Speciation and adaptation in Antarctic sea spiders: Assessing the relative importance of genetic drift and natural selection using comparative population genomic and morphological approaches

• Applicants: Professor Dr. Florian Leese; Professor Dr. Roland Melzer
• Subject Area: Oceanography
• Term: from 2014 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 257866065

The analysis of speciation and adaptation processes is fundamental to understand how biodiversity is generated and how it responds to changing environments. An ideal ecosystem to study such processes is the Southern Ocean. The cold waters around Antarctica host a rich and highly endemic benthic fauna. Recent studies have found that many of these species consist of groups of discrete genetic clades interpreted as previously unrecognized species that radiated in the Pleistocene. These cryptic species can be distinguished with molecular methods (e.g. DNA barcoding) and in some cases with meticulous morphological analyses. Typically the speciation events are interpreted as being driven by recent glaciations. The assumption is that speciation occurred by chance when formerly large populations were isolated into small allopatric refugia during glacial maxima, where they were then subjected to the strong effects of random genetic drift. Alternative models of speciation are rarely explored due to previous limitations in molecular techniques. Studies from other ecosystems suggest that ecological speciation, i.e., divergence due to strong differential selection, is one obvious alternative model. Here we propose to use high-resolution population genomic approaches coupled with morphological analyses to test competing Southern Ocean speciation models in a natural and non-laboratory setting. As a test case we will study patterns of genetic drift and selection in a particularly successful group of Southern Ocean benthic species: the sea spiders (Pycnogonida). Building on our earlier studies, we will analyse genomic patterns of neutral and non-neutral markers in two species flocks: Colossendeis megalonyx and Pallenopsis patagonica. These two sea spider species complexes are good models to address the objectives of this proposal because of several characteristics: 1) The presence of many genetically divergent cryptic species, 2) morphological support some of these species, 3) their broad distribution range including both Antarctic continental shelf and sub-Antarctic regions less affected by past glaciations, 4) their low mobility. If allopatric speciation driven largely by genetic drift in glacial refugial populations is the primary evolutionary force, we expect random fixation of neutral alleles and signatures of population bottlenecks to be highest in heavily ice-affected regions. Alternatively, if natural selection has been the central evolutionary force promoting speciation, we expect strong signatures of selection on geno- and phenotype level in particular in sympatry (ecological character displacement). We will investigate variation across whole genomes to assess random vs. non-random variation. The proposed study will make an important first step in systematically testing for the relative importance of natural selection in evolution in the Southern Ocean.

• DFG Programme: Infrastructure Priority Programmes
• Subproject of: SPP 1158: Infrastructure area - Antarctic Research with Comparative Investigations in Arctic Sea Ice Areas
• International Connection: Australia, Chile, United Kingdom, USA
• Participating Persons: Claudia Arango, Ph.D.; Dr. Huw Griffiths; Professor Kenneth M. Halanych, Ph.D.; Christoph Held, Ph.D.; Dr. Verena Häussermann; Professor Andrew Mahon, Ph.D.