Carlon David B, Budd Ann F
Wrigley Institute for Environmental Studies, University of Southern California, P. O. Box 5069, Avalon, California 90704, USA.
Evolution. 2002 Nov;56(11):2227-42. doi: 10.1111/j.0014-3820.2002.tb00147.x.
A few marine cases have demonstrated morphological and genetic divergence in the absence of spatial barriers to gene flow, suggesting that the initial phase of speciation is possible without geographic isolation. In the Bocas del Toro Archipelago of the Atlantic Coast of Panama, we found two morphotypes of the scleractinian coral Favia fragum with opposing depth distributions. One morphotype fit the classical description of F. fragum and was most abundant at 3 m depth. A second morphotype was distinguished by raised corallites and was restricted to < or = 1 m depth. The two morphotypes overlapped in distribution at 1 m depth. Multivariate analysis of polyp-level characters (shape and distribution of septa within corallites) divided samples into two groups corresponding to initial qualitative observations of colony shape and corallite relief. To determine whether reduced gene flow maintains morphological variation, we measured the frequencies of alleles at five allozyme loci in both morphotypes at three sites 1-2 km distant. While there were significant differences in allele frequencies between morphotypes within sites, there were also frequency differences among sites at most loci, with the exception of nearly fixed alleles at the PGM locus. Extremely low heterozygosity permitted us to use haplotypes to compare genetic distance between morphotypes and among sites. Comparisons between haplotype data and a null model assuming gene flow between morphotypes showed that the two morphotypes shared significantly fewer haplotypes than expected, and average genetic distance between morphotypes was significantly greater than expected. Partitioning haplotype variation with analysis of molecular variance demonstrated that 35% of the variation was explained by morphotype, whereas 28% of the variation was explained by site. Two PGM heterozygotes and several individuals homozygous for rare PGM alleles are consistent with hybridization, and perhaps introgression by selfing within morphotypes. We consider three hypotheses for this morphological and genetic divergence in F. fragum: (1) intraspecific polymorphism, (2) incipient species, (3) biological species; and discuss the role of reproductive characters in a divergence-with-gene flow mechanism of speciation.
一些海洋案例表明，在不存在基因流动空间障碍的情况下，也会出现形态和遗传分化，这表明物种形成的初始阶段在没有地理隔离的情况下也是可能的。在巴拿马大西洋海岸的博卡斯德尔托罗群岛，我们发现了石珊瑚 Favites fragum 的两种形态类型，它们具有相反的深度分布。一种形态类型符合 Favites fragum 的经典描述，在3米深度处最为丰富。第二种形态类型的特征是珊瑚杯凸起，且仅限于深度小于或等于1米的区域。这两种形态类型在1米深度处分布重叠。对珊瑚虫水平特征（珊瑚杯内隔膜的形状和分布）进行多变量分析，将样本分为两组，这与最初对群体形状和珊瑚杯凸起的定性观察结果相对应。为了确定基因流动减少是否维持形态变异，我们在相距1 - 2公里的三个地点测量了两种形态类型中五个等位酶位点的等位基因频率。虽然位点内形态类型之间的等位基因频率存在显著差异，但除了磷酸葡萄糖变位酶（PGM）位点几乎固定的等位基因外，大多数位点在位点之间也存在频率差异。极低的杂合度使我们能够使用单倍型来比较形态类型之间以及位点之间的遗传距离。单倍型数据与假设形态类型之间存在基因流动的零模型进行比较表明，两种形态类型共享的单倍型明显少于预期，并且形态类型之间的平均遗传距离明显大于预期。通过分子方差分析对单倍型变异进行划分表明，35%的变异由形态类型解释，而28%的变异由位点解释。两个PGM杂合子以及几个罕见PGM等位基因的纯合个体与杂交现象一致，也许是形态类型内通过自交发生了基因渗入。我们考虑了关于 Favites fragum 这种形态和遗传分化的三种假设：（1）种内多态性，（2）初始物种，（3）生物物种；并讨论了生殖特征在具有基因流动的物种形成分化机制中的作用。
