Biomedical subjects

Michael A McCartney

Publications and source records attributed to Michael A McCartney.

4 recordsLinked to original sources

PubMed2005

Sea urchin bindin divergence predicts gamete compatibility.

Studies on the evolution of reproductive proteins have shown that they tend to evolve more rapidly than other proteins, frequently under positive selection. Progress on understanding the implications of these patterns is possible for marine invertebrates, where molecular evolution can be linked to gamete compatibility. In this study, we surveyed data from the literature from five genera of sea urchins for which there was information on gamete compatibility, divergence of the sperm-egg recognition protein bindin, and mitochondrial divergence. We draw three conclusions: (1) bindin divergence at nonsynonymous sites predicts gamete compatibility, whereas (2) bindin divergence at synonymous sites and mitochondrial DNA divergence do not, and (3) as few as 10 amino acid changes in bindin can lead to complete gamete incompatibility between species. Using mitochondrial divergence as a proxy for time, we find that complete gamete incompatibility can evolve in approximately one and a half million years, whereas sister species can maintain complete gamete compatibility for as long as five million years.

Amino Acid Sequence↗

PubMed2004

Adaptive evolution of sperm bindin tracks egg incompatibility in neotropical sea urchins of the genus Echinometra.

Bindin is a gamete recognition protein known to control species-specific sperm-egg adhesion and membrane fusion in sea urchins. Previous analyses have shown that diversifying selection on bindin amino acid sequence is found when gametically incompatible species are compared, but not when species are compatible. The present study analyzes bindin polymorphism and divergence in the three closely related species of Echinometra in Central America: E. lucunter and E. viridis from the Caribbean, and E. vanbrunti from the eastern Pacific. The eggs of E. lucunter have evolved a strong block to fertilization by sperm of its neotropical congeners, whereas those of the other two species have not. As in the Indo-West Pacific (IWP) Echinometra, the neotropical species show high intraspecific bindin polymorphism in the same gene regions as in the IWP species. Maximum likelihood analysis shows that many of the polymorphic codon sites are under mild positive selection. Of the fixed amino acid replacements, most have accumulated along the bindin lineage of E. lucunter. We analyzed the data with maximum likelihood models of variation in positive selection across lineages and codon sites, and with models that consider sites and lineages simultaneously. Our results show that positive selection is concentrated along the E. lucunter bindin lineage, and that codon sites with amino acid replacements fixed in this species show by far the highest signal of positive selection. Lineage-specific positive selection paralleling egg incompatibility provides support that adaptive evolution of sperm proteins acts to maintain recognition of bindin by changing egg receptors. Because both egg incompatibility and bindin divergence are greater between allopatric species than between sympatric species, the hypothesis of selection against hybridization (reinforcement) cannot explain why adaptive evolution has been confined to a single lineage in the American Echinometra. Instead, processes acting to varying degrees within species (e.g., sperm competition, sexual selection, and sexual conflict) are more promising explanations for lineage-specific positive selection on bindin.

Adaptation, Biological↗

PubMed2003

Genetic mosaic in a marine species flock.

We used molecular approaches to study the status of speciation in coral reef fishes known as hamlets (Serranidae: Hypoplectrus). Several hamlet morphospecies coexist on Caribbean reefs, and mate assortatively with respect to their strikingly distinct colour patterns. We provide evidence that, genetically, the hamlets display characteristics common in species flocks on land and in freshwaters. Substitutions within two mitochondrial DNA (mtDNA) protein-coding genes place hamlets within a monophyletic group relative to members of two related genera (Serranus and Diplectrum), and establish that the hamlet radiation must have been very recent. mtDNA distances separating hamlet morphospecies were slight (0.6 +/- 0.04%), yielding a coalescent estimate for the age of the hamlet flock of approximately 430 000 years. Morphospecies did not sort into distinct mtDNA haplotype phylogroups, and alleles at five hypervariable microsatellite loci were shared broadly across species boundaries. None the less, molecular variation was not distributed at random. Analyses of mtDNA haplotype frequencies and nested clades in haplotype networks revealed significant genetic differences between geographical regions and among colour morphospecies. We also observed significant microsatellite differentiation between geographical regions and in Puerto Rico, among colour morphospecies; the latter providing evidence for reproductive isolation between colour morphospecies at this locale. In our Panama collection, however, colour morphospecies were mostly genetically indistinguishable. This mosaic pattern of DNA differentiation implies a complex interaction between population history, mating behaviour and geography and suggests that porous boundaries separate species in this flock of brilliantly coloured coral reef fishes.

Animals↗

PubMed2002

Quantitative analysis of gametic incompatibility between closely related species of neotropical sea urchins.

Species of the sea urchin genus Echinometra found on the two coasts of Panamá are recently diverged and only partially isolated by incomplete barriers to interspecific fertilization. This study confirms previous work that revealed incompatibility between the eggs of the Atlantic E. lucunter and the sperm of the other two neotropical species, whereas eggs of its sympatric congener E. viridis and allopatric E. vanbrunti are largely compatible with heterospecific sperm. Here we quantify fertilization using a range of sperm dilutions. We demonstrate a much stronger block to cross-species fertilization of E. lucunter eggs than was previously shown at fixed sperm concentrations, and mild incompatibility of the other two species' eggs where previous crosses between species were not distinguishable from within-species controls. Additionally, we present evidence for intraspecific variation in egg receptivity towards heterospecific sperm. Our findings here again discount the "reinforcement model" as a viable explanation for the pattern of prezygotic isolation. Gamete incompatibility in these Echinometra has appeared recently-within the last 1.5 million years-but is weaker in sympatry than in allopatry. Accidents of history may help explain why incompatibility of eggs emerged in one species and not in others. Compensatory sexual selection on sperm in this species could follow, and promote divergence of proteins mediating sperm-egg recognition.

Animals↗