Intralocus sexual conflict can maintain alternative reproductive tactics

Alternative reproductive tactics (ARTs) are ubiquitous throughout the animal kingdom and widely regarded as an outcome of high variance in reproductive success. Proximate mechanisms underlying ARTs include genetically based polymorphisms, environmentally induced polymorphisms, and those mediated by a combination of genetic and environmental factors. However, few ultimate mechanisms have been proposed to explain the maintenance of ARTs over time, the most important of which have been disruptive and negative frequency-dependent selection. Here we explore the role that intralocus sexual conflict may play in the maintenance of sex-specific ARTs. We use a genetically explicit individual-based model in which body size influences both female fecundity and male tactic through a shared genetic architecture. By modeling ART maintenance under varying selection regimes and levels of sex-specific gene expression, we explore the conditions under which intralocus sexual conflict can maintain a hypothetical ART defined by larger (alpha) and smaller (beta) tactics. Our models consistently revealed that sexual conflict can result in the persistence of a sex-specific polymorphism over hundreds of generations, even in the absence of negative frequency-dependent selection. ARTs were maintained through correlated selection when one male ART has lower fitness but produces daughters with higher fitness. These results highlight the importance of understanding selection on both sexes when attempting to explain the maintenance of ARTs. Our results are consistent with a growing literature documenting genetic correlations between male ARTs and female fitness, suggesting that the maintenance of sex-specific ARTs through intralocus sexual conflict may be common and widespread in nature.

Selective Interference and the Evolution of Sex

Selection acts upon genes linked together on chromosomes. This physical connection reduces the efficiency by which selection can act because, in the absence of sex, alleles must rise and fall together in frequency with the genome in which they are found. This selective interference underlies such phenomena as clonal interference and Muller’s Ratchet and is broadly termed Hill-Robertson interference. In this review, I examine the potential for selective interference to account for the evolution and maintenance of sex, discussing the positive and negative evidence from both theoretical and empirical studies, and highlight the gaps that remain.

When does parasitism maintain sex in the absence of Red Queen Dynamics?

Parasites can select for sexual reproduction in host populations, preventing replacement by faster growing asexual lineages. This is usually attributed to so-called “Red Queen Dynamics” (RQD), where antagonistic coevolution causes fluctuating selection in allele frequencies, which provides sex with an advantage over asex. However, parasitism may also maintain sex in the absence of RQD when sexual populations are more genetically diverse – and hence more resistant, on average – than clonal populations, allowing sex and asex to stably coexist. While the maintenance of sex due to RQD has been studied extensively, the conditions that allow sex and asex to stably coexist have yet to be explored in detail. In particular, we lack an understanding of how host demography and parasite epidemiology affect the maintenance of sex in the absence of RQD. Here, I use an eco-evolutionary model to show that both population density and the type and strength of virulence are important for maintaining sex, which can be understood in terms of their effects on disease prevalence and severity. In addition, I show that even in the absence of heterozygote advantage, asexual heterozygosity affects coexistence with sex due to variation in niche overlap. These results reveal which host and parasite characteristics are most important for the maintenance of sex in the absence of RQD, and provide empirically testable predictions for how demography and epidemiology mediate competition between sex and asex.

Quantifying the prevalence of assortative mating in a human population

For the first time, empirical evidence allowed to construct the frequency distribution of a genetic relatedness index between the parents of about half a million individuals living in the UK. The results suggest that over 30% of the population is the product of parents mating assortatively. The rest is probably the offspring of parents matching the genetic composition of their partners randomly. High degrees of genetic relatedness between parents, i.e. extreme inbreeding, was rare. This result shows that assortative mating is likely to be highly prevalent in human populations. Thus, assuming only random mating among humans, as widely done in ecology and population genetic studies, is not an appropriate approximation to reality. The existence of assortative mating has to be accounted for. The results suggest the conclusion that both, assortative and random mating, are evolutionary stable strategies. This improved insight allows to better understand complex evolutionary phenomena, such as the emergence and maintenance of sex, the speed of adaptation, runaway adaptation, maintenance of cooperation, and many others in human and animal populations.

Evidence for “inter- and intraspecific horizontal genetic transfers” between anciently asexual bdelloid rotifers is explained by cross-contamination

SummaryBdelloid rotifers are microscopic invertebrates thought to have evolved for millions of years without sexual reproduction. They have attracted the attention of biologists puzzled by the maintenance of sex among nearly all other eukaryotes. Bdelloid genomes have an unusually high proportion of horizontally acquired non-metazoan genes. This well-substantiated finding has invited speculation that homologous horizontal transfer between rotifers also may occur, perhaps even 'replacing' sex. A 2016 study in Current Biology claimed to supply evidence for this hypothesis. The authors sampled rotifers of the genus Adineta from natural populations and sequenced one mitochondrial and four nuclear loci. For several samples, species assignments were incongruent among loci, which the authors interpreted as evidence of "interspecific genetic exchanges". Here, we use sequencing chromatograms supplied by the authors to demonstrate that samples treated as individuals actually contained two or more divergent mitochondrial and ribosomal sequences, indicating contamination with DNA from additional animals belonging to the supposed “donor species”. We also show that “exchanged” molecules share only 75% sequence homology, a degree of divergence incompatible with established mechanisms of recombination and genomic features of Adineta. These findings are parsimoniously explained by cross-contamination of tubes with animals or DNA from different species. Given the proportion of tubes contaminated in this way, we show by calculation that evidence for "intraspecific horizontal exchange" in the same dataset is explained by contamination with conspecific DNA. On the clear evidence of these analyses, the 2016 study provides no reliable support for the hypothesis of horizontal genetic transfer between or within these bdelloid species.