Abstract
Sperm and egg proteins constitute a remarkable paradigm in evolutionary biology: despite their fundamental role in mediating fertilization (suggesting stasis), some of these molecules are among the most rapidly evolving ones known, and their divergence can lead to reproductive isolation. Because of strong selection to maintain function among interbreeding individuals, interacting fertilization proteins should also exhibit a strong signal of correlated divergence among closely related species. We use evidence of such molecular co-evolution to target biochemical studies of fertilization in North Pacific abalone (Haliotis spp.), a model system of reproductive protein evolution. We test the evolutionary rates (d N/d S) of abalone sperm lysin and two duplicated egg coat proteins (VERL and VEZP14), and find a signal of co-evolution specific to ZP-N, a putative sperm binding motif previously identified by homology modeling. Positively selected residues in VERL and VEZP14 occur on the same face of the structural model, suggesting a common mode of interaction with sperm lysin. We test this computational prediction biochemically, confirming that the ZP-N motif is sufficient to bind lysin and that the affinities of VERL and VEZP14 are comparable. However, we also find that on phylogenetic lineages where lysin and VERL evolve rapidly, VEZP14 evolves slowly, and vice versa. We describe a model of sexual conflict that can recreate this pattern of anti-correlated evolution by assuming that VEZP14 acts as a VERL mimic, reducing the intensity of sexual conflict and slowing the co-evolution of lysin and VERL.
Highlights
Reproductive genes are often identified in genome-wide scans as targets of positive selection, and some are among the most rapidly evolving proteins known [1,2,3,4]
Abalone sperm de-condense the Vitelline Envelope (VE) fibers to create a hole in the VE via a non-enzymatic mechanism that involves binding between positively charged,16 kiloDalton sperm lysin [13] and a large (.1000 kDa) VE glycoprotein [10]
We model a novel explanation for this pattern whereby Vitelline Envelope Zona Pellucida 14 (VEZP14) acts as a decoy of VERL in order to decrease the effective amount of sperm lysin and slow the rate of fertilization
Summary
Reproductive genes are often identified in genome-wide scans as targets of positive selection, and some are among the most rapidly evolving proteins known [1,2,3,4]. Because cognate sperm and egg proteins must coevolve to maintain compatibility, their divergence should result in correlated evolutionary rates – on lineages where females evolve rapidly, males should evolve rapidly This expectation of correlated evolution between males and females underlies a twopronged approach to investigating molecular interactions at fertilization: we can use evolutionary signals of co-evolution to focus genetic and biochemical assays on molecules that are likely to functionally interact. Stoichiometry of VE dissolution indicates that two lysin molecules bind each repeat [10], in support of a model whereby lysin dimers out compete hydrophobic interactions among intermolecular VERL repeats and unravel VE fibers in a zipper-like fashion through surface structure and electrostatic interactions [16]
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