Specific regulatory mechanism or selection? Discussion of one hypothesis about the mechanisms of reproduction of interspecific hybrids of green frogs. Column for Kompyuterrа #110

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Dmytro Shabanov
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Social Utopia: How an Unbiased Society Free from Ideologies Should Relate to the Diversity of Human Sexuality Specific Mechanism of Regulation or Selection? Discussion of a Hypothesis on the Mechanisms of Reproduction of Green Frog Inter-species Hybrids The Creed of a Zoologist, or What We Can Learn from Our Own Animal Nature?

Column for Kompyuterra #109 Column for Kompyuterra #110 Column for Kompyuterra #111

A few days ago, I argued with my colleague. Discussing the same facts, we came to different conclusions. We realized that our specialties were reflected in this: the difference in our assumptions stemmed from the difference in the tasks we were accustomed to solving. My interlocutor is a respected professor who has dedicated decades of his life to the study of biochemistry and physiology. "His" sciences focus on the mechanisms of functioning of organisms and cells. I am a population ecologist with twenty years of experience. "My" tasks are related to the study of selection, primarily – the selection of organisms within populations. Perhaps that's why I tend to look for manifestations of both population selection within a species' range and cell selection within an organism. I couldn't convince my colleague. We ended with me saying: "I will write down my arguments, and we will discuss them again." I started writing – and hope seized me that our dispute might interest someone else. The fact is that the issue we were arguing about is part of a more general problem. Let me explain. I have written more than once that the main object of my interest is the hybridogenic complex of green frogs. I have already drawn a diagram of the hemiclonal (semi-clonal) reproduction of hybrid green frogs. To avoid repetition, I will provide a black-and-white drawing taken from Marina Kravchenko's dissertation and add its mirror-symmetric half to it. Look. In both cases, at the top are representatives of the parental species. Hybrids appear from their crossing. The genomes (chromosome sets, each with 13 chromosomes) of the two parental species are denoted by the letters L and R. Hybrids carry one genome from each of the parental species. The ovals show the sets of genomes in the frogs' cells, and the circles show them in their gametes. Now let's compare the two halves of the diagram. For Western Europe, it is typical that hybrid frogs live together with the parental species LL, and in Eastern Europe (for example, in the ponds of Kharkiv) – with parents RR. When producing gametes, hybrids perform miracles. They discard one of the parental sets of chromosomes (this is called elimination of the non-clonal genome), and then double the remaining set (this is called endoreduplication of the clonal genome). Why clonal? Because it is transmitted as a whole, without "shuffling" parts. We denote the clonality of a genome by enclosing its symbol in parentheses – look at the diagram, and you will understand which genomes are clonal and which are not. In the two cases shown in the figure, hybrids discard different genomes – that of the species with which they will have to mate. The offspring obtained from such crosses turns out to be hybrid. It again crosses with representatives of the parental species, again discards the genome of its partner, and forms a chain of generations that transmit the clonal genome of the species absent in this habitat. And now – the main question. Why, where the partners of the hybrids turn out to be LL frogs, do the hybrids discard the L genome and transmit (R), and where their partners are RR, do they discard R and transmit (L)? There is no definitive answer to this question, because the mechanism of genome elimination has not yet been studied. But how to look for it depends on which hypothesis will be tested during such searches. My colleague and I formulated completely different hypotheses. Of course, in the main, these hypotheses are unified: the observed expediency in the reproduction of hybrid frogs is considered a consequence of selection. The whole question is when this selection occurred and how long it lasted. I will conditionally call these hypotheses "physiological" and "ecological." The "physiological" hypothesis is based on the fact that expedient reactions in biology are usually the result of a mechanism created by selection during the evolutionary history of a species. It consists in the fact that precursor cells of gametes receive information about which partners hybrid frogs will have to mate with in the future. For example, it can be assumed that tadpoles excrete regulators species-specifically into the water. The genome of the species whose regulators predominate in the water where the hybrids develop will be eliminated. Several circumstances support the "physiological" hypothesis. One can recall the views of one of the classics of Russian ecology, S.S. Schwartz. The release of regulators into the external environment (into water, for example) that inhibit the growth of similar organisms (and similar stages of their development) is one of the fundamental regulatory mechanisms. By the way, Schwartz and his colleagues studied this method of regulation precisely in tadpoles... The "ecological" hypothesis explains the observed expediency by selection that occurs directly during the observed events. According to this logic, when parental species cross, different lines of hybrids arise that produce different gametes. The population environment in which they live will preserve suitable lines for it and discard all others. The filter that will leave a suitable line of hybrids and filter out the other will be the partners with whom the hybrid frogs will meet. Return to the figure. Imagine that hybrids that produce offspring with the genome (L) cross with LL frogs. All offspring from such a cross will belong to the parental species LL. Such a line of hybrids will disappear, while the one that produces gametes with the genome R will be preserved and can reproduce for many generations! It might seem that this hypothesis predicts an even more incredible event than its alternative. It is necessary to explain not just the origin of one line of semi-clonal hybrids, but the simultaneous origin of at least two lines. How can this be explained? By selection, of course! Now is the time to tell about one important fact. First-generation hybrids resulting from the crossing of parental species are often sterile, and even if they can leave offspring, they still have abnormalities in the development of their gonads. I clearly remember a respectable lady in an authoritative scientific center dissecting frogs. While dissecting a male hybrid, she exclaimed: "What are these testicles? Ugh, not testicles: small, crooked, and black... But there (in the male of the parental species) it was a pleasure to look at: large, even, yellow." This is not the end of it. Even if first-generation hybrids form gametes, a significant portion of their sperm and eggs are filled with genetic debris: non-integer chromosome sets. An offspring cannot develop from such gametes. However, cells into which a complete set (L) or set (R) has entered have a chance to give rise to another frog, and possibly a whole line of descendants, like those shown in the figure. We see two levels of selection here. Selection between different cell lines within an individual affects the ability to form viable chromosome sets of gametes. Selection between individuals – on the ability to produce normal gametes. And a third level – selection by the population environment, which affects the ability of hybrid lines to reproduce themselves in the crosses that turn out to be most likely. Remember, I wrote about multilevel selection? This is exactly the case. And what is the probability that with the random loss of some number of chromosomes, a gamete with a viable set will arise? In the cells of hybrids, there are 26 chromosomes, of which 13 sets are L and 13 sets are R. If the chromosome distribution is random, the chances of getting a complete L or R set will be insignificant... Well, a few more facts should be recalled. The diagram in the figure above does not exhaust the diversity of green frog hybrids. In many regions, a significant proportion of hybrids have three sets of chromosomes, LLR or LRR. And the most interesting thing is hybrids that have two sets, LR, and produce a mixture of gametes (L) and (R). Such frogs are common, for example, in the North Donetsk center of green frog diversity (a significant part of which is located in the Kharkiv region). In the North Donetsk center (practically native to me), there are no adult LL frogs at all; all (L) genomes are transmitted through hybrids. And when an individual (L)(L) appears from the crossing of two hybrids, each transmitting the (L) genome, it turns out to be short-lived and dies before reaching sexual maturity. The same happens in Western Europe when hybrids transmitting the (R) genome cross with each other. What does this mean? That over time, the transmission of clonal genomes changes, and the increase in their stability, which we have already discussed, is part of this process. Look at the figure. This happens in one frog. Can it be that the same cells develop differently, some remove the L genome and leave (R), while others remove R and leave (L)? My colleague, with whom I began the column describing our dispute, finds it difficult to believe that frogs producing a mixture of gametes (L) and (R) consist of identical cells. It is easier for him to assume that these are cellular chimeras, a mixture of genetically different cells. In physiological logic, a special mechanism is responsible for a certain effect. But I still tend to explain this phenomenon by selection acting directly in the cells of such frogs. Before explaining what, in my opinion, happens in the cells of such frogs, I will give two analogies. Yesterday, when I had already started writing this column, I was riding a minibus and listening to a conversation on a mobile phone by a confident man sitting next to me. He was talking about a match between two German football teams. In his opinion (how accurate, I have no idea), the teams were equal in strength, but one was lucky enough to score first. "You see," he told his interlocutor, "preparation and luck influence the outcome. Even if the preparation is different, luck can help the weaker team: it scores by chance, its players get a boost, and the opponents weaken. And if the preparation is the same, luck decides everything." The second analogy is more academic. A ball rolls down a convex inclined surface. Potentially, it could roll along its midline, along the "crest." However, random influences will almost inevitably deviate its trajectory to the right or left. This deviation will increase the force that will further deviate the ball's movement to the side. Eventually, the ball will end up either on the right or on the left. Let's return to the frogs. Remember: with an increase in the number of generations through which the clonal genome has passed, the stability of its transmission increases (gonads develop more correctly, the proportion of viable gametes increases). So, the clonal genome changes during transmission! And, by the way, it changes in such a way that two clonal genomes of the same species are already unable to ensure the normal development of an individual... I believe that the main thing in these changes is adaptation to discarding (elimination) the genome of another species. The clonal genome (L), transmitted over many generations, becomes specialized in discarding the R genome, and the long successful path through the bodies of semi-clonal hybrids makes the clonal genome (R) "sharpened" on discarding the L genome as a result of selection. And what happens if highly specialized "opponents" genomes (L) and (R) collide? Their encounter will be similar to the meeting of equally strong teams (except that a football team consists of 11 players, and a frog genome – of 13 chromosomes). The chromosomes of the (L) genome will disrupt the transmission of the chromosomes of the (R) genome during cell division, and the latter will act similarly on the former. If any of these chromosomes are eliminated, the genome it belonged to will weaken. Most likely, after the first "expelled" chromosome, others from the same set will also be removed. The "ball" will roll to one of the sides. How can such a "struggle" of chromosome sets occur? During cell division, chromosomes interact with the spindle. Some proteins recognize and specifically bind others... In different species, these proteins can differ. Each set can induce the synthesis of molecules that disrupt such interaction in the protagonist set... I have redrawn the figure above, showing that, according to my hypothesis, all genomes in such frogs are clonal. Here's what happened. As you understand, I assume that at the stage of elimination, there is a struggle between two genomes, and random factors influence the outcome of this struggle in each cell line. I started by comparing two hypotheses: the "physiological" and the "ecological." In the column, they are presented to varying degrees. Nothing surprising: I am convinced of the correctness of the "ecological" version (while acknowledging the certain logic of its alternative). I like the hypothesis. It remains to understand how to confirm it...

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Dmytro Shabanov
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Social Utopia: How an Unbiased Society Free from Ideologies Should Relate to the Diversity of Human Sexuality Specific Mechanism of Regulation or Selection? Discussion of a Hypothesis on the Mechanisms of Reproduction of Green Frog Inter-species Hybrids The Creed of a Zoologist, or What We Can Learn from Our Own Animal Nature?

Column for Kompyuterra #109 Column for Kompyuterra #110 Column for Kompyuterra #111