Department of Biology, Sewanee, The University of the South, Sewanee, Tennessee 37383, USA.
Evolution. 2012 Jun;66(6):1695-708. doi: 10.1111/j.1558-5646.2012.01609.x. Epub 2012 Apr 10.
Marine species with high dispersal potential often have huge ranges and minimal population structure. Combined with the paucity of geographic barriers in the oceans, this pattern raises the question as to how speciation occurs in the sea. Over the past 20 years, evidence has accumulated that marine speciation is often linked to the evolution of gamete recognition proteins. Rapid evolution of gamete recognition proteins in gastropods, bivalves, and sea urchins is correlated with gamete incompatibility and contributes to the maintenance of species boundaries between sympatric congeners. Here, we present a counterexample to this general pattern. The sea urchins Pseudoboletia indiana and P. maculata have broad ranges that overlap in the Indian and Pacific oceans. Cytochrome oxidase I sequences indicated that these species are distinct, and their 7.3% divergence suggests that they diverged at least 2 mya. Despite this, we suspected hybridization between them based on the presence of morphologically intermediate individuals in sympatric populations at Sydney, Australia. We assessed the opportunity for hybridization between the two species and found that (1) individuals of the two species occur within a meter of each other in nature, (2) they have overlapping annual reproductive cycles, and (3) their gametes cross-fertilize readily in the laboratory and in the field. We genotyped individuals with intermediate morphology and confirmed that many were hybrids. Hybrids were fertile, and some female hybrids had egg sizes intermediate between the two parental species. Consistent with their high level of gamete compatibility, there is minimal divergence between P. indiana and P. maculata in the gamete recognition protein bindin, with a single fixed amino acid difference between the two species. Pseudoboletia thus provides a well-characterized exception to the idea that broadcast spawning marine species living in sympatry develop and maintain species boundaries through the divergence of gamete recognition proteins and the associated evolution of gamete incompatibility.
具有高扩散能力的海洋物种通常具有广泛的分布范围和最小的种群结构。再加上海洋中地理障碍的缺乏，这就提出了一个问题，即在海洋中物种是如何形成的。在过去的 20 年中，有证据表明海洋物种的形成通常与配子识别蛋白的进化有关。腹足动物、双壳类动物和海胆中配子识别蛋白的快速进化与配子不相容性有关，并有助于维持同域近缘种之间的物种边界。在这里，我们提出了一个与此普遍模式相反的例子。印度太平洋石斑鱼和 P. maculata 的分布范围广泛，在印度洋和太平洋重叠。细胞色素氧化酶 I 序列表明这些物种是不同的，它们的 7.3%的差异表明它们至少在 200 万年前就已经分化了。尽管如此，我们还是根据澳大利亚悉尼同域种群中形态中间个体的存在，怀疑它们之间存在杂交。我们评估了这两个物种之间杂交的可能性，发现：（1）在自然界中，两个物种的个体之间相距不到一米；（2）它们有重叠的年度繁殖周期；（3）它们的配子在实验室和野外很容易受精。我们对具有中间形态的个体进行了基因分型，并证实了许多是杂种。杂种是可育的，一些雌性杂种的卵大小介于两个亲种之间。与它们高水平的配子相容性一致，在配子识别蛋白结合蛋白中，P. indiana 和 P. maculata 之间的差异很小，两个物种之间只有一个固定的氨基酸差异。因此，Pseudoboletia 为一个很好的例子，它表明生活在同域的、具有广泛繁殖能力的海洋物种通过配子识别蛋白的分化以及配子不相容性的相关进化来发展和维持物种边界的观点并非普遍适用。
