran et al., 2009) worked with a worm already familiar to us C. elegans. These beautiful animals seem to have been specially created to test hypotheses about the benefits of sex. As we remember, they do not have females. Populations consist of males and hermaphrodites, with the latter being more numerous. Hermaphrodites produce sperm and eggs and can reproduce unaided by self-fertilization. Males produce only sperm and can fertilize hermaphrodites. As a result of self-fertilization, only hermaphrodites are born. When cross-fertilization occurs, half of the offspring are hermaphrodites and half are males. Frequency of cross-fertilization in populations C. elegans usually does not exceed a few percent. To determine this frequency, you do not need to monitor intimate life worms - it is enough to know the percentage of males in the population.

In roundworms Caenorhabditis elegans there are no females, only males(up) and hermaphrodites(at the bottom) . Hermaphrodites can be distinguished by their thin, long tail.

It should be clarified that self-fertilization is not exactly the same as asexual (clonal) reproduction, but the differences between them quickly disappear in a series of self-fertilizing generations. After this, the offspring ceases to differ from their parents genetically, in the same way as during clonal reproduction.

U C. elegans mutations are known that affect the frequency of cross-fertilization. One of them, xol-1, is fatal to males and actually leads to the fact that only hermaphrodites remain in the population. Another, fog-2, deprives hermaphrodites of the ability to produce sperm and effectively turns them into females. A population in which all individuals carry this mutation becomes a normal dioecious population, like most animals.

Scheme of the experimental setup. Young worms of each new generation are placed in the left half of the dish(white circle) . To get to food(gray oval) , they must overcome the barrier. Weak individuals, overloaded with harmful mutations, cannot cope with this task. From Morran et al., 2009 .

The authors, using classical methods (through crossings, not genetic engineering), developed two breeds of worms with almost identical genomes, differing only in the presence of mutations xol-1 And fog-2. The first breed had a mutation xol-1, and nematodes reproduced only by self-fertilization. The second one had a mutation fog-2, so these worms reproduced only by cross-fertilization. Each breed was accompanied by a third, devoid of both mutations ( wild type, DT). In DT, the cross-fertilization rate is about 5%. The following two series of experiments were carried out with these triplets.

In the first episode The hypothesis was tested that cross-fertilization helps get rid of harmful mutations. The experiment lasted 50 generations (of the worms, of course, not the experimenters). Each generation of worms was exposed to a chemical mutagen - ethyl methanesulfonate. This increased the mutation rate fourfold. The young animals were placed in a Petri dish divided in half by a barrier of tiny bricks (see picture), with the worms being placed in one half of the dish and their food (bacteria) E. coli) was in the other half. Thus, in order to get to food, and therefore have a chance to survive and leave offspring, the worms had to overcome the barrier. Thus, experimenters increased the efficiency of purifying selection, which weeds out harmful mutations. Under normal laboratory conditions, selection efficiency is low because the worms are surrounded by food on all sides. This allows even very weak animals, overloaded with harmful mutations, to survive. In the new experimental setup, this leveling was put to an end. To overcome the wall, the worm must be healthy and strong.

Scientists compared the fitness of worms before and after the experiment, that is, in individuals of the first and fiftieth generations. Worms C. elegans can be stored frozen. It is very comfortable. While the experiment lasted, a sample of the first generation of worms lay quietly in the freezer. Fitness was measured as follows: the worms were mixed in equal proportions with control wild-type worms, into whose genome the gene for the glowing protein was inserted, and planted in an experimental setup. The animals were given time to overcome the barrier and reproduce, and then the percentage of non-luminous individuals in the offspring was determined. If this percentage increased in the fiftieth generation compared to the first, it means that fitness increased during the experiment; if it decreased, it means that degeneration took place. As a result, it turned out that the artificially increased mutation rate led to degeneration (decrease in fitness) of all breeds of worms, except for the “obligate crossbreeders”. The experiment showed that cross-fertilization is a powerful means of combating the “genetic load”.

Even for those lines in which mutagenesis was not artificially accelerated, high frequency cross-fertilization gave an advantage. Under normal laboratory conditions, this advantage does not occur because the worms do not have to climb over walls to get to the food. However, under experimental conditions, “obligate self-fertilizers” degenerated even without an increase in the mutation rate.

In addition, the frequency of cross-fertilization in “wild” breeds during the experiment increased by 5% compared to the original ones. This is perhaps the most important result. It means that under harsh conditions, individuals that reproduce by cross-fertilization have an advantage. Their offspring turns out to be more viable, and therefore, during the experiment, selection automatically occurs for a tendency to cross-fertilization.

Thus, this experiment confirmed the hypothesis that sex is beneficial to the population, despite the "double price of males." It helps the population get rid of harmful mutations and effectively adapt to life's troubles.

In the second episode experiments tested whether cross-fertilization helps produce adaptations by accumulating beneficial mutations. This time, the worms had to overcome an area populated by bacteria to get to food. Serratia. These bacteria enter the digestive tract C. elegans, cause a fatal disease in the worm.

To survive, the worms either had to learn not to ingest harmful bacteria or develop resistance to them. Which option they chose is unknown, but over 40 generations, sex-practicing worms adapted perfectly to new conditions, wild-type worms adapted less well, and obligate hermaphrodites did not adapt at all: their survival in an environment with harmful bacteria remained at the original low level. Again, the rate of cross-fertilization in wild worms increased during the experiment.

Thus, cross-fertilization actually helps a population adapt to changing conditions, in this case the emergence of a pathogen. The fact that cross-fertilization rates increased in the wild type over the course of the experiment means that mating with males (as opposed to self-fertilization) gives hermaphrodites an advantage that outweighs the “double price” they have to pay in producing males.

As you can see, the conclusions coincide with the results of experiments on yeast, in which it was also shown that sex promotes both the rejection of harmful mutations and the accumulation of beneficial ones. Most likely, these two effects are interrelated and should not be contrasted with each other. Sex shuffles genes, allowing selection to “separate the wheat from the chaff”—spreading out genes with beneficial mutations while getting rid of genes with harmful mutations. These are two sides of the same coin, and which one will be more important in a given place and in given time, depends on many factors, including the rate of mutagenesis and favorable conditions.

The reader may object that artificial mutant populations and the difficulties imagined for them are far from real natural conditions. But nature, as field studies show, is against abandoning males; it orders us to treat them with care. This is sadly proven known facts from the history of species that were actively hunted. One such species is the Asian saiga. Until the mid-twentieth century, hunters killed males and females equally. After the decline in numbers, hunting for this species was sharply limited, but poachers still continued to shoot male saigas - manufacturers of esoteric oriental potions paid handsomely for their horns. But only the best males with beautiful and big horns. Others, the poorer ones, remained, and it was they who had the honor of leaving their genes to their offspring. In this case, there is no loss in the number of offspring, since neither females nor cubs are shot. There is only seemingly ephemeral and unquantifiable damage from a decrease in the quality of genes brought by males. It is clear that the gene pool of the population did not receive the highest quality part; the quality was managed by poachers. As a result, where poachers were especially active, the degeneration of animals and a catastrophic decline in their numbers became apparent. Where regulated legal hunting has taken place, there are no signs of decline and population numbers, although declining in the 1970s, remain relatively stable ( Melnikov, Sidorov, 2009).

Only the transition to genome-wide mutual genetic exchange could enable intensive interorganismal recombination evolutionarily stable, i.e. protected from selfish alleles like tr−. This is exactly what we think happened in ancient eukaryotes.

An intermediate link on the path from HGT to amphimixis could be mutual conjugation with the formation of cytoplasmic bridges and recombination of the genomic DNA of two cells ( Gross, Bhattacharya, 2010). Something similar to such an intermediate link, namely conjugation with the formation of cytoplasmic bridges, transfer of genomic DNA and the ability of each cell to be both a donor and a recipient, was found in halophilic (salt-loving) archaea Haloferax (Halobacterium) volcanii (Rosenshine et al., 1989; Ortenberg et al., 1998).

So, sexual reproduction in eukaryotes may be not just an analogue, but a direct descendant of prokaryotic sex.

We have found that sex is extremely beneficial for life. It is the key to stability in our unstable world. But, as you know, free cheese only comes in a mousetrap. What do living beings pay for the ability to quickly adapt?

Microbes that occasionally swap a few of their genes for copies borrowed from their neighbors may not pay much for sex. Especially when you consider that the mechanism of homologous recombination, based on complementarity, reduces the risk of something completely inappropriate getting into the genome, and the ability to use someone else's DNA simply as food is an additional bonus. The price is low, but the gain from such sex is small. It is higher in amphictic, dioecious organisms. But they also pay much more for sex. You have to pay for males, and the price is doubled.

The point is that, all other things being equal, asexual clonal reproduction (or self-fertilization) is exactly twice as efficient as cross-fertilization involving males (see figure). This problem was addressed by the eminent evolutionist John Maynard Smith (1920–2004) in his book The Evolution of Sex (1978).

Diagram illustrating the "double price of males." In dioecious organisms, half of the offspring of each female are males, who themselves cannot produce any offspring. In asexual reproduction, all offspring consist of females (in self-fertilization, self-reproducing hermaphrodites). Therefore, other things being equal, reproduction without the participation of males is twice as effective as with males. The figure shows a situation where each female produces exactly two offspring.

It turns out that males are prohibitively expensive for the population. Refusal of them gives a significant gain in the rate of reproduction. We already know that the transition from dioeciousness and cross-fertilization to asexual reproduction or self-fertilization is technically quite possible. There are many examples of this in both plants and animals. However, for some reason, asexual races and populations of self-fertilizing hermaphrodites have not yet supplanted those who reproduce in the “normal” way, with the participation of males.

It follows from this that sex in general (and dioecious sex in particular) should provide such important advantages that they cover even a double loss in reproductive efficiency. Moreover, these benefits should appear quickly, and not sometime in a million years. Let us repeat that natural selection does not care about long-term prospects.

Let's consider an example that shows that the gains from sex in dioecious organisms still outweigh the double losses in the number of offspring. Such an example must be selected especially carefully, because you need a good control. In this case, we need two groups of organisms (two populations), identical in everything except one - the ability to have sex. And biologists managed to create such populations.

Biologists from the University of Oregon ( Morran et al., 2009) worked with a worm already familiar to us C. elegans. These beautiful animals seem to have been specially created to test hypotheses about the benefits of sex. As we remember, they do not have females. Populations consist of males and hermaphrodites, with the latter being more numerous. Hermaphrodites produce sperm and eggs and can reproduce unaided by self-fertilization. Males produce only sperm and can fertilize hermaphrodites. As a result of self-fertilization, only hermaphrodites are born. With cross-fertilization, half of the offspring are hermaphrodites, half are males. Frequency of cross-fertilization in populations C. elegans usually does not exceed a few percent. To determine this frequency, you do not need to monitor the intimate life of the worms - it is enough to know the percentage of males in the population.

In roundworms Caenorhabditis elegans there are no females, only males(up) and hermaphrodites(at the bottom) . Hermaphrodites can be distinguished by their thin, long tail.

It should be clarified that self-fertilization is not exactly the same as asexual (clonal) reproduction, but the differences between them quickly disappear in a series of self-fertilizing generations. After this, the offspring ceases to differ from their parents genetically, in the same way as during clonal reproduction.

U C. elegans mutations are known that affect the frequency of cross-fertilization. One of them, xol-1, is fatal to males and actually leads to the fact that only hermaphrodites remain in the population. Another, fog-2, deprives hermaphrodites of the ability to produce sperm and effectively turns them into females. A population in which all individuals carry this mutation becomes a normal dioecious population, like most animals.

Scheme of the experimental setup. Young worms of each new generation are placed in the left half of the dish(white circle) . To get to food(gray oval) , they must overcome the barrier. Weak individuals, overloaded with harmful mutations, cannot cope with this task. From Morran et al., 2009.

The authors, using classical methods (through crossings, not genetic engineering), developed two breeds of worms with almost identical genomes, differing only in the presence of mutations xol-1 And fog-2. The first breed had a mutation xol-1, and nematodes reproduced only by self-fertilization. The second one had a mutation fog-2, so these worms reproduced only by cross-fertilization. Each breed was accompanied by a third, devoid of both mutations (wild type, WT). In DT, the cross-fertilization rate is about 5%. The following two series of experiments were carried out with these triplets.

In the first episode The hypothesis was tested that cross-fertilization helps get rid of harmful mutations. The experiment lasted 50 generations (of the worms, of course, not the experimenters). Each generation of worms was exposed to a chemical mutagen, ethyl methanesulfonate. This increased the mutation rate fourfold. The young animals were placed in a Petri dish divided in half by a barrier of tiny bricks (see picture), with the worms being placed in one half of the dish and their food (bacteria) E. coli) was in the other half. Thus, in order to get to food, and therefore have a chance to survive and leave offspring, the worms had to overcome the barrier. Thus, experimenters increased the efficiency of purifying selection, which weeds out harmful mutations. Under normal laboratory conditions, selection efficiency is low because the worms are surrounded by food on all sides. This allows even very weak animals, overloaded with harmful mutations, to survive. In the new experimental setup, this leveling was put to an end. To overcome the wall, the worm must be healthy and strong.

appear.
The famous evolutionist John Maynard Smith drew attention to the seriousness of this problem in

his book The Evolution of Sex (1978). Maynard Smith examined in detail the paradox, which he gave the name “double

price of sex" (two-fold cost of sex). Its essence is that, other things being equal, asexual reproduction (or

self-fertilization) is exactly twice as effective as cross-fertilization with the participation of males (see figure).

In other words, males are prohibitively expensive for the population. Refusal of them gives immediate and very significant

gain in reproduction speed. We know that, purely technically, the transition from dioeciousness and crossbreeding

fertilization to asexual reproduction or self-fertilization is quite possible, there are many examples of this, such as

plants and animals (see, for example: Female giant komodo dragon reproduce without the participation of males,

"Elements", 12/26/2006). Nevertheless, for some reason, asexual races and populations of self-fertilizing hermaphrodites

So far, they have not supplanted those who reproduce in the “normal” way, with the participation of males.
Why are they still needed?
From the above it follows that cross-fertilization must provide some advantages, so significant that

that they cover even the double gain in reproductive efficiency provided by the abandonment of males. Moreover, these

the benefits should appear immediately, not sometime in a million years. Natural selection don't care

distant prospects.
There are many hypotheses about the nature of these advantages (see: Evolution of sexual reproduction). We will look at two of them.

The first is known as the “Muller ratchet” (see: Muller's ratchet). A ratchet is a device in which the axis

can only spin in one direction. The idea is that if a harmful mutation occurs in an asexual organism,

his descendants can no longer get rid of it. She'll be like generational curse, transmitted to all his descendants forever

(unless a reverse mutation occurs, and the likelihood of this is very low). U asexual organisms selection may

discard only entire genomes, not individual genes. Therefore, in a series of generations of asexual organisms it may (with

subject to certain conditions), a steady accumulation of harmful mutations occurs. One of these conditions is

enough big size genome a. Roundworms, by the way, have small genomes compared to others

animals. Maybe that's why they can afford self-fertilization.
If organisms reproduce sexually and practice cross-fertilization, then individual genomes

constantly scatter and mix, and new genomes are formed from fragments that previously belonged to different

organisms. As a result, a special new entity arises, which asexual organisms do not have - the gene pool of the population.

Genes are given the opportunity to reproduce or be eliminated independently of each other. A gene with an unfortunate mutation can

be rejected by selection, and the remaining (“good”) genes of a given parent organism can safely

cargo", that is, it helps to get rid of constantly occurring harmful mutations, preventing degeneration (reduction

general fitness of the population).
The second idea is similar to the first: it suggests that sexual reproduction helps organisms adapt more effectively

to changing conditions due to the accelerated accumulation of mutations useful in a given environment. Suppose one individual

one beneficial mutation has arisen, and another has another. If these organisms are asexual, they have virtually no chance

wait for the combination of both mutations in one genome e. Sexual reproduction gives such an opportunity. It actually

makes all beneficial mutations that arise in the population a “common property.” It is clear that the speed of adaptation to

Under changing conditions, organisms with sexual reproduction should be higher.
All these theoretical constructions, however, are based on certain assumptions. Mathematical results

simulations suggest that the extent to which cross-fertilization is beneficial or harmful compared

with asexual reproduction or self-fertilization depends on a number of parameters. These include population size;

mutation rate; genome size a; quantitative distribution of mutations depending on their degree

harmfulness/usefulness; the number of offspring produced by one female; selection efficiency (degree of dependence of the number

the descendants left are not from random, but from genetic factors), etc. Some of these parameters are very difficult

measure not only in natural, but also in laboratory populations.
Therefore, all hypotheses of this kind urgently need not so much theoretical justification and mathematical models

(all this is already in abundance), how much in direct experimental verification. However, such experiments are still

not much has been done (Colegrave, 2002. Sex releases the speed limit on evolution // Nature. V. 420. P. 664-

666; Goddard et al., 2005. Sex increases the efficacy of natural ion in experimental yeast populations //

Nature. V. 434. P. 636-640). A new study carried out by biologists from the University of Oregon roundworm

Caenorhabditis elegans, clearly illustrated the effectiveness of both mechanisms considered, ensuring

advantage to those populations that do not refuse males, despite their “double price”.
A unique object for studying the role of males
The Caenorhabditis elegans worms seem to have been deliberately created to experimentally test the above-mentioned hypotheses. These

there are no female worms. Populations consist of males and hermaphrodites, with the latter numerically predominant. U

hermaphrodites have two X chromosomes, males have only one (X0 sex determination system, like Drosophila). Hermaphrodites

produce sperm and eggs and can reproduce unaided through self-fertilization. Males

produce only sperm and can fertilize hermaphrodites. As a result of self-fertilization into the world

only hermaphrodites appear. When cross-fertilization occurs, half of the offspring are hermaphrodites.

half are males. Typically, the frequency of cross-fertilization in C. elegans populations does not exceed several

percent. To determine this frequency, it is not necessary to observe the intimate life of worms - it is enough to know

percentage of males in a population.
It should be clarified that self-fertilization is not exactly the same as asexual (clonal) reproduction, however

the differences between them quickly disappear in a series of self-fertilizing generations. Self-fertilizing organisms

over several generations they become homozygous for all loci. After this, the offspring ceases to differ from

parents genetically, in the same way as during clonal reproduction.
Mutations are known in C. elegans that affect the frequency of cross-fertilization. One of them, xol-1, is lethal to

males and actually leads to the fact that only hermaphrodites remain in the population, reproducing by

self-fertilization. Another, fog-2, removes the ability of hermaphrodites to produce sperm and effectively turns them into

females A population in which all individuals carry this mutation becomes a normal dioecious population, like

almost identical genomes, differing only in the presence of xol-1 and fog-2 mutations. The first breed in each pair, with

xol-1 mutation, reproduces only by self-fertilization (obligate selfing, OS). The second, with the fog-2 mutation, may

reproduce only by cross-fertilization (obligate outcrossing, OO). Each pair of breeds was accompanied by

the third, with the same genetic “background”, but lacking both mutations (wild type, WT). In WT breeds the frequency

cross-fertilization under standard laboratory conditions does not exceed 5%.
Two series of experiments were carried out with these triplets of rocks.
In the first series, the hypothesis was tested that cross-fertilization helps get rid of “genetic

cargo." The experiment continued for 50 generations (of the worms, of course, not the experimenters). Each

a generation of worms was exposed to a chemical mutagen - ethyl methanesulfonate. This led to an increase

the mutation rate is approximately fourfold. Young animals were placed in a Petri dish divided in half by a wall made of

vermiculite, and the worms were planted in one half of the cup, and their food - E. coli bacteria - was in the other

half. When transplanting, the worms were treated with an antibiotic to remove any accidentally adhering bacteria. IN

As a result, in order to get to food, and therefore have a chance to survive and leave offspring, the worms had to

overcome the obstacle. Thus, experimenters increased the efficiency of “purifying” selection, which eliminates

harmful mutations. Under normal laboratory conditions, selection efficiency is very low because the worms are surrounded by food

from all sides. In such a situation, even very weak ones, overloaded with harmful mutations, can survive and reproduce.

animals. In the new experimental setup, this leveling was put to an end. To crawl over the wall

fiftieth generation. C. elegans worms can be stored frozen for a long time. This makes things like this much easier

experiments. While the experiment lasted, a sample of 1st generation worms lay quietly in the freezer.

Fitness was measured as follows. Worms were mixed in equal proportions with control worms into a genome

which the glowing protein gene was inserted into and placed in an experimental setup. The animals were given time to

overcome the barrier and reproduce, and then the percentage of non-luminescent individuals in the offspring was determined. If this percentage

increased in the fiftieth generation compared to the first, which means that fitness increased during the experiment,

if it has decreased, it means that degeneration has taken place.
The results of the experiment are shown in the figure. They clearly indicate that cross-fertilization

is a powerful tool fight against genetic load. The higher the frequency of cross-fertilization, the better

final result (all lines in the figure increase from left to right). Artificially increased mutation rate

led to degeneration (decrease in fitness) of all breeds of worms, except OO - “obligate crossbreeders”.
Even for those breeds in which mutagenesis has not been artificially accelerated, the frequency of cross-fertilization is high

gave an advantage. Under normal laboratory conditions, this advantage does not occur because worms do not need to

climb over walls to get to food.
Interestingly, in one of the two control OS breeds ("obligate self-fertilizers"), even without an increase in speed

mutations, refusal of cross-fertilization led to degeneration (the left square in the upper pair of curves on

the figure is below zero).
The figure also shows that the frequency of cross-fertilization in most wild breeds (WT) during

experiment turned out to be noticeably higher than the original 5%. This is perhaps the most important result. It means that in tough

conditions (meaning both the need to climb over the barrier and the increased rate of mutagenesis) natural

selection gives a clear advantage to individuals that reproduce by cross-fertilization. The offspring of such individuals

turns out to be more viable, and therefore, during the experiment, selection occurs for a tendency to cross

fertilization.
Thus, the first experiment convincingly confirmed the hypothesis that cross-fertilization helps

populations to get rid of harmful mutations.
The second series of experiments tested whether cross-fertilization helps develop new adaptations

through the accumulation of beneficial mutations. This time, the worms had to overcome the area to get to the food.

colonized by pathogenic Serratia bacteria. These bacteria, entering the digestive tract of C. elegans, cause

worm dangerous disease which could end in death. To survive in this situation, the worms had to either

learn not to ingest harmful bacteria, or develop resistance to them. Which option did the subjects choose?

worm populations are unknown, but over 40 generations the OO breeds have adapted perfectly to new conditions, the WT breeds

adapted somewhat worse, and the OS breeds did not adapt at all (their survival in an environment with harmful bacteria

remained at the original low level). And again, during the experiment, WT in breeds increased sharply under the influence of selection

frequency of cross-fertilization.
Thus, cross-fertilization actually helps the population adapt to changing

conditions, in this case - to the appearance of a pathogenic microbe. The fact that during the experiment the WT breeds

the frequency of cross-fertilization increased, meaning that mating with males (as opposed to

self-fertilization) gives hermaphrodites an immediate adaptive advantage, which apparently outweighs

the “double price” they have to pay when producing males.
It should be noted that cross-fertilization occurs not only in dioecious organisms. For example,

Many invertebrates are hermaphrodites, fertilizing not themselves, but cross-fertilizing each other. U

In plants, cross-pollination of bisexual (“hermaphroditic”) individuals is also, to put it mildly, not uncommon. Both hypotheses are

tested in this work are quite applicable to such hermaphrodites. In other words, this work did not prove that

“cross-hermaphroditism” is in some ways inferior to dioeciousness. But for the first of these two options you don’t need

pay the notorious “double price”. Therefore, the problem still remains.
The experiments carried out revealed the disadvantages of self-fertilization compared to cross-fertilization, but they did not explain

why many organisms preferred dioeciousness to “cross-breeding hermaphroditism.” The key to solving this riddle is

Most likely it is sexual selection. Dioeciousness allows females to choose their partners meticulously,

and this can serve additional way increasing the efficiency of rejecting harmful and accumulating useful

mutations. Perhaps this hypothesis will someday receive experimental confirmation.

Alexander Markov

Self-fertilizing animals reproduce, all other things being equal, twice as fast as dioecious animals. Why does dioecy prevail in nature? To answer this question, breeds of roundworms were artificially bred. Caenorhabditis elegans, some of which practice only cross-fertilization, others only self-fertilization. Experiments with these worms confirmed two hypotheses about the benefits of cross-fertilization. One advantage is more effective cleansing gene pool from harmful mutations, the second is the accelerated accumulation of beneficial mutations, which helps the population adapt to changing conditions.

Double price for males

Why is sexual reproduction necessary, why are males needed? The answers to these questions are not at all as obvious as they might seem.

Diagram illustrating the "double price of sex" (or "double price of males"). In dioecious organisms, half of the offspring of each female are males, who themselves cannot produce any offspring. In asexual reproduction, all offspring consist of females (in self-fertilization, self-reproducing hermaphrodites). Therefore, other things being equal, reproduction without the participation of males is twice as effective as with males. The figure shows a situation where each female produces exactly two offspring. Picture from en.wikipedia.org

The famous evolutionist John Maynard Smith drew attention to the seriousness of this problem in his book The Evolution of Sex(1978). Maynard Smith examined in detail the paradox, which he gave the name “two-fold cost of sex.” Its essence is that, all other things being equal, asexual reproduction (or self-fertilization) is exactly twice as effective as cross-fertilization with the participation of males (see figure). In other words, males are prohibitively expensive for the population. Refusal of them gives an immediate and very significant gain in the rate of reproduction. We know that, purely technically, the transition from dioeciousness and cross-fertilization to asexual reproduction or self-fertilization is quite possible; there are many examples of this in both plants and animals. Nevertheless, for some reason, asexual races and populations of self-fertilizing hermaphrodites have not yet supplanted those who reproduce in the “normal” way, with the participation of males.

Why are they still needed?

From the above it follows that cross-fertilization must provide certain advantages, so significant that they even cover the double gain in reproductive efficiency provided by the abandonment of males. Moreover, these advantages should appear immediately, and not sometime in a million years. Natural selection does not care about long-term prospects.

There are many hypotheses about the nature of these advantages (see: Evolution of sexual reproduction). We will look at two of them. The first is known as the “Muller ratchet” (see: Muller's ratchet). A ratchet is a device in which the axis can only rotate in one direction. The essence of the idea is that if a harmful mutation occurs in an asexual organism, its descendants will already cannot get rid of it. It will, like a family curse, be passed on to all his descendants forever (unless a reverse mutation occurs, and the probability of this is very small). In asexual organisms, selection can only cull entire genomes, but not individual genes. Therefore, in Over a succession of generations of asexual organisms, a steady accumulation of harmful mutations can (if certain conditions are met), one of which is a sufficiently large genome size. , by the way, genomes are small compared to other animals. Maybe that's why they can afford self-fertilization (see below).

If organisms reproduce sexually and practice cross-fertilization, then individual genomes are constantly scattered and mixed, and new genomes are formed from fragments that previously belonged to different organisms. As a result, a special new entity arises, which asexual organisms do not have - gene pool populations. Genes are given the opportunity to reproduce or be eliminated independently of each other. A gene with an unsuccessful mutation can be rejected by selection, and the remaining (“good”) genes of a given parent organism can be safely preserved in the population.

Thus, the first idea is that sexual reproduction helps to cleanse genomes of “genetic load”, that is, it helps to get rid of constantly occurring harmful mutations, preventing degeneration (a decrease in the overall fitness of the population).

The second idea is similar to the first: it suggests that sexual reproduction helps organisms more effectively adapt to changing conditions by accelerating the accumulation of mutations that are useful in a given environment. Let’s say one individual has one beneficial mutation, and another has another. If these organisms are asexual, they have virtually no chance of waiting for both mutations to combine in one genome. Sexual reproduction provides this opportunity. It effectively makes all beneficial mutations that arise in a population a “common property.” It is clear that the rate of adaptation to changing conditions in organisms with sexual reproduction should be higher.

Diagram showing how sexual reproduction can accelerate the spread of beneficial mutations through a population. During sexual reproduction ( top picture) two new beneficial alleles (A and B) are quickly combined by crossing individuals that each have only one of these alleles. With asexual reproduction ( bottom picture) you have to wait until both mutations randomly occur in the same clone. Picture from en.wikipedia.org

All these theoretical constructs, however, are based on certain assumptions. The results of mathematical modeling indicate that the degree of benefit or harm of cross-fertilization compared to asexual reproduction or self-fertilization depends on a number of parameters. These include population size; mutation rate; genome size; quantitative distribution of mutations depending on the degree of their harmfulness/usefulness; the number of offspring produced by one female; selection efficiency (the degree of dependence of the number of offspring left not on random, but on genetic factors), etc. Some of these parameters are very difficult to measure not only in natural, but also in laboratory populations.

Therefore, all hypotheses of this kind urgently need not so much theoretical justifications and mathematical models (all of which are already in abundance), but rather direct experimental verification. However, not many such experiments have been carried out so far (Colegrave, 2002. Sex releases the speed limit on evolution // Nature. V. 420. P. 664–666; Goddard et al., 2005. Sex increases the efficacy of natural selection in experimental yeast populations. Nature. V. 434. P. 636–640). New research carried out by biologists from the University of Oregon on roundworms Caenorhabditis elegans, clearly illustrated the effectiveness of both mechanisms considered, providing an advantage to those populations that do not refuse males, despite their “double price.”

A unique object for studying the role of males

Worms Caenorhabditis elegans as if they were deliberately created for experimental testing of the above-mentioned hypotheses. These worms do not have females. Populations consist of males and hermaphrodites, with the latter numerically predominant. Hermaphrodites have two X chromosomes, while males have only one (X0 sex determination system, like Drosophila). Hermaphrodites produce sperm and eggs and can reproduce unaided by self-fertilization. Males produce only sperm and can fertilize hermaphrodites. As a result of self-fertilization, only hermaphrodites are born. When cross-fertilization occurs, half of the offspring are hermaphrodites and half are males. Typically, the frequency of cross-fertilization in populations C. elegans does not exceed a few percent. To determine this frequency, it is not necessary to observe the intimate life of the worms - it is enough to know the percentage of males in the population.

It should be clarified that self-fertilization is not exactly the same as asexual (clonal) reproduction, but the differences between them quickly disappear in a series of self-fertilizing generations. Self-fertilizing organisms become homozygous at all loci over several generations. After this, the offspring ceases to differ from their parents genetically, in the same way as during clonal reproduction.

U C. elegans mutations are known that affect the frequency of cross-fertilization. One of them, xol-1, is fatal to males and actually leads to the fact that only hermaphrodites remain in the population, reproducing by self-fertilization. Another, fog-2, deprives hermaphrodites of the ability to produce sperm and effectively turns them into females. A population in which all individuals carry this mutation becomes a normal dioecious population, like most animals.

The authors, using classical methods (through crossings, not genetic engineering), developed two pairs of worm breeds with almost identical genomes, differing only in the presence of mutations xol-1 And fog-2. The first breed in each pair, with a mutation xol-1, reproduces only by self-fertilization (obligate selfing, OS). Second, with mutation fog-2, can reproduce only by cross-fertilization (obligate outcrossing, OO). Each pair of breeds was accompanied by a third, with the same genetic “background”, but lacking both mutations (wild type, WT). In WT breeds, the cross-fertilization rate under standard laboratory conditions does not exceed 5%.

Males are needed! Tested experimentally

Two series of experiments were carried out with these triplets of rocks.

In the first episode The hypothesis that cross-fertilization helps to get rid of the “genetic load” was tested. The experiment continued for 50 generations (of the worms, of course, not the experimenters). Each generation of worms was exposed to a chemical mutagen - ethyl methanesulfonate. This resulted in an approximately fourfold increase in the mutation rate. Young animals were placed in a Petri dish divided in half by a wall of vermiculite (see picture), with worms placed in one half of the dish, and their food was bacteria E. coli- was in the other half. When transplanting, the worms were treated with an antibiotic to remove any accidentally adhering bacteria. As a result, in order to get to food, and therefore have a chance to survive and leave offspring, the worms had to overcome an obstacle. Thus, experimenters increased the efficiency of “purifying” selection, which eliminates harmful mutations. Under normal laboratory conditions, selection efficiency is very low because the worms are surrounded by food on all sides. In such a situation, even very weak animals overloaded with harmful mutations can survive and reproduce. In the new experimental setup, this leveling was put to an end. To crawl over the wall, the worm must be healthy and strong.

Scheme of the experimental setup. Young worms of each new generation are placed in the left half of the dish ( blue circle). To get to food ( yellow oval), they must overcome the vermiculite barrier. Weak individuals, overloaded with harmful mutations, rarely cope with this task. Rice. from additional materials to the article discussed in Nature

The authors compared fitness in worms before and after the experiment, that is, in individuals of the first and fiftieth generations. Worms C. elegans Can be stored frozen for a long time. This greatly facilitates such experiments. While the experiment lasted, a sample of 1st generation worms lay quietly in the freezer. Fitness was measured as follows. The worms were mixed in equal proportions with control worms, into whose genome the glowing protein gene was inserted, and planted in an experimental setup. The animals were given time to overcome the barrier and reproduce, and then the percentage of non-luminous individuals in the offspring was determined. If this percentage increased in the fiftieth generation compared to the first, it means that fitness increased during the experiment; if it decreased, it means that degeneration took place.

The results of the experiment are shown in the figure. They clearly indicate that cross-fertilization is a powerful means of combating genetic load. The higher the frequency of cross-fertilization, the better the final result (all lines in the figure increase from left to right). The artificially increased rate of mutation led to degeneration (decrease in fitness) of all breeds of worms, except for OO - “obligate crossbreeders”.

Even for those breeds in which mutagenesis was not artificially accelerated, the high frequency of cross-fertilization gave an advantage. Under normal laboratory conditions, this advantage does not occur because the worms do not have to climb over walls to get to the food.

It is curious that in one of the two control OS breeds (“obligate self-fertilizers”), even without increasing the mutation rate, the refusal of cross-fertilization led to degeneration (the left square in the upper pair of curves in the figure is located below zero).

Results of the first experiment. Horizontal axis- frequency of cross-fertilization. The leftmost position is occupied by points corresponding to OS worm breeds, the rightmost - OO. The intermediate position is occupied by the points corresponding to WT rocks. Triangles and squares correspond to two triplets of worm breeds with the same “genetic background”. Vertical axis- change in fitness during the experiment. Positive values ​​mean an increase in fitness, negative values ​​mean degeneration. Solid lines connected dots corresponding to worm breeds in which the mutation rate was not increased. Dotted line- breeds exposed to chemical mutagen. Rice. from the article discussed in Nature

The figure also shows that the cross-fertilization rate of most wild breeds (WT) during the experiment was significantly higher than the original 5%. This is perhaps the most important result. It means that under harsh conditions (meaning both the need to climb over a barrier and an increased rate of mutagenesis), natural selection gives a clear advantage to individuals that reproduce by cross-fertilization. The offspring of such individuals turns out to be more viable, and therefore, during the experiment, selection occurs for a tendency to cross-fertilization.

Thus, the first experiment convincingly confirmed the hypothesis that cross-fertilization helps a population get rid of harmful mutations.

In the second episode experiments tested whether cross-fertilization helps develop new adaptations by accumulating beneficial mutations. This time, in order to get to food, the worms had to overcome an area populated by pathogenic bacteria Serratia. These bacteria enter the digestive tract C. elegans, cause a dangerous disease in the worm that can result in death. To survive in this situation, the worms either had to learn not to ingest harmful bacteria or develop resistance to them. It is unknown which of the options the experimental worm populations chose, but over 40 generations, the OO breeds adapted perfectly to the new conditions, the WT breeds adapted somewhat worse, and the OS breeds did not adapt at all (their survival in an environment with harmful bacteria remained at the original low level). Again, during the experiment, the frequency of cross-fertilization increased sharply in the WT breeds under the influence of selection.

Thus, cross-fertilization actually helps a population adapt to changing conditions, in this case the emergence of a pathogen. The fact that cross-fertilization rates increased in the WT breeds over the course of the experiment means that mating with males (as opposed to self-fertilization) gives hermaphrodites an immediate adaptive advantage that apparently outweighs the "double price" they have to pay in producing males.

It should be noted that cross-fertilization occurs not only in dioecious organisms. For example, many invertebrates are hermaphrodites, fertilizing not themselves, but cross-fertilizing each other. In plants, cross-pollination of bisexual (“hermaphroditic”) individuals is also, to put it mildly, not uncommon. Both hypotheses tested in this work are quite applicable to such hermaphrodites. In other words, this work did not prove that “cross-hermaphroditism” is in any way inferior to dioeciousness. But for the first of these two options you do not need to pay the notorious “double price”. Therefore, the problem still remains.

The experiments revealed the disadvantages of self-fertilization compared to cross-fertilization, but they did not explain why many organisms preferred dioeciousness to “cross-fertilization.” The key to solving this mystery most likely lies in sexual selection. Dioeciousness allows females to choose their partners meticulously, and this can serve as an additional way to increase the efficiency of rejecting harmful mutations and accumulating beneficial mutations. Perhaps this hypothesis will someday receive experimental confirmation.