MBE Advance Access published online on January 22, 2007
Molecular Biology and Evolution, doi:10.1093/molbev/msm009
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Research Article |
Rapid Evolution of Outer Egg Membrane Proteins in the Drosophila melanogaster Subgroup: A Case of Ecologically Driven Evolution of Female Reproductive Traits
Department of Biology, McMaster University, Hamilton, Ontario, L8S 4K1, Canada
Corresponding Author: Rama S. Singh. Department of Biology, McMaster University, Hamilton, Ontario, L8S 4K1. Canada, Phone: (905) 525-9140x24378, Fax: (905) 522-6066, Email: singh{at}mcmaster.ca
Accepted for publication January 9, 2007.
Although sexual selection has been predominantly used to explain the rapid evolution of sexual traits, eggs of oviparous organisms directly face both the challenges of sexual selection as well as natural selection (environmental challenges, survival in niches, etc). Being the outermost membrane in most insect eggs, the chorion layer is the interface between the embryo and the environment, thereby serving to protect the egg. Adaptive ecological radiations such as divergence in ovipositional substrate usage and host-plant specializations can therefore influence the evolution of eggshell proteins. We can hypothesize that proteins localized on the outer eggshell may be affected to a greater degree by ecological challenges compared to inner eggshell proteins, and therefore, proteins localized in the outer eggshell (chorion membrane) may evolve differently (faster) than proteins localized in the inner egg membrane (vitelline membrane). We compared the evolutionary divergence of vitelline and chorion membrane proteins in species of the melanogaster subgroup and found that chorion proteins as a group are indeed evolving faster than vitelline membrane proteins. At least one vitelline membrane protein (Vm32E) specifically localized on the outer eggshell, is also evolving faster than other vitelline membrane proteins suggesting that all proteins localized on the outer eggshell may be evolving rapidly. We also found evidence that specific codons in chorion proteins cp15, cp16 are evolving under positive selection. Polymorphism surveys of cp16 revealed inflated levels of divergence relative to polymorphism in specific regions of the gene, indicating that these regions are under strong selection. At the morphological level, we found notable difference in eggshell surface morphologies between specialist (D. sechellia and D. erecta) and generalist species of Drosophila. We do not know if any of the chorion proteins actually interact with spermatozoids, therefore leaving the possibility of rapid evolution through gametic interaction wide open. At this point however, our results support previous suggestions that divergences in ecology, particularly ovipositional substrate divergences may be a strong force driving the evolution of eggshell proteins.
Key Words: speciation • gametic surface proteins • sex and reproduction related genes • ecological diversification • positive selection • chorion proteins • vitelline proteins
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