ABSTRACT
Mate choice is hypothesized to play an important role in maintaining high diversity at major histocompatibility complex (MHC) genes in vertebrates. Many studies have revealed that females across taxa prefer the scent of males with MHC genotypes different to their own. In this study we tested the "opposites-attract" hypothesis in two species of darter with known differences in female criteria used in mate choice: in the fantail darters (a paternal-care species), females prefer males with visual traits related to nest guarding and egg tending, while in rainbow darters (not a paternal-care species) female mate choice criteria are unknown. In dichotomous mate-choice trials, we presented females of both species with the scents of conspecific males with MHC class IIb genotypes that were either similar or dissimilar to that of the focal female. We evaluated the proportion of time each female spent with each male and calculated the average strength of female preference for both species. Female fantail darters demonstrated a preference for the scent of males with similar (rather than dissimilar) MHC genotypes, but this result was not statistically significant. Rainbow darter females showed no preference for the scent of males with similar or dissimilar MHC genotypes. Our results do not support the "opposites-attract" hypothesis in darters.
ABSTRACT
Major Histocompatibility Complex (MHC) genes are one of the most polymorphic gene groups known in vertebrates. MHC genes also exhibit allelic variants that are shared among taxa, referred to as trans-specific polymorphism (TSP). The role that selection plays in maintaining such high diversity within species, as well as TSP, is an ongoing discussion in biology. In this study, we used deep-sequencing techniques to characterize MHC class IIb gene diversity in three sympatric species of darters. We found at least 5 copies of the MHC gene in darters, with 126 genetic variants encoding 122 unique amino acid sequences. We identified four supertypes based on the binding properties of proteins encoded by the sequences. Although each species had a unique pool of variants, many variants were shared between species pairs and across all three species. Phylogenetic analysis showed that the variants did not group together monophyletically based on species identity or on supertype. An expanded phylogenetic analysis showed that some darter alleles grouped together with alleles from other percid fishes. Our findings show that TSP occurs in darters, which suggests that balancing selection is acting at the genotype level. Supertypes, however, are most likely evolving convergently, as evidenced by the fact that alleles do not form monophyletic groups based on supertype. Our research demonstrates that selection may be acting differently on MHC genes at the genotype and supertype levels, selecting for the maintenance of high genotypic diversity while driving the convergent evolution of similar MHC phenotypes across different species.
ABSTRACT
A growing body of research suggests that many clonal populations maintain genetic diversity even without occasional sexual reproduction. The purpose of our study was to document variation in single-nucleotide polymorphism (SNP) diversity, DNA content, and pathogen susceptibility in clonal lineages of the New Zealand freshwater snail, Potamopyrgus antipodarum. We studied snails that were collected from multiple field sites around a single lake (Lake Alexandrina), as well as isofemale clonal lineages that had been isolated and maintained in the laboratory. We used the kompetitive allele specific PCR (KASP) method to genotype our samples at 46 nuclear SNP sites, and we used flow cytometry to estimate DNA content. We found high levels of SNP diversity, both in our field samples and in our clonal laboratory lines. We also found evidence of high variation in DNA content among clones, even among clones with identical genotypes across all SNP sites. Controlled pathogen exposures of the laboratory populations revealed variation in susceptibility among distinct clonal genotypes, which was independent of DNA content. Taken together, these results show high levels of diversity among asexual snails, especially for DNA content, and they suggest rapid genome evolution in asexuals.
Subject(s)
Ploidies , Polymorphism, Single Nucleotide , Reproduction, Asexual/genetics , Snails/genetics , Animals , Genetics, Population , Genotype , New ZealandABSTRACT
The striated darter (Etheostoma striatulum Page & Braasch) is a rare fish species restricted to the Duck River system in Maury County, Tennessee, USA. In this paper we describe Aethycteron striatuli n. sp., a gill parasite from a genus that has specialised on darters as the parasites' preferred hosts. Aethycteron striatuli morphologically resembles A. malleus (Mueller, 1938), A. robisoni Cloutman & McAllister, 2017, A. chlorosomus (Harrises & Vickery, 1970) and A. micropercae Suriano & Beverley-Burton, 1982. It is distinguished from other species in that it is one of the smallest described parasites in this genus to date and possesses a small male copulatory organ with a distinct accessory piece. This is the first monogenean parasite reported from E. striatulum, a species that is currently under threat due to its restricted range and the potential for habitat loss. It is essential to expand our knowledge of the host and the pressures it faces in its limited habitat, including disease and parasite pressures, so that measures can be taken to preserve this vulnerable native species.
