Genetic sex determination in three closely related hydrothermal vent gastropods, including one species with intersex individuals.

Molluscs have undergone many transitions between separate sexes and hermaphroditism, which is of interest in studying the evolution of sex determination and differentiation. Here, we combined multi-locus genotypes obtained from restriction site-associated DNA (RAD) sequencing with anatomical observations of the gonads of three deep-sea hydrothermal vent gastropods of the genus Alviniconcha living in the southwest Pacific. We found that all three species (Alviniconcha boucheti, Alviniconcha strummeri, and Alviniconcha kojimai) share the same male-heterogametic XY sex-determination system but that the gonads of XX A. kojimai individuals are invaded by a variable proportion of male reproductive tissue. The identification of Y-specific RAD loci (found only in A. boucheti) and the phylogenetic analysis of three sex-linked loci shared by all species suggested that X-Y recombination has evolved differently within each species. This situation of three species showing variation in gonadal development around a common sex-determination system provides new insights into the reproductive mode of poorly known deep-sea species and opens up an opportunity to study the evolution of recombination suppression on sex chromosomes and its association with mixed or transitory sexual systems.

Inter-Specific Genetic Exchange Despite Strong Divergence in Deep-Sea Hydrothermal Vent Gastropods of the Genus Alviniconcha.

Deep hydrothermal vents are highly fragmented and unstable habitats at all temporal and spatial scales. Such environmental dynamics likely play a non-negligible role in speciation. Little is, however, known about the evolutionary processes that drive population-level differentiation and vent species isolation and, more specifically, how geography and habitat specialisation interplay in the species history of divergence. In this study, the species range and divergence of Alviniconcha snails that occupy active Western Pacific vent fields was assessed by using sequence variation data of the mitochondrial Cox1 gene, RNAseq, and ddRAD-seq. Combining morphological description and sequence datasets of the three species across five basins, we confirmed that A. kojimai, A. boucheti, and A. strummeri, while partially overlapping over their range, display high levels of divergence in the three genomic compartments analysed that usually encompass values retrieved for reproductively isolated species with divergences rang from 9% to 12.5% (mtDNA) and from 2% to 3.1% (nuDNA). Moreover, the three species can be distinguished on the basis of their external morphology by observing the distribution of bristles and the shape of the columella. According to this sampling, A. boucheti and A. kojimai form an east-to-west species abundance gradient, whereas A. strummeri is restricted to the Futuna Arc/Lau and North Fiji Basins. Surprisingly, population models with both gene flow and population size heterogeneities among genomes indicated that these three species are still able to exchange genes due to secondary contacts at some localities after a long period of isolation.

Subtle limits to connectivity revealed by outlier loci within two divergent metapopulations of the deep-sea hydrothermal gastropod Ifremeria nautilei.

Hydrothermal vents form archipelagos of ephemeral deep-sea habitats that raise interesting questions about the evolution and dynamics of the associated endemic fauna, constantly subject to extinction-recolonization processes. These metal-rich environments are coveted for the mineral resources they harbour, thus raising recent conservation concerns. The evolutionary fate and demographic resilience of hydrothermal species strongly depend on the degree of connectivity among and within their fragmented metapopulations. In the deep sea, however, assessing connectivity is difficult and usually requires indirect genetic approaches. Improved detection of fine-scale genetic connectivity is now possible based on genome-wide screening for genetic differentiation. Here, we explored population connectivity in the hydrothermal vent snail Ifremeria nautilei across its species range encompassing five distinct back-arc basins in the Southwest Pacific. The global analysis, based on 10,570 single nucleotide polymorphism (SNP) markers derived from double digest restriction-site associated DNA sequencing (ddRAD-seq), depicted two semi-isolated and homogeneous genetic clusters. Demogenetic modeling suggests that these two groups began to diverge about 70,000 generations ago, but continue to exhibit weak and slightly asymmetrical gene flow. Furthermore, a careful analysis of outlier loci showed subtle limitations to connectivity between neighbouring basins within both groups. This finding indicates that migration is not strong enough to totally counterbalance drift or local selection, hence questioning the potential for demographic resilience at this latter geographical scale. These results illustrate the potential of large genomic data sets to understand fine-scale connectivity patterns in hydrothermal vents and the deep sea.

Global 16S rRNA diversity of provannid snail endosymbionts from Indo-Pacific deep-sea hydrothermal vents.

Symbioses between invertebrate animals and chemosynthetic bacteria build the foundation of deep-sea hydrothermal ecosystems worldwide. Despite the importance of these symbioses for ecosystem functioning, the diversity of symbionts within and between host organisms and geographic regions is still poorly understood. In this study we used 16S rRNA amplicon sequencing to determine the diversity of gill endosymbionts in provannid snails of the genera Alviniconcha and Ifremeria, which are key species at deep-sea hydrothermal vents in the Indo-Pacific Ocean. Our analysis of 761 snail samples across the distributional range of these species confirms previous findings that symbiont lineages are strongly partitioned by host species and broad-scale geography. Less structuring was observed within geographic regions, probably due to insufficient strain resolution of the 16S rRNA gene. Symbiont richness in individual hosts appeared to be unrelated to host size, suggesting that provannid snails might acquire their symbionts only during a permissive time window in early developmental stages in contrast to other vent molluscs that obtain their symbionts throughout their lifetime. Despite the extent of our dataset, symbiont accumulation curves did not reach saturation, highlighting the need for increased sampling efforts to uncover the full diversity of symbionts within these and other hydrothermal vent species.