Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Panama.
Integr Comp Biol. 2011 Sep;51(3):456-65. doi: 10.1093/icb/icr039. Epub 2011 Jun 22.
Research on speciation of marine organisms has lagged behind that of terrestrial ones, but the study of the evolution of molecules involved in the adhesion of gametes in free-spawning invertebrates is an exception. Here I review the function, species-specificity, and molecular variation of loci coding for bindin in sea urchins, lysin in abalone and their egg receptors, in an effort to assess the degree to which they contribute to the emergence of reproductive isolation during the speciation process. Bindin is a protein that mediates binding of the sperm to the vitelline envelope (VE) of the egg and the fusion of the gametes' membranes, whereas lysin is a protein involved only in binding to the VE. Both of these molecules are important in species recognition by the gametes, but they rarely constitute absolute blocks to interspecific hybridization. Intraspecific polymorphism is high in bindin, but low in lysin. Polymorphism in bindin is maintained by frequency-dependent selection due to sexual conflict arising from the danger of polyspermy under high densities of sperm. Monomorphism in lysin is the result of purifying selection arising from the need for species recognition. Interspecific divergence in lysin is due to strong positive selection, and the same is true for bindin of four out of seven genera of sea urchins studied to date. The differences between the sea urchin genera in the strength of selection can only partially be explained by the hypothesis of reinforcement. The egg receptor for lysin (VERL) is a glycoprotein with 22 repeats, 20 of which have evolved neutrally and homogenized by concerted evolution, whereas the first two repeats are under positive selection. Selection on lysin has been generated by the need to track changes in VERL, permitted by the redundant structure of this molecule. Both lysin and bindin are important in reproductive isolation, probably had a role in speciation, but it is hard to determine whether they meet the strictest criteria of "speciation loci," defined as genes whose differentiation has caused speciation.
海洋生物的物种形成研究落后于陆地生物，但对自由产卵无脊椎动物配子黏附分子进化的研究是一个例外。本文综述了海胆结合蛋白（bindin）、鲍鱼溶菌酶（lysin）及其卵受体编码基因的功能、种特异性和分子变异，以评估它们在物种形成过程中对生殖隔离形成的贡献程度。Bindin 是一种介导精子与卵黄膜（VE）结合以及配子膜融合的蛋白质，而 lysin 是一种仅与 VE 结合的蛋白质。这两种分子在配子的物种识别中都很重要，但它们很少构成种间杂交的绝对障碍。Bindin 种内多态性高，lysine 种内多态性低。Bindin 的多态性是由于性冲突引起的频率依赖性选择而维持的，性冲突是由于高密度精子下的多精受精危险所致。Lysin 的单态性是由于物种识别的需要而产生的净化选择的结果。Lysin 的种间差异是由于强烈的正选择造成的，到目前为止研究的七种海胆属中的四种 bindin 也是如此。海胆属之间选择强度的差异只能部分用强化假说来解释。lysine 的 egg receptor（VERL）是一种具有 22 个重复的糖蛋白，其中 20 个重复经历了中性进化和协同进化的同质化，而前两个重复受到正选择的影响。lysine 的选择是由 VERL 变化的需要产生的，这种分子的冗余结构允许这种变化。Lysin 和 bindin 在生殖隔离中都很重要，可能在物种形成中起作用，但很难确定它们是否符合“物种形成基因”的最严格标准，即分化导致物种形成的基因。
