Molecular evolution of sperm-egg recognition and species boundaries in closely related sea urchins

Abstract

Among marine organisms with high dispersal capabilities and limited geographic barriers, new species have arisen in abundance. Such organisms may diversify in ways that differ from strict allopatric speciation as understood from studies of terrestrial systems. The work reported here is a study of the evolution of fertilization barriers between species of Indo-Pacific sea urchins in the genus Echinometra. These urchins form a complex of closely related sibling species with broad overlapping ranges. Barriers to cross-fertilization between the species are pronounced and reciprocal. Electron microscopy and electrophysiology showed that fertilization barriers between two Hawaiian species occur during sperm-egg attachment and fusion. In urchins, these events involve interactions of a sperm protein called bindin and its receptor on the egg surface. Echinometra bindin proteins showed greater binding of conspecific eggs than heterospecific eggs, indicating that this interaction is species-selective. In order to examine the molecular evolution of reproductive isolation, bindin gene sequences were obtained from three Echinometra species. Species-specific amino acid replacements between bindin proteins change the charge or polarity of the molecule, and there is evidence that positive selection has favored such replacements. Extensive polymorphism of bindin alleles within species indicates that the variation required for the evolution of reproductive isolation exists in natural populations, and suggests parallels with previous models of the evolution of assortative mating. Rare phenotypically-intermediate urchins possess bindin alleles characteristic of two different species, confirming a hybrid origin. Hybrid fitness was studied by measuring fertilization; the results suggest a hybrid advantage when sperm are in excess, but not when sperm are limiting. Mate recognition behaviors have parallels at the molecular level in the interactions of sperm-egg recognition proteins. Reproductive isolation may depend on few gamete recognition loci that evolve rapidly in a process of sexual selection. This type of species boundary is potentially important in a very large number of taxa in which mate recognition is focused on gametes, including unicellular organisms and free-spawning plants and animals. Fertilization barriers provide an alternative to long-term geographic barriers, and are likely to be an important aspect of speciation and biodiversity in marine organisms.