Methodologies for testing a pluralist idea for the maintenance of sex

The advantage of sex: Reinserting fluctuating selection in the pluralist approach

The advantage of sex, and its fixation in some clades and species all over the eukaryote tree of life, is considered an evolutionary enigma, especially regarding its assumed two-fold cost. Several likely hypotheses have been proposed such as (1) a better response to the negative frequency-dependent selection imposed by the “Red Queen” hypothesis; (2) the competition between siblings induced by the Tangled Bank hypothesis; (3) the existence of genetic and of (4) ecological factors that can diminish the cost of sex to less than the standard assumed two-fold; and (5) a better maintenance of genetic diversity and its resulting phenotypic variation, providing a selective advantage in randomly fluctuating environments. While these hypotheses have mostly been studied separately, they can also act simultaneously. This was advocated by several studies which presented a pluralist point of view. Only three among the five causes cited above were considered yet in such a framework: the Red Queen hypothesis, the Tangled Bank and the genetic factors lowering the cost of sex. We thus simulated the evolution of a finite mutating population undergoing negative frequency-dependent selection on phenotypes and a two-fold (or less) cost of sexuality, experiencing randomly fluctuating selection along generations. The individuals inherited their reproductive modes, either clonal or sexual. We found that exclusive sexuality begins to fix in populations exposed to environmental variation that exceeds the width of one ecological niche (twice the standard deviation of a Gaussian response to environment). This threshold was lowered by increasing negative frequency-dependent selection and when reducing the two-fold cost of sex. It contributes advocating that the different processes involved in a short-term advantage of sex and recombination can act in combination to favor the fixation of sexual reproduction in populations.

Evidence from automixis with inverted meiosis for the maintenance of sex by loss of complementation

The adaptive value of sexual reproduction is still debated. A short-term disadvantage of asexual reproduction is loss of heterozygosity, which leads to the unmasking of recessive deleterious mutations. The cost of this loss of complementation is predicted to be higher than the twofold cost of meiosis for most types of asexual reproduction. Automixis with terminal fusion of sister nuclei is especially vulnerable to the effect of loss of complementation. It is found, however, in some taxa including oribatid mites, the most prominent group of ancient asexuals. Here, I show that automixis with terminal fusion is stable if it is associated with inverted meiosis and that this appears to be the case in nature, notably in oribatid mites. The existence of automixis with terminal fusion, and its co-occurrence with inverted meiosis, therefore, is consistent with the hypothesis that loss of complementation is important in the evolution of sexual reproduction.

Can resource costs of polyploidy provide an advantage to sex?

The predominance of sexual reproduction despite its costs indicates that sex provides substantial benefits, which are usually thought to derive from the direct genetic consequences of recombination and syngamy. While genetic benefits of sex are certainly important, sexual and asexual individuals, lineages, or populations may also differ in physiological and life history traits that could influence outcomes of competition between sexuals and asexuals across environmental gradients. Here, we address possible phenotypic costs of a very common correlate of asexuality, polyploidy. We suggest that polyploidy could confer resource costs related to the dietary phosphorus demands of nucleic acid production; such costs could facilitate the persistence of sex in situations where asexual taxa are of higher ploidy level and phosphorus availability limits important traits like growth and reproduction. We outline predictions regarding the distribution of diploid sexual and polyploid asexual taxa across biogeochemical gradients and provide suggestions for study systems and empirical approaches for testing elements of our hypothesis.

Why are there so many theories for sex, and what do we do with them?

It is widely known that there exists a multitude of possible explanations for the maintenance of sex; however, it is less clear how to handle such an explanatory pluralism. In this paper, we address one older and one more recent discussion on what might constitute a good theory for sex and find that they reflect a trade-off between maximizing the scientific virtues of generalism, realism, and precision. A historical analysis indicates that varying research interests and research backgrounds of the different biologists shape the trade-off. We use the reflection on the trade-offs in order to understand the existence of the diversity of theories in the field and discuss how to address the explanatory pluralism. We find that the existence of multiple theories for sex, that is, explanatory pluralism, is not surprising or embarrassing but can be seen as a resource. Still, it is important to clarify the possibilities of integration of different theories. Integration between certain theories might be complex, however, as they involve models and theories from different disciplines that have diverged historically in both conceptual and methodological aspects.

Male sex drive and the maintenance of sex: evidence from Drosophila

The resolution of the paradoxes surrounding the evolutionary origins and maintenance of sexual reproduction has been a major focus in biology. The operation of sexual selection-which is very common among multicellular organisms-has been proposed as an important factor in the maintenance of sex, though in order for this hypothesis to hold, the strength of sexual selection must be stronger in males than in females. Sexual selection poses its own series of evolutionary questions, including how genetic variability is maintained in the face of sustained directional selection (known as the "paradox of the lek"). In this short review, we present evidence obtained from recent comparative genomics projects arguing that 1) the genomic consequences of sexual selection clearly show that its effect is stronger in males and 2) this sustained selection over evolutionary timescales also has an effect of capturing de novo genes and expression patterns influencing male fitness, thus providing a mechanism via which new genetic variation can be input into to male traits. Furthermore, we argue that this latter process of genomic "masculinization" has an additional effect of making males difficult to purge from populations, as evidence from Drosophila indicates that, for example, many male sexually selected seminal fluid factors are required to ensure maximally efficient reproduction. Newly arising parthenogenic mutations would suffer an immediate reproductive rate disadvantage were these proteins lost. We show that recent studies confirm that genomic masculinization, as a result of "male sex drive," has important consequences for the evolution of sexually dimorphic species.