Ho Phuong-Thao, Park Eunji, Hong Soon Gyu, Kim Eun-Hye, Kim Kangchon, Jang Sook-Jin, Vrijenhoek Robert C, Won Yong-Jin
Interdisciplinary Program of EcoCreative, The Graduate School, Ewha Womans University, Seoul, 03760, Korea.
Division of EcoScience, Ewha Womans University, Seoul, 03760, Korea.
BMC Evol Biol. 2017 May 30;17(1):121. doi: 10.1186/s12862-017-0966-3.
Chemolithoautotrophic primary production sustains dense invertebrate communities at deep-sea hydrothermal vents and hydrocarbon seeps. Symbiotic bacteria that oxidize dissolved sulfur, methane, and hydrogen gases nourish bathymodiolin mussels that thrive in these environments worldwide. The mussel symbionts are newly acquired in each generation via infection by free-living forms. This study examined geographical subdivision of the thiotrophic endosymbionts hosted by Bathymodiolus mussels living along the eastern Pacific hydrothermal vents. High-throughput sequencing data of 16S ribosomal RNA encoding gene and fragments of six protein-coding genes of symbionts were examined in the samples collected from nine vent localities at the East Pacific Rise, Galápagos Rift, and Pacific-Antarctic Ridge.
Both of the parapatric sister-species, B. thermophilus and B. antarcticus, hosted the same numerically dominant phylotype of thiotrophic Gammaproteobacteria. However, sequences from six protein-coding genes revealed highly divergent symbiont lineages living north and south of the Easter Microplate and hosted by these two Bathymodiolus mussel species. High heterogeneity of symbiont haplotypes among host individuals sampled from the same location suggested that stochasticity associated with initial infections was amplified as symbionts proliferated within the host individuals. The mussel species presently contact one another and hybridize along the Easter Microplate, but the northern and southern symbionts appear to be completely isolated. Vicariance associated with orogeny of the Easter Microplate region, 2.5-5.3 million years ago, may have initiated isolation of the symbiont and host populations. Estimates of synonymous substitution rates for the protein-coding bacterial genes examined in this study were 0.77-1.62%/nucleotide/million years.
Our present study reports the most comprehensive population genetic analyses of the chemosynthetic endosymbiotic bacteria based on high-throughput genetic data and extensive geographical sampling to date, and demonstrates the role of the geographical features, the Easter Microplate and geographical distance, in the intraspecific divergence of this bacterial species along the mid-ocean ridge axes in the eastern Pacific. Altogether, our results provide insights into extrinsic and intrinsic factors affecting the dispersal and evolution of chemosynthetic symbiotic partners in the hydrothermal vents along the eastern Pacific Ocean.
化能无机自养初级生产维持着深海热液喷口和烃类渗漏处密集的无脊椎动物群落。氧化溶解的硫、甲烷和氢气的共生细菌滋养着在全球这些环境中繁盛的深海贻贝。贻贝共生体在每一代中通过自由生活形式的感染而新获得。本研究调查了生活在东太平洋热液喷口的深海贻贝所携带的硫营养型内共生体的地理细分情况。在从东太平洋海隆、加拉帕戈斯裂谷和太平洋 - 南极海岭的九个喷口地点采集的样本中，检测了编码16S核糖体RNA基因和共生体六个蛋白质编码基因片段的高通量测序数据。
同域分布的姊妹物种嗜热深海贻贝和南极深海贻贝都携带了硫营养型γ - 变形菌中数量上占主导的系统发育型。然而，来自六个蛋白质编码基因的序列显示，在复活节微板块以北和以南生活的、由这两种深海贻贝物种携带的共生体谱系高度分化。从同一地点采集的宿主个体中，共生体单倍型的高度异质性表明，与初始感染相关的随机性在共生体在宿主个体内增殖时被放大。贻贝物种目前在复活节微板块沿线相互接触并杂交，但北部和南部的共生体似乎完全隔离。与250万至530万年前复活节微板块地区造山运动相关的地理隔离，可能启动了共生体和宿主种群的隔离。本研究中检测的蛋白质编码细菌基因的同义替换率估计为每核苷酸每百万年0.77 - 1.62%。
我们目前的研究报告了基于高通量遗传数据和广泛地理采样对化学合成内共生细菌进行的最全面的种群遗传分析，并证明了地理特征、复活节微板块和地理距离在这种细菌物种沿东太平洋洋中脊轴的种内分化中的作用。总之，我们的结果为影响东太平洋热液喷口化学合成共生伙伴的扩散和进化的外在和内在因素提供了见解。
