Adaptive evolution of gamete-recognition proteins in birds.
Gamete-recognition proteins have been shown to evolve by positive selection in diverse organism groups, such as marine invertebrates and mammals, although underlying evolutionary mechanisms driving this rapid divergence are poorly understood. However, several hypotheses have been put forward to explain the observed pattern, including different forms of sexual conflict and sperm competition. Because female gametes require more energy to produce than male gametes, female organisms suffer more when fertilisation goes wrong. One process that results in a failed mammalian fertilisation is polyspermy, when >1 sperm fertilises the egg. However in birds, there is no such sexual conflict because multiple sperm typically bind and fuse with the egg. If sexual conflict driven by polyspermy avoidance is important for the evolution of gamete-recognition proteins in vertebrates, we expect to find positive selection in the genes to be less pronounced in birds. We therefore sequenced six genes (ZP1, ZP2, ZP4, ZPAX, CD9, and Acrosin) encoding gamete-recognition proteins in several bird species to test for positive selection. For comparison, we also analysed ortologous sequences in a set of mammalian species. We found no major differences in the occurrence of adaptive evolution and the strength of selection between bird and mammal orthologs. From this we conclude that polyspermy avoidance does not act as the main underlying evolutionary force shaping the rate of evolution in these genes. We discuss other possible processes that could explain positive selection of gamete-recognition proteins in birds and mammals, such as hybridisation avoidance, cryptic female choice, and postcopulatory sperm competition.
Department of Evolutionary Biology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18 D, 752 36, Uppsala, Sweden.
Sofia
S
Berlin
Lujiang
L
Qu
Hans
H
Ellegren
D000176:Acrosin / Q000235:genetics*; D000595:Amino Acid Sequence; D000818:Animals; D015703:Antigens, CD / Q000737:chemistry / Q000235:genetics*; D030161:Avian Proteins / Q000235:genetics*; D001717:Birds / Q000235:genetics* / Q000502:physiology; D004527:Egg Proteins / Q000235:genetics*; D019143:Evolution, Molecular; D005260:Female; D005306:Fertilization / Q000235:genetics; D008297:Male; D008322:Mammals / Q000235:genetics; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000235:genetics*; D008969:Molecular Sequence Data; D010053:Ovary / Q000737:chemistry; D011956:Receptors, Cell Surface / Q000235:genetics*; D012641:Selection, Genetic; D017421:Sequence Analysis; D017422:Sequence Analysis, DNA; D013154:Spleen / Q000737:chemistry; D013737:Testis / Q000737:chemistry; D060245:Tetraspanin 29; D000072216:Zona Pellucida Glycoproteins
D015703:Antigens, CD; D030161:Avian Proteins; D004527:Egg Proteins; D008562:Membrane Glycoproteins; D011956:Receptors, Cell Surface; D060245:Tetraspanin 29; D000072216:Zona Pellucida Glycoproteins; D000176:Acrosin