Population substructure and signals of divergent adaptive selection despite admixture in the sponge Dendrilla antarctica from shallow waters surrounding the Antarctic Peninsula.

Gonzalo Giribet,Nathan J. Kenny,David Combosch,Sergi Taboada,Thibaut Jombart,Carlos Leiva,Ana Riesgo

doi:10.1111/mec.15135

Abstract

Antarctic shallow-water invertebrates are exceptional candidates to study population genetics and evolution, because of their peculiar evolutionary history and adaptation to extreme habitats that expand and retreat with the ice sheets. Among them, sponges are one of the major components, yet population connectivity of none of their many Antarctic species has been studied. To investigate gene flow, local adaptation and resilience to near-future changes caused by global warming, we sequenced 62 individuals of the sponge Dendrilla antarctica along the Western Antarctic Peninsula (WAP) and the South Shetlands (spanning ~900km). We obtained information from 577 double digest restriction site-associated DNA sequencing (ddRADseq)-derived single nucleotide polymorphism (SNP), using RADseq techniques for the first time with shallow-water sponges. In contrast to other studies in sponges, our 389 neutral SNPs data set showed high levels of gene flow, with a subtle substructure driven by the circulation system of the studied area. However, the 140 outlier SNPs under positive selection showed signals of population differentiation, separating the central-southern WAP from the Bransfield Strait area, indicating a divergent selection process in the study area despite panmixia. Fourteen of these outliers were annotated, being mostly involved in immune and stress responses. We suggest that the main selective pressure on D.antarctica might be the difference in the planktonic communities present in the central-southern WAP compared to the Bransfield Strait area, ultimately depending on sea-ice control of phytoplankton blooms. Our study unveils an unexpectedly long-distance larval dispersal exceptional in Porifera, broadening the use of genome-wide markers within nonmodel Antarctic organisms.

Highlights

The gene flow and phylogeographic patterns of Southern Ocean shallow-water marine invertebrates in general, and sponges in particular, are interesting for a number of reasons
The most likely number of clusters was K = 2, with K = 5 as the second most likely number of clusters. These results indicate a lack of geographic structure in the dataset under positive selection, which may be the result of the high migration and gene flow detected in the neutral dataset
Our results revealed high admixture and lack of population differentiation, supported by the low global FST of 0.011 and the non-significant pairwise FST values (Table 3), suggesting high connectivity and dispersal capability of D. antarctica throughout the sampling area, which covered most of the species distribution

Summary

| RESULTS

The total length of the mitochondrial genome obtained from our gDNA reads from Deception. Both the DAPC and the Barrier groupings appeared to be non-significant portions of the genetic variance (p-values = 0.468 and 0.248, respectively), the two of them representing less than. The most likely number of clusters was K = 2, with K = 5 as the second most likely number of clusters (inferred from delta K, shown in Supplementary Material 4c) These results indicate a lack of geographic structure in the dataset under positive selection, which may be the result of the high migration and gene flow detected in the neutral dataset. Samples from the Bransfield Strait stations (i.e. South Shetland Islands, O’Higgins Bay, and Cierva Cove) were grouped together, while Paradise Bay and Adelaide Is. appeared as the most divergent sites

| DISCUSSION

Findings

743 ACKNOWLEDGEMENTS