Gene duplication is the primary mechanism leading to new genes and phenotypic novelty, but the pr... more Gene duplication is the primary mechanism leading to new genes and phenotypic novelty, but the proximate evolutionary processes underlying gene family origin, maintenance, and expansion are poorly understood. Although sub-and neofunctionalization provide clear long-term advantages, selection does not act with foresight, and unless a redundant gene copy provides an immediate fitness advantage, the copy will most likely be lost. Many models for the evolution of genes immediately following duplication have been proposed, but the robustness and applicability of these models is unclear because of the lack of data at the population-level. We used qPCR, protein expression data, genome sequencing, and hybrid enrichment to test three competing models that differ in whether selection favoring the spread of duplicates acts primarily on expression level or sequence diversity for specific toxin-encoding loci in the eastern diamondback rattlesnake (Crotalus adamanteus). We sampled 178 individuals and identified significant inter-and intrapopulation variation in copy number, demonstrated that copy number was significantly and positively correlated with protein expression, and found little to no sequence variation across paralogs in all populations. Collectively, these results demonstrate that selection for increased expression, not sequence diversity, was the proximate evolutionary process underlying gene family origin and expansion, providing data needed to resolve the debate over which evolutionary processes govern the fates of gene copies immediately following duplication.
Maladaptive hybridization is hypothesized to be an important force driving the evolution of repro... more Maladaptive hybridization is hypothesized to be an important force driving the evolution of reproductive isolation between closely related species. Because the magnitude and direction of selection can vary across a life cycle, an accurate understanding of the ubiquity of reinforcement requires fitness to be estimated across the life cycle, but the literature is surprisingly depauperate of such studies. We present fitness estimates of laboratory-raised hybrids between the chorus frogs Pseudacris feriarum and Pseudacris nigrita—two species that have undergone reproductive character displacement where they come into secondary contact. By studying viability, mating success, and fertility across the life cycle, we find strong support for reinforcement as the force driving displacement in this system. Specifically, we find hybrid fitness is reduced by 44%. This reduction results from both sexual selection against hybrid males and natural selection on male fertility, but not viability selection. Sexual selection against hybrid males is four times stronger than natural selection. Hybrid female fitness is not reduced, however, suggesting that Haldane's rule may be operating in this system if males are heterogametic. We also found higher variation in hybrid male fertilization success relative to P. feriarum males, suggesting that the hybrid incompatibility genes are polymorphic within one or both of the parent species.
Gene duplication is the primary mechanism leading to new genes and phenotypic novelty, but the pr... more Gene duplication is the primary mechanism leading to new genes and phenotypic novelty, but the proximate evolutionary processes underlying gene family origin, maintenance, and expansion are poorly understood. Although sub-and neofunctionalization provide clear long-term advantages, selection does not act with foresight, and unless a redundant gene copy provides an immediate fitness advantage, the copy will most likely be lost. Many models for the evolution of genes immediately following duplication have been proposed, but the robustness and applicability of these models is unclear because of the lack of data at the population-level. We used qPCR, protein expression data, genome sequencing, and hybrid enrichment to test three competing models that differ in whether selection favoring the spread of duplicates acts primarily on expression level or sequence diversity for specific toxin-encoding loci in the eastern diamondback rattlesnake (Crotalus adamanteus). We sampled 178 individuals and identified significant inter-and intrapopulation variation in copy number, demonstrated that copy number was significantly and positively correlated with protein expression, and found little to no sequence variation across paralogs in all populations. Collectively, these results demonstrate that selection for increased expression, not sequence diversity, was the proximate evolutionary process underlying gene family origin and expansion, providing data needed to resolve the debate over which evolutionary processes govern the fates of gene copies immediately following duplication.
Maladaptive hybridization is hypothesized to be an important force driving the evolution of repro... more Maladaptive hybridization is hypothesized to be an important force driving the evolution of reproductive isolation between closely related species. Because the magnitude and direction of selection can vary across a life cycle, an accurate understanding of the ubiquity of reinforcement requires fitness to be estimated across the life cycle, but the literature is surprisingly depauperate of such studies. We present fitness estimates of laboratory-raised hybrids between the chorus frogs Pseudacris feriarum and Pseudacris nigrita—two species that have undergone reproductive character displacement where they come into secondary contact. By studying viability, mating success, and fertility across the life cycle, we find strong support for reinforcement as the force driving displacement in this system. Specifically, we find hybrid fitness is reduced by 44%. This reduction results from both sexual selection against hybrid males and natural selection on male fertility, but not viability selection. Sexual selection against hybrid males is four times stronger than natural selection. Hybrid female fitness is not reduced, however, suggesting that Haldane's rule may be operating in this system if males are heterogametic. We also found higher variation in hybrid male fertilization success relative to P. feriarum males, suggesting that the hybrid incompatibility genes are polymorphic within one or both of the parent species.
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Papers by Emily Lemmon