Evolution of bindin in the pantropical sea urchin Tripneustes: comparisons to bindin of other genera

Bindin, a sea urchin sperm protein, mediates sperm-egg attachment and membrane fusion and is thus important in species recognition and speciation. Patterns of bindin variation differed among three genera that had been studied previously. In two genera of the superorder Camarodonta, Echinometra and Strongylocentrotus, both of which contain sympatric species, bindin is highly variable within and between species; a region of the molecule evolves at high rates under strong positive selection. In Arbacia, which belongs to the superorder Stirodonta and whose extant species are all allopatric, bindin variation is low, and there is no evidence of positive selection. We cloned and sequenced bindin from Tripneustes, a sea urchin that belongs to the Camarodonta but whose three species are found in different oceans. Worldwide sampling of bindin alleles shows that the bindin of Tripneustes (1) contains the highly conserved core characteristic of all other bindins characterized to date, (2) has an intron in the same position, and (3) has approximately the same length. Its structure is more like that of bindin from other camarodont sea urchins than to bindin from the stirodont ARBACIA: The resemblances to other camarodonts include a glycine-rich repeat structure upstream of the core and lack of a hydrophobic domain 3' of the core, a characteristic of Arbacia bindin. Yet the mode of evolution of Tripneustes bindin is more like that of Arbacia. Differences between bindins of the Caribbean Tripneustes ventricosus and the eastern Pacific T. depressus, separated for 3 my by the Isthmus of Panama, are limited to four amino acid changes and a single indel. There are no fixed amino acid differences or indels between T. depressus from the eastern Pacific and T. gratilla from the Indo-Pacific. Bindin of Tripneustes, like that of Arbacia, also shows no evidence of diversifying selection that would manifest itself in a higher proportion of amino acid replacements than of silent nucleotide substitutions. When the rate of intrageneric bindin divergence is standardized by dividing it by cytochrome oxidase I (COI) divergence, Tripneustes and Arbacia show a lower ratio of bindin to COI substitutions between the species of each genus than exists between the species of either Echinometra or Strongylocentrotus. Thus, mode of bindin evolution is not correlated with phylogenetic affinities or molecular structure, but rather with whether the species in a genus are allopatric or sympatric. For a molecule involved in gametic recognition, this would suggest a pattern of evolution via reinforcement. However, in bindin the process that gave rise to this pattern is not likely to have been selection to avoid hybridization, because there is no excess of amino acid replacements between species versus within species in the bindins of Echinometra and Strongylocentrotus, as would have been expected if specific recognition were the driving force in their evolution. We suggest instead that the pattern of reinforcement is a secondary effect of the ability of species with rapidly evolving bindins to coexist in sympatry.