Chromosome inheritance and reproductive barriers in backcrosses between two hybridizing Viviparus snail species

Hybridization must be followed by repeated backcrossing of the subsequent hybrid generations to the parental species for gene exchange between species to occur. Due to meiotic failures, first-generation hybrids of some species produce unreduced gametes. Their progeny in backcrosses with a diploid parental species are polyploid and functionally sterile. Polyploidy of the backcross generation may therefore act as an instantaneous barrier to gene flow between hybrids and the parental species. Here we determined chromosome inheritance in backcrosses of two hybridizing freshwater caenogastropod snail species to assess whether gene introgression is inhibited in the first backcross generation. Viviparus ater and V. contectus intermate in nature and produce viable F1 hybrid progeny, although offspring sex ratio is strongly male biased. Despite the different chromosome numbers of the two parental species (V. ater, 2n = 18; V. contectus, 2n = 14), the F1 hybrids are able to reproduce. Allozyme data from natural populations are compatible with gene exchange between the two species, although there is also evidence suggesting that some alleles may be shared because of common ancestry. Our study revealed that all viable backcross progeny were homoploid as they inherited between seven and nine chromosomes from the hybrid father. The siring success of the karyotypically different hybrid sperm was skewed against one sperm karyotype depending on the non-hybrid mother in the cross. In backcross broods of V. ater females, the observed distribution of the karyotypes conformed with an assumption of random segregation of two unpaired chromosomes at meiosis in hybrid males. In contrast, when backcrossing hybrid males to V. contectus females, post-copulatory processes ultimately determined the karyotype distribution of the backcross progeny. Homoploidy of all backcross progeny together with the presence of sperm and embryos in their gonads makes gene exchange between the two parental species through hybridization possibl

The Karyotype of the Yellow Dung Fly, Scathophaga stercoraria, a Model Organism in Studies of Sexual Selection

Knowledge of karyotypical characteristics of a species is essential for understanding how sexually selected and sexually antagonistic traits evolve. The yellow dung fly Scathophaga stercoraria L. (Diptera: Scathophagidae) is an established model system for studies of sexual selection and sexual conflict, but karyotypical data are lacking to date. Here, the karyotype of S. stercoraria was characterized using conventional Giemsa-staining and C-banding techniques. The diploid chromosome set consists of 6 pairs of bi-armed meta- or submetacentric chromosomes. The sex chromosomes are the largest chromosomes and constitute 30% of the total length of the diploid set in females and about 25% in males. Males are the heterogametic sex, and the length of the Y chromosome is about three-quarters of that of the X chromosome. C-banding revealed that both sex chromosomes are largely heterochromatic. In contrast, in the five autosome pairs, heterochromatin is limited to narrow bands in the centromeric regions. This karyotypic information will help provide a more profound understanding of the inheritance of phenotypic variation in reproductive traits and the chromosomal locations of underlying genes

Paternal condition affects offspring reproduction and life history in a sex-specific manner in Drosophila melanogaster

Nongenetic parental effects can contribute to the adaptation of species to changing environments by circumventing some of the limitations of genetic inheritance. A clearer understanding of the influence of nongenetic inheritance and its potentially sex-specific responses in daughters and sons is needed to better predict the evolutionary trajectories of species. However, whereas nongenetic maternal effects have long been recognized and widely studied, comparatively little is known about corresponding paternal effects. Here, by following 30 isogenic lines of Drosophila melanogaster across two generations, each reared under two dietary regimes in each generation, we tested how protein restriction during larval development of the fathers affects the fitness and health of their daughters and sons. We then quantified genetic and non-genetic paternal, and direct environmental, effects across multiple axes of offspring fitness. Daughters and sons responded differently to their father’s developmental history. While isolines differed in mean trait values, their specific responses to protein restriction generally varied little. The sex- and trait-specific responses to paternal effects emphasize the complexity of inter-generational parental effects, which raise important questions about their mode of transmission and adaptive value, including the potential for conflict between the sexes

Experimental Removal of Sexual Selection Reveals Adaptations to Polyandry in Both Sexes

Polyandrous mating is extremely common, yet for many species the evolutionary significance is not fully resolved. In order to understand the evolution of mating systems, it is crucial that we investigate the adaptive consequences across many facets of reproduction. We performed experimental evolution with the naturally polygamous flour beetle Tribolium castaneum subjected to either polyandry or enforced monogamy, creating contrasting selection regimes associated with the presence or absence of sexual selection. After 36 generations, we investigated male and female adaptations by mating beetles with an unselected tester strain to exclude potential effects of male-female coevolution. Reproductive success of focal monogamous and polyandrous beetles from each sex was assessed in separate single male and multiple male experiments emulating the different selection backgrounds. Males and females from the polyandrous regime had more offspring in the experiments with multiple males present than monogamous counterparts. However, in single male experiments, neither females nor males differed between selection regimes. Subsequent mating trials with multiple males suggested that adaptations to polyandry in both sexes provide benefits when choice and competition were allowed to take place. Polyandrous females delayed the first copulation when given a choice of males and polyandrous males were quicker to achieve copulation when facing competition. In conclusion, we show that the expected benefits of evolutionary adaptation to polyandry in T. castaneum depended on the availability of multiple mates. This context-dependent effect, which concerned both sexes, highlights the importance of realistic competition and choice experiments

Genetic variance in fitness and its cross-sex covariance predict adaptation during experimental evolution

The additive genetic variation (VA) of fitness in a population is of particular importance to quantify its adaptive potential and predict its response to rapid environmental change. Recent statistical advances in quantitative genetics and the use of new molecular tools have fostered great interest in estimating fitness VA in wild populations. However, the value of VA for fitness in predicting evolutionary changes over several generations remains mostly unknown. In our study, we addressed this question by combining classical quantitative genetics with experimental evolution in the model organism Tribolium castaneum (red flour beetle) in three new environmental conditions (Dry, Hot, Hot‐Dry). We tested for potential constraints that might limit adaptation, including environmental and sex genetic antagonisms captured by negative genetic covariance between environments and female and male fitness, respectively. Observed fitness changes after 20 generations mainly matched our predictions. Given that body size is commonly used as a proxy for fitness, we also tested how this trait and its genetic variance (including nonadditive genetic variance) were impacted by environmental stress. In both traits, genetic variances were sex and condition dependent, but they differed in their variance composition, cross‐sex and cross‐environment genetic covariances, as well as in the environmental impact on VA

Chromosome inheritance and reproductive barriers in backcrosses between two hybridizing Viviparus snail species

Hybridization must be followed by repeated backcrossing of the subsequent hybrid generations to the parental species for gene exchange between species to occur. Due to meiotic failures, first-generation hybrids of some species produce unreduced gametes. Their progeny in backcrosses with a diploid parental species are polyploid and functionally sterile. Polyploidy of the backcross generation may therefore act as an instantaneous barrier to gene flow between hybrids and the parental species. Here we determined chromosome inheritance in backcrosses of two hybridizing freshwater caenogastropod snail species to assess whether gene introgression is inhibited in the first backcross generation. Viviparus ater and V. contectus intermate in nature and produce viable F1 hybrid progeny, although offspring sex ratio is strongly male biased. Despite the different chromosome numbers of the two parental species (V. ater, 2n = 18; V. contectus, 2n = 14), the F1 hybrids are able to reproduce. Allozyme data from natural populations are compatible with gene exchange between the two species, although there is also evidence suggesting that some alleles may be shared because of common ancestry. Our study revealed that all viable backcross progeny were homoploid as they inherited between seven and nine chromosomes from the hybrid father. The siring success of the karyotypically different hybrid sperm was skewed against one sperm karyotype depending on the non-hybrid mother in the cross. In backcross broods of V. ater females, the observed distribution of the karyotypes conformed with an assumption of random segregation of two unpaired chromosomes at meiosis in hybrid males. In contrast, when backcrossing hybrid males to V. contectus females, post-copulatory processes ultimately determined the karyotype distribution of the backcross progeny. Homoploidy of all backcross progeny together with the presence of sperm and embryos in their gonads makes gene exchange between the two parental species through hybridization possibl

Condition‐dependent interaction between mating success and competitive fertilization success in Drosophila melanogaster

Dietary restriction during development can affect adult body size and condition. In many species, larger (high‐condition) males gain higher mating success through male‐male competition and female choice, and female condition can affect the extent of both female mate choice and male investment in courtship or ejaculates. However, few studies have examined the joint effects and interplay of male and female condition during both the pre‐ and the postcopulatory phases of sexual selection. We therefore manipulated the larval diet of male and female Drosophila melanogaster to study how body size variation in both sexes biases competitive outcomes at different reproductive stages, from mating to paternity. We did not find a difference in mate preference or mating latency between females of different conditions, nor any interaction between male and female conditions. However, large males were more successful in gaining matings, but only when in direct competition, whereas mating latencies were shorter for low‐condition males in noncompetitive settings. Small males also transferred more sperm to nonvirgin females, displaced a larger proportion of resident sperm, and achieved higher paternity shares per mating than large males. In agreement with existing theory, we suggest that small males might partially compensate for their low mating success by strategically investing in larger sperm numbers and potentially other, unmeasured ejaculate traits, when they do have a mating opportunity

Impacts of starvation on male reproductive success in Tribolium castaneum

ISSN:1522-0613ISSN:1937-379