The composition and diversity of animal microbiomes is shaped by a variety of factors, many of them interacting, such as host traits, the environment, and biogeography. Hybrid zones, in which the ranges of two host species meet and hybrids are found, provide natural experiments for determining the drivers of microbiome communities, but have not been well studied in marine environments. Here, we analysed the composition of the symbiont community in two deep-sea, Bathymodiolus mussel species along their known distribution range at hydrothermal vents on the Mid-Atlantic Ridge, with a focus on the hybrid zone where they interbreed. In-depth metagenomic analyses of the sulphur-oxidising symbionts of 30 mussels from the hybrid zone, at a resolution of single nucleotide polymorphism analyses of ~2500 orthologous genes, revealed that parental and hybrid mussels (F2–F4 generation) have genetically indistinguishable symbionts. While host genetics does not appear to affect symbiont composition in these mussels, redundancy analyses showed that geographic location of the mussels on the Mid-Atlantic Ridge explained most of the symbiont genetic variability compared to the other factors. We hypothesise that geographic structuring of the free-living symbiont population plays a major role in driving the composition of the microbiome in these deep-sea mussels.

动物微生物组的组成和多样性受到多种因素的影响，其中许多因素相互作用，例如宿主特征、环境和生物地理学。两个宿主物种的范围相遇并发现杂交种的混合区，为确定微生物群落的驱动因素提供了自然实验，但在海洋环境中尚未得到很好的研究。在这里，我们分析了两个深海共生群落的组成，Bathymodiolus在大西洋中脊的热液喷口沿其已知分布范围的贻贝物种，重点是它们杂交的混合区。对来自杂交区的 30 种贻贝的硫氧化共生体进行的深入宏基因组分析，在对约 2500 个直系同源基因的单核苷酸多态性分析的分辨率下，表明亲本和杂交贻贝（F2-F4 代）具有遗传上无法区分的共生体。虽然宿主遗传学似乎不会影响这些贻贝中的共生体组成，但冗余分析表明，与其他因素相比，贻贝在大西洋中脊的地理位置解释了大部分共生体遗传变异性。