Nucleotide sequences were analyzed by using macvector (Accelrys, San Diego). GenBank accession numbers for VERL repeats 1–3 are AF490760 (white abalone, Haliotis sorenseni), AF490761 (pinto abalone, Haliotis kamtschatkana), AF490762 (flat abalone, Haliotis walallensis), AF490763 (Japanese abalone, Haliotis discus), AF490764 (pink abalone, Haliotis corrugata), AF490765 (black abalone, Haliotis cracherodii), and AF490766 (green abalone, Haliotis fulgens). (Two other primers occasionally used were sense 5′-CCAGAGAGTTGTCTTTCAAGT-3′ and antisense 5′-GGTTACAAGACG ACTGAT-3′.) PCR products were cloned into pCR 2.1-TOPO and sequenced by using vector primers (TOPO TA Cloning Kit, Invitrogen). A sense primer 5′ to repeat 1 (5′-GATACCCCAGACCCCAGAGTG-3′) and an antisense primer (5′-TTGGCTGGAATGCTCTC-3′) within the 5′ portion of repeat 4 were based on the full-length red abalone VERL sequence (GenBank accession no. The result of this continuous coevolution of the gamete recognition system could be the splitting of one population into two that are reproductively isolated (speciation).īecause full-length VERL has no introns within the array of 22 repeats, repeats 1–3 were amplified from purified genomic DNA of the seven abalone species by using standard PCR methods ( 7). These data provide molecular support for theoretical models showing that the two sexes are locked in a “coevolutionary chase” that could be driven by processes such as sexual selection, sexual conflict, or microbial attack (pathogen avoidance). Here we report that the sequence diversity of the amino-terminal end of the egg vitelline envelope receptor for lysin has been promoted by positive Darwinian selection. Several mathematical simulations predict that both male and female reproductive proteins should evolve rapidly by positive selection. The egg vitelline envelope receptor for lysin had previously been shown to evolve neutrally and be subjected to concerted evolution. In abalone, sperm lysin evolves rapidly by positive Darwinian selection. In free-spawning animals with no complex mating behavior, prezygotic reproductive isolation (speciation) could result from the rapid divergence of genes coding for sperm and egg proteins that bind each other during fertilization. The mechanism of speciation is a central problem in evolutionary biology.
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