Abstract
BackgroundDispersal ability, population genetic structure and species divergence in marine nematodes are still poorly understood, especially in remote areas such as the Southern Ocean. We investigated genetic differentiation of species and populations of the free-living endobenthic nematode genera Sabatieria and Desmodora using nuclear 18S rDNA, internal transcribed spacer (ITS) rDNA, and mitochondrial cytochrome oxidase I (COI) gene sequences. Specimens were collected at continental shelf depths (200–500 m) near the Antarctic Peninsula, Scotia Arc and eastern side of the Weddell Sea. The two nematode genera co-occurred at all sampled locations, but with different vertical distribution in the sediment. A combination of phylogenetic (GMYC, Bayesian Inference, Maximum Likelihood) and population genetic (AMOVA) analyses were used for species delimitation and assessment of gene flow between sampling locations.ResultsSequence analyses resulted in the delimitation of four divergent species lineages in Sabatieria, two of which could not be discriminated morphologically and most likely constitute cryptic species. Two species were recognised in Desmodora, one of which showed large intraspecific morphological variation. Both genera comprised species that were restricted to one side of the Weddell Sea and species that were widely spread across it. Population genetic structuring was highly significant and more pronounced in the deeper sediment-dwelling Sabatieria species, which are generally less prone to resuspension and passive dispersal in the water column than surface Desmodora species.ConclusionsOur results indicate that gene flow is restricted at large geographic distance in the Southern Ocean, which casts doubt on the efficiency of the Weddell gyre and Antarctic Circumpolar Current in facilitating circum-Antarctic nematode species distributions. We also show that genetic structuring and cryptic speciation can be very different in nematode species isolated from the same geographic area, but with different habitat preferences (surface versus deeper sediment layers).
Highlights
Dispersal ability, population genetic structure and species divergence in marine nematodes are still poorly understood, especially in remote areas such as the Southern Ocean
We investigate the phylogeographic and population genetic structure of species from two marine nematode genera (Sabatieria Rouville, 1903 and Desmodora de Man, 1889) in the Antarctic using mitochondrial and nuclear (internal transcribed spacer (ITS) ribosomal DNA (rDNA) and small subunit (18S) rDNA) markers
Bayesian and maximum likelihood trees inferred from ITS haplotypes separated the sequences into four highly differentiated and relatively well-supported clades according to morphotype and/or geographic location
Summary
Population genetic structure and species divergence in marine nematodes are still poorly understood, especially in remote areas such as the Southern Ocean. Marine nematodes are the most abundant metazoan inhabitants of seafloor sediments and estimates of total marine species numbers (including parasites) are believed to exceed 50,000 [1]. Most of this diversity remains undescribed due to the difficult and time-consuming taxonomy, and logistically challenging sampling in remote (e.g., deep-sea) environments [2, 3]. A profound understanding of species-specific preferences and life history traits (e.g., habitat preference, dispersal ability), in combination with knowledge on physical drivers of connectivity among marine populations (e.g., hydrodynamic forces, habitat characteristics) is imperative in the study of nematode species distribution patterns across various spatial scales and habitats
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