Two new hypotheses have been proposed that address the “double price of sex”: one of the greatest mysteries in the evolution of sexual reproduction. The evolution of sexual reproduction in living things is one of the great mysteries of biology. Two methods of reproduction are known: asexual, when the organism creates its own clones, and sexual, when gametes from two individuals unite to produce offspring. There are many hypotheses that address various aspects of the evolution of sexual reproduction; however, there are also many questions that are still unanswered.
The biggest issue in studying the evolution of sexual reproduction is the issue of cost. Sexual reproduction requires exponentially more energy than asexual reproduction. However, sexual reproduction has two main advantages over asexual reproduction: 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 the study of the evolution of sexual reproduction. Their hypotheses were published in the Journal of Ethology.
Scientists have proposed hypotheses that address the “double cost of sex”: the cost of meiosis and the cost of producing large numbers 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 using computer modeling.
The first hypothesis they proposed is a “seesaw effect” whereby a large number of harmful mutations are eliminated. The first individual to have the sex-control gene which allowed meiosis to occur produced four gametes. Only gametes with the sex-control gene could fuse, fixing it in the population and erasing the cost of meiosis. In addition, any deleterious mutations were diluted or eliminated depending on whether they were associated with a sex-control gene.
The second hypothesis, the evolution of anisogamy through “inflated isogamy”, was developed from the first hypothesis. They suggest that multicellular organisms with higher energy production originally evolved; thereafter, gamete size increased (“inflated isogamy”) as increased resources in larger gametes increased offspring survival. Then the male gametes shrank to fertilize more female gametes – depending on the inflated female gametes to provide resources for survival.
This strategy does not involve any additional costs on the part of the woman; in fact, it may have triggered their counter-adaptation to the current female meiosis, which results in only one female gamete (oocyte) per gametocyte. With these hypotheses, the authors addressed the question of the “double price of sex” and also hypothesized that the first sexual reproduction required only one individual and was self-fertilization. However, these two hypotheses are still in their early stages and further work is needed to resolve the specific assumptions and conclusions on which they are based.
Source Reference: Yukio Yasui, Eisuke Hasegawa. The origination events of gametic sexual reproduction and anisogamy. Journal of Ethology, 2022; DOI: 10.1007/s10164-022-00760-3
