Geyer Laura B, Palumbi Stephen R
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
Evolution. 2003 May;57(5):1049-60. doi: 10.1111/j.0014-3820.2003.tb00315.x.
Reproductive character displacement occurs when sympatric and allopatric populations of a species differ in traits crucial to reproduction, and it is commonly thought of as a signal of selection acting to limit hybridization. Most documented cases of reproductive character displacement involve characters that are poorly understood at the genetic level, and rejecting alternative hypotheses for biogeographic shifts in reproductive traits is often very difficult. In sea urchins, the gamete recognition protein bindin evolves under positive selection when species are broadly sympatric, suggesting character displacement may be operating in this system. We sampled sympatric and allopatric populations of two species in the sea urchin genus Echinometra for variation in bindin and for the mitochondrial cytochrome oxidase I to examine patterns of population differentiation and molecular evolution at a reproductive gene. We found a major shift in bindin alleles between central Pacific (allopatric) and western Pacific (sympatric) populations of E. oblonga. Allopatric populations of E. oblonga are polyphyletic with E. sp. C at bindin, whereas sympatric populations of the two species are reciprocally monophyletic. There is a strong signal of positive selection (P(N)/P(S) = 4.5) in the variable region of the first exon of bindin, which is associated with alleles found in sympatric populations of E. oblonga. These results indicate that there is a strong pattern of reproductive character displacement between E. oblonga and E. sp. C and that the divergence is driven by selection. There is much higher population structure in sympatric populations at the bindin locus than at the neutral mitochondrial locus, but this difference is not seen in allopatric populations. These data suggest a pattern of speciation driven by selection for local gamete coevolution as a result of interactions between sympatric species. Although this pattern is highly suggestive of speciation by reinforcement, further research into hybrid fitness and egg-sperm interactions is required to address this potential mechanism for character displacement.
当一个物种的同域和异域种群在对繁殖至关重要的性状上存在差异时，就会发生生殖性状替代，人们通常认为这是选择作用以限制杂交的信号。大多数有记录的生殖性状替代案例涉及在遗传水平上了解甚少的性状，并且拒绝关于生殖性状生物地理变化的替代假设通常非常困难。在海胆中，当物种广泛同域分布时，配子识别蛋白结合素在正选择下进化，这表明性状替代可能在这个系统中起作用。我们对海胆属棘海胆的两个物种的同域和异域种群进行了采样，以研究结合素的变异以及线粒体细胞色素氧化酶I，从而检查一个生殖基因的种群分化模式和分子进化。我们发现长刺棘海胆在中太平洋（异域）和西太平洋（同域）种群之间结合素等位基因发生了重大转变。长刺棘海胆的异域种群在结合素方面与C种棘海胆是多系的，而这两个物种的同域种群是相互单系的。在结合素第一个外显子的可变区域存在强烈的正选择信号（P(N)/P(S)=4.5），这与长刺棘海胆同域种群中发现的等位基因相关。这些结果表明，长刺棘海胆和C种棘海胆之间存在强烈的生殖性状替代模式，并且这种分化是由选择驱动的。在结合素基因座上，同域种群的种群结构比中性线粒体基因座上的种群结构高得多，但在异域种群中没有看到这种差异。这些数据表明，由于同域物种之间的相互作用，选择导致局部配子共同进化从而推动了物种形成模式。尽管这种模式强烈暗示了通过强化实现物种形成，但需要对杂种适合度和精卵相互作用进行进一步研究，以探讨这种性状替代的潜在机制。
