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Sex Ratio Distortion By: Dr. Leo Buss Submitted: 3/30/2006 (V. Parnell) Anyone who routinely breeds Betta will eventually encounter a spawn with vastly more males than females or the converse. If one listens to breeder's lore, one finds no lack of explanations: pH too high, pH too low, the age at which fish are jarred, the phase of the moon, the temperature at dawn today, the price of beer yesterday, and so on. What is immediately evident, though, is that sex ratio can vary from nearly 100% female to nearly 100% male; sex is unquestionably malleable. Answers to hard questions sometimes come in examining the answer to a seemingly different question. In this spirit, let's first ask why we expect sex ratio to be 1:1 in the first place. While we are conditioned to believe this to be "normal" by virtue of the 1:1 sex ratio in humans, we are all familiar with social insects like ants and termites that display quite different sex ratios. What then maintains a sex ratio at 1:1? If we know the answer to this question, perhaps we can figure out a way in which that same mechanism can explain why a 1:1 ratio might be violated. Imagine a population of animals with an equal number of males and females. We will explore the fate of mutations that arise that distort the sex ratio from 1:1. Mutations of this sort happen all the time. A classic case is that responsible for the devastating corn blight of 1970. A mutant strain of corn arose that was male-sterile. This cultivar had the notable virtue that the crop did not have to be hand-tassled and, accordingly, the cultivar was widely planted. Unfortunately, this male-sterile cultivar also had the feature that the plant was unusually susceptible to a fungus and much of the 1970 crop was lost. Mutants that disrupt sex ratio, as well as mutants that restore it, are well known. Now what would be the fate of a naturally occurring mutation that led to the production of an excess of males? The mutation would lead to a population with more males than females. Some males would inevitably fail to obtain a mate. Thus males will, on average, do worse than females in the mating game and so mutations that produce excess males will be disadvantaged relative to those that produce equal numbers. Parents that make more males leave fewer descendants than parents that produce a 1:1 ratio. The argument goes both ways. If the mutation yields an excess of females, then some females will lack the chance to breed and any such mutation will lose out to a gene which enforces the 1:1 ratio. Now that we have the answer to the question not asked, we can return to the question we really want to answer. The key comes in looking closely at the argument on 1:1 sex ratios. In formulating that answer, I asked you to imagine a population with an equal sex ratio at the outset. What if we relax this assumption? What if there was deficit of males, a condition for example that pertained at the end of World War II? If that war had gone on for a human generation such that the deficit in males was chronic, then it would be an advantage to a female to produce an excess of males. The advantage, however, would be fleeting. When the war ended, the male population would inevitably return to normal and excess males would again be disadvantaged. When sex ratios are far from 1:1 there is a real advantage to producing an excess of the limited sex, albeit an advantage that will eventually be eliminated. Is there a way an animal can grasp this fleeting advantage in the mating game? Yes, but only if the organism can somehow assess whether the sex ratio in his/her neighborhood was imbalanced and alter sex ratio accordingly. Imagine that Betta, or any other animal, was capable to detecting that there were too many males around and produce an excess of females (or vice versa) in response. Any such organism would do much better than an organism that, like ourselves, slavishly produces a 1:1 ratio. The rub here, of course, is that the organism has to be able to assess with some accuracy the sex ratio in their pond, puddle, or stream. Betta, presumably, can't do simple arithmetic, so how might they calculate sex ratios? This turns out to be not such a hard problem. We should expect that males will always look for big strong females and females always look for big strong males, for the obvious reasons that large females produce more eggs and large males have greater capacity to defend nests. If a female Betta encounters only small males in her neighborhood, she might "infer" that the population is experiencing a deficit of males. If in mating with one of these runts, she produces an excess of males, she might leave more offspring to future generations than if she produced a 1:1 sex ratio in the spawn. Similarly, if a small female finds herself courted by the finest of large and robust males, she might likewise "infer" a deficit of females in the neighborhood and correspondingly produce a higher frequency of females in her spawn. If Betta have the biological capacity to adjust sex ratio, then they need only to be able to assess the size of their mates to manipulate it adaptively. Do they do so? To answer a question like this, one sets up a simple experiment. In this case one might hypothesize that small males + large females yield an excess of males, small females + large males yields an excess of females, and roughly equivalent-sized breeders yields a 1:1 ratio. One would need only to determine the sex ratio of a minimum of ten large (>100 fry) spawns of male/female pairings of maximally disparate and similar sizes using animals of a single (preferably inbred) line. You could do it inside of a year with 50 or so tanks and few thousand jars! Before embarking on such a project, however, it is always prudent to review the existing scientific literature on the topic at hand. Until quite recently this was a bit of a chore, requiring countless hours at a major university library just to get an idea of what is known. The premier hobbyist organization, International Betta Congress, however, has greatly simplified the matter. A partially annotated bibliography of the scientific literature on the genus Betta from 1969-2004 can be downloaded from an IBC website www.ibcbettas.org/ResGrants.htm Perusal of this literature will reveal - as is so often the case in scientific matters of interest to hobbyists - that the question has already been asked and answered by Dr. Gene Lucas. As part of his doctoral thesis at the Iowa State University, Gene made a large number of crosses and kept careful records of the relative size of the breeders and the sex ratio of the spawns. Gene's work is available to most hobbyists via this very column. Many reading this column may be forgiven for not having mastered the full body of Gene's work, especially by virtue of the fact that a significant fraction of his readership was likely not yet born when the column first appeared. As yet another service, the IBC has provided a bibliography of Gene's FAMA contribution from January of 1978 through 2001, which can also be downloaded at the address provided above. The answer to the question we are posing can be found in a table presented in a February 1979 FAMA column. Gene summarizes, "Young males mated to old females gave a 34:2 advantage to males. The old male x young female combination gave only a 5:12 ratio." Gene's results, therefore, are precisely that which would be expected if Betta adaptively modify the sex ratio of their spawns. While Gene's data on the plasticity of sex ratio in Betta spawns is that which evolutionary theory would predict, that fact does not preclude other factors as modifying sex ratio as well. Issues of sufficiency aside, there are clear lessons here for both the commercial breeder and hobbyist. If you are a commercial breeder, you may expect a higher frequency of the more marketable males by mating young males to older females. And hobbyists, who are seeking to continue to develop or maintain a line, should be careful in their use of backcrosses. Backcrosses of mother to son are likely to leave you with few females with which to work and those from father to daughter, few males. Evolutionary first principles suggest that breeders make both backcrosses or make the brother-to-sister cross to best ensure themselves of having a choice of breeders of both sexes for the next generation. Source: FAMA

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