Two new hypotheses have been proposed that address the “double cost of sex”: one of the biggest enigmas in the evolution of sexual reproduction.
The evolution of sexuality in living things is one of the greatest mysteries in biology. There are two known modes of copying: asexual, where the organism creates clones of itself, and sexual, where the gametes of two individuals fuse to give rise to offspring. There are many hypotheses that address various factors in the evolution of sexual replication however, there are also many questions that remain unanswered.
The biggest question in the study of the evolution of sexual reproduction is the problem of cost. Sexual reproduction requires exponentially more energy than asexual reproduction. Nevertheless, the sexual replica has two major advantages over the asexual copy: it leads to genetic diversity in the offspring and it eliminates harmful mutations.
Associate Professor Eisuke Hasegawa of Hokkaido University and Associate Professor Yukio Yasui of Kagawa University proposed and modeled two new hypotheses that address two open questions in studying the evolution of sexual reproduction. Their hypotheses were published in the Journal of Ethology.
The researchers proposed hypotheses to address the “double cost of sex”: the cost of meiosis and the cost of manufacturing of a large number of male gametes. Sexual reproduction can be isogamous, where the gametes are all the same size, or it can be anisogamous, where the female gametes are large, while the male gametes are small and numerous. The hypotheses were tested by computer modelling.
The first hypothesis they proposed was the “toggle effect” whereby a large number of harmful mutations are eliminated. The first individual to have a sex-controlling gene – which allowed meiosis to occur – produced four gametes. Only gametes with the sex-controlling gene could fuse, fixing it in the populace and erasing the cost of meiosis. Additionally, all harmful mutations were diluted or discarded depending on whether or not they were associated with the gene controlling sex.
The second hypothesis, the development of anisogamy via “inflated isogamy”, was developed from the first hypothesis. They suggest that originally multicellular organisms with higher energy generation evolved then gamete size increased (“bloated isogamy”) as the resources increased in larger gametes. offspring survival rate. Then the male gametes reduced in size to fertilize more female gametes – depending on the swollen female gametes to provide the resources needed for survival. This strategy does not involve any additional cost of the sectioning of the female in fact, it may have triggered their counter-adaptation to the current meiosis in females which results in only one female gamete (the oocyte) per gametocyte.
With these assumptions, the authors addressed the “double cost of sex” dilemma and also issued the assumption that the first sexual copy required only one individual and was a self-fertilizing event. However, both hypotheses are still in their infancy and further work is needed to address the specific hypotheses and conclusions that underlie them.