Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039-9644, USA.
BMC Evol Biol. 2013 Jan 24;13:21. doi: 10.1186/1471-2148-13-21.
The inhabitants of deep-sea hydrothermal vents occupy ephemeral island-like habitats distributed sporadically along tectonic spreading-centers, back-arc basins, and volcanically active seamounts. The majority of vent taxa undergo a pelagic larval phase, and thus varying degrees of geographical subdivision, ranging from no impedance of dispersal to complete isolation, often exist among taxa that span common geomorphological boundaries. Two lineages of Bathymodiolus mussels segregate on either side of the Easter Microplate, a boundary that separates the East Pacific Rise from spreading centers connected to the Pacific-Antarctic Ridge.
A recent sample from the northwest flank of the Easter Microplate contained an admixture of northern and southern mitochondrial haplotypes and corresponding alleles at five nuclear gene loci. Genotypic frequencies in this sample did not fit random mating expectation. Significant heterozygote deficiencies at nuclear loci and gametic disequilibria between loci suggested that this transitional region might be a 'Tension Zone' maintained by immigration of parental types and possibly hybrid unfitness. An analysis of recombination history in the nuclear genes suggests a prolonged history of parapatric contact between the two mussel lineages. We hereby elevate the southern lineage to species status as Bathymodiolus antarcticus n. sp. and restrict the use of Bathymodiolus thermophilus to the northern lineage.
Because B. thermophilus s.s. exhibits no evidence for subdivision or isolation-by-distance across its 4000 km range along the EPR axis and Galápagos Rift, partial isolation of B. antarcticus n. sp. requires explanation. The time needed to produce the observed degree of mitochondrial differentiation is consistent with the age of the Easter Microplate (2.5 to 5.3 million years). The complex geomorphology of the Easter Microplate region forces strong cross-axis currents that might disrupt self-recruitment of mussels by removing planktotrophic larvae from the ridge axis. Furthermore, frequent local extinction events in this tectonically dynamic region might produce a demographic sink rather than a source for dispersing mussel larvae. Historical changes in tectonic rates and current patterns appear to permit intermittent contact and introgression between the two species.
深海热液喷口的居民占据了分散在构造扩张中心、弧后盆地和火山活跃海山周围的短暂岛屿状栖息地。大多数喷口分类群经历了浮游幼虫阶段，因此，跨越常见地貌边界的分类群之间存在着不同程度的地理划分，从没有扩散障碍到完全隔离。两种 Bathymodiolus 贻贝谱系在 Easter 微板块的两侧分离，该板块将东太平洋海隆与连接太平洋-南极脊的扩张中心分开。
最近从 Easter 微板块的西北侧翼采集的一个样本中，含有北方和南方线粒体单倍型的混合物，以及五个核基因座的相应等位基因。该样本中的基因型频率不符合随机交配的预期。在核基因座中存在显著的杂合子缺失和基因座之间的配子不平衡，表明这个过渡区域可能是一个由亲代类型的移民和可能的杂种不适维持的“张力带”。对核基因中重组历史的分析表明，这两个贻贝谱系之间存在着长期的并育接触历史。我们特此将南方谱系提升为 Bathymodiolus antarcticus n. sp. 种，并将 Bathymodiolus thermophilus 限制在北方谱系中使用。
由于 B. thermophilus s.s. 在沿着 EPR 轴和加拉帕戈斯裂谷的 4000 公里范围内没有表现出任何细分或隔离的证据，因此 B. antarcticus n. sp. 的部分隔离需要解释。产生观察到的线粒体分化程度所需的时间与 Easter 微板块的年龄（250 万至 530 万年）一致。 Easter 微板块地区复杂的地貌迫使强烈的跨轴电流，这些电流可能通过将浮游幼虫从脊轴上移除，破坏贻贝的自我补充。此外，在这个构造活跃的区域频繁发生的局部灭绝事件可能产生一个人口汇而不是散布贻贝幼虫的源。构造速率的历史变化和当前模式似乎允许两个物种之间的间歇性接触和基因渗透。
