Variation in the vertebrate major histocompatibility complex (MHC) genes is crucial for fighting pathogen assault. Because new alleles confer a selective advantage, MHC should readily introgress between species, even under limited hybridization. Using replicated transects through two hybrid zones between strongly reproductively isolated European newts, Lissotriton montandoni and L. vulgaris, we demonstrated recent and ongoing MHC class I and II introgression in the Carpathian region. The extent of introgression correlated with the age of contact. In the older zone, MHC similarity between species within transects exceeded similarity between transects within species, implying pervasive introgression ‐ a massive exchange of MHC genes, not limited to specific variants. In simulations, the observed pattern emerged under the combined action of balancing selection and hybridization, but not when these processes acted separately. Thus, massive introgression at advanced stages of divergence can introduce novel and restore previously lost MHC variation, boosting the adaptive potential of hybridizing taxa. In consequence, MHC genes may be the last to stop introgressing between incipient species.

Selection acting on males can reduce mutation load of sexual relative to asexual populations, thus mitigating the twofold cost of sex, provided that it seeks and destroys the same mutations as selection acting on females, but with higher efficiency. This could happen due to sexual selection-a potent evolutionary force that in most systems predominantly affects males. We used replicate populations of red flour beetles (Tribolium castaneum) to study sex-specific selection against deleterious mutations introduced with ionizing radiation. We found no evidence for selection being stronger in males than in females; in fact, we observed a nonsignificant trend in the opposite direction. This suggests that selection on males does not reduce mutation load below the level expected under the (hypothetical) scenario of asexual reproduction. Additionally, we employed a novel approach, based on a simple model, to quantify the relative contributions of sexual and offspring viability selection to the overall selection observed in males. We found them to be similar in magnitude; however, only the offspring viability component was statistically significant. In summary, we found no support for the hypothesis that selection on males in general, and sexual selection in particular, contributes to the evolutionary maintenance of sex.

However, the evolutionary significance of introgression is less clear.Is it primarily a manifestation of porous species boundaries, akin to intraspecific gene flow, or is it a process of adaptive significance? As reasonably well-documented examples of adaptively introgressed variants accumulate (Hedrick, 2013;Jones et al., 2018), the field moves towards identification of its general properties. The broad

Sexual selection may increase population‐level fitness by facilitating the removal of deleterious mutations with pleiotropic effects on competition for fertilizations as well as other fitness components in both sexes. Under inbreeding, this could promote purging selection, that is the removal of deleterious recessive alleles exposed in homozygotes via matings between closely related individuals. Here, in two independent experiments, we found no evidence for short‐term purging of the inbreeding load from severely bottlenecked populations of red flour beetles, Tribolium castaneum. We hypothesize that sexual selection may have dual effects on purging, corresponding to good‐genes versus compatible‐genes mechanisms. Whereas the former should facilitate the removal of inbreeding load from bottlenecked populations, the latter may actually hamper this process while simultaneously limiting inbreeding depression by preventing the expression of deleterious recessives.

It is the process of gene exchange between differentiated populations, including species that are not fully reproductively isolated, through hybridisation and repeated backcrossing of hybrid offspring into the parental populations (Aguillon et al., 2022;Anderson, 1953).Introgression is often limited by reduced fitness of hybrids stemming

10 Variation in the vertebrate major histocompatibility complex (MHC) genes is crucial for 11 fighting pathogen assault. Because new alleles confer a selective advantage, MHC should 12 readily introgress between species, even under limited hybridization. Using replicated 13 transects through two hybrid zones between strongly reproductively isolated newts, we 14 demonstrated recent and ongoing MHC introgression. Its extent correlated with the age of 15 contact. In the older zone, MHC similarity between species within transects exceeded that of 16 between transects within species, implying pervasive introgression -a massive exchange of 17

According to theory, sexual selection in males may efficiently purge mutation load of sexual populations, reducing or fully compensating 'the cost of males'. For this to occur, mutations not only need to be deleterious to both sexes, they also must affect males more than females. A frequently overlooked problem is that relative strength of selection on males versus females may vary between environments, with social conditions being particularly likely to affect selection in males and females differently. Here, we induced mutations in red flour beetles (Tribolium castaneum) and tested their effect in both sexes under three different operational sex ratios (1:2, 1:1 and 2:1). Induced mutations decreased fitness of both males and females, but their effect was not stronger in males. Surprisingly, operational sex ratio did not affect selection against deleterious mutations nor its relative strength in the sexes. Thus, our results show no support for the role of sexual selection in the evolutionary maintenance of sex.

Selection acting on males can reduce mutation load of sexual relative to asexual populations, thus mitigating the two-fold cost of sex. This requires that it seeks and destroys the same mutations as selection acting on females, but with higher efficiency, which could happen due to sexual selection-a potent evolutionary force that in most systems predominantly affects males. We used replicate populations of red flour beetles (Tribolium castaneum) to study sex-specific selection against deleterious mutations introduced with ionizing radiation. Additionally, we employed a novel approach to quantify the relative contribution of sexual selection to the overall selection observed in males. The induced mutations were selected against in both sexes, with decreased sexual competitiveness contributing, on average, over 40% of the total decline in male fitness. However, we found no evidence for selection being stronger in males than in females; in fact, we observed a non-significant trend in the opposite direction. These results suggest that selection on males does not reduce mutation load below the level expected under the (hypothetical) scenario of asexual reproduction. Thus, we found no support for the hypothesis that sexual selection contributes to the evolutionary maintenance of sex.