Lee C E
Marine Molecular Biotechnology Laboratory, School of Oceanography, University of Washington, Seattle 98195-7940, USA.
Evolution. 2000 Dec;54(6):2014-27. doi: 10.1111/j.0014-3820.2000.tb01245.x.
The copepod Eurytemora affinis has a broad geographic range within the Northern Hemisphere, inhabiting coastal regions of North America, Asia, and Europe. A phylogenetic approach was used to determine levels of genetic differentiation among populations of this species, and interpopulation crosses were performed to determine reproductive compatibility. DNA sequences from two mitochondrial genes, large subunit (16S) rRNA (450 bp) and cytochrome oxidase I (COI, 652 bp), were obtained from 38 populations spanning most of the species range and from two congeneric species, E. americana and E. herdmani. Phylogenetic analysis revealed a polytomy of highly divergent clades with maximum sequence divergences of 10% in 16S rRNA and 19% in COI. A power test (difference of a proportion) revealed that amount of sequence data collected was sufficient for resolving speciation events occurring at intervals greater than 300,000 years, but insufficient for determining whether speciation events were approximately simultaneous. Geographic and genetic distances were not correlated (Mantel's test; r = 0.023, P = 0.25), suggesting that populations had not differentiated through gradual isolation by distance. At finer spatial scales, there was almost no sharing of mtDNA haplotypes among proximate populations, indicating little genetic exchange even between nearby sites. Interpopulation crosses demonstrated reproductive incompatibility among genetically distinct populations, including those that were sympatric. Most notably, two geographically distant (4000 km) but genetically proximate (0.96% 16S, 0.15% COI) populations exhibited asymmetric reproductive isolation at the F2 generation. Large genetic divergences and reproductive isolation indicate that the morphologically conservative E. affinis constitutes a sibling species complex. Reproductive isolation between genetically proximate populations underscores the importance of using multiple measures to examine patterns of speciation.
桡足类的近缘真宽水蚤在北半球具有广泛的地理分布范围，栖息于北美、亚洲和欧洲的沿海地区。采用系统发育方法来确定该物种不同种群间的遗传分化水平，并进行种群间杂交以确定生殖兼容性。从跨越该物种大部分分布范围的38个种群以及两个同属物种美洲真宽水蚤和赫氏真宽水蚤中获取了两个线粒体基因的DNA序列，即大亚基（16S）核糖体RNA（450碱基对）和细胞色素氧化酶I（COI，652碱基对）。系统发育分析揭示了一个由高度分化的分支组成的多歧分支，16S核糖体RNA的最大序列差异为10%，COI的最大序列差异为19%。一项功效检验（比例差异检验）表明，所收集的序列数据量足以解析间隔大于30万年发生的物种形成事件，但不足以确定物种形成事件是否大致同时发生。地理距离和遗传距离不相关（Mantel检验；r = 0.023，P = 0.25），这表明种群并非通过距离导致的逐渐隔离而分化。在更精细的空间尺度上，相邻种群之间几乎没有线粒体DNA单倍型的共享，这表明即使在相邻地点之间，遗传交换也很少。种群间杂交表明，在遗传上不同的种群之间存在生殖不相容性，包括那些同域分布的种群。最值得注意的是，两个地理距离遥远（4000公里）但遗传距离相近（16S为0.96%，COI为0.15%）的种群在F2代表现出不对称生殖隔离。巨大的遗传差异和生殖隔离表明，形态上保守的近缘真宽水蚤构成了一个姐妹种复合体。遗传距离相近的种群之间的生殖隔离强调了使用多种方法来研究物种形成模式的重要性。
