Background
Blue-mussels from the Mytilus species-complex (M. edulis x M. trossulus x M. galloprovincialis ) are an abundant component of the benthos community, found in the high latitude habitats. Aside from their ecological value (Norling & Kautsky, 2007), these foundation species are of extreme relevance to the aqu aculture industry, with 2 149 534 tonnes produced globally in 2019 (FAO, 2021) .
Blue mussels are able to withstand a wide range of environmental conditions and significant effort has been made to understand the physiological consequences of environmental stress in this species (e.g Lesser, 2016; Melzner, 2011; Riisgard, 2013) . O ur understanding on the genomic mechanisms underlying species distribution , hybridisation and local adaptation remains incomplete , as the development of genetic resources began to be emphasized in recent years. To help overcome this, we developed a novel genomic tool, a medium-density SNP array, for rapid, high-throughput genotyping of individuals in the Mytilus species complex . The array contains globally polymorphic SNPs, which capture the genetic diversity present in mussel populations thriving across a gradient of diverse environmental conditions (e.g. temperature, salinity and CO2) , and showing high population divergence in response to habitat, facilitating the investigation of genomic structure in this species complex. Whilst genetics and breeding technologies have yet to be widely applied to improve blue mussel aquaculture production. This multipurpose tool can also benefit shellfish aquaculture, facilitating genome wide association studies (GWAS) for key production traits, and testing genomic selection approaches .
Material and Methods
For the creation of the array, 23 Mytilus spp populations were sampled from across their global distribution, incorporating 4 species in this complex (M. edulis , M trossulus M. galloprovincialis and M. chilensis), as well as hybrids. P opulations were selected based on prevailing environmental conditions to capture habitat diversity (e.g. upwelling regions, low salinity regions; Figure 1), and subsequently genetic diversity associated with local adaptation . Samples were sequenced using a low coverage (4-6x) whole-genome resequencing approach, with s equences aligned to the Mytilus galloprovincialis genome prior to filtering and calling of SNPs. In addition, existing sets of published and validated SNPs were included on the array as informative for speciation, as well as diagnosis of transmissible cancer. The final SNP set was selected and incorporated on the in medium-density SNP-array, developed in collaboration with Affymetrix.
Results and future perspectives
In this study , we have identified SNPs representative of global and within species diversity and explored levels of introgression in Mytilus spp populations distributed worldwide . A subset of 60K SNPs was used to develop a SNP array the Mytilus species complex. This tool will allow the consistent , fast and affordable genotyping of individuals, facilitating the investigation of ecological and evolutionary processes in these taxa. Unravelling global genetic diversity in Mytilus spp populations will contribute to our understanding on ecological and biological processes ongoing in these taxa. The applications of this array extends to shellfish aquaculture, contributing to the optimisation of this industry via (a) genomic selection of blue mussels,; (b) parentage assignment; (c) inbreeding level assessment and (d) species/product identification and provenance.
Subsequently, we will undertake analyses to assess to the heritability and mechanisms of stress tolerant phenotypes, as well as conserved mechanisms of adaptation/divergence among populations inhabiting challenging environments. By combining ‘omic approaches with physiology, we can investigate the genetic basis of phenotype and characteris e animals resilient to environmental stress. Such research is especially relevant to safeguard aquaculture production under climate change.
References
FAO (2021). Global production statistics (http://www.fao.org/figis/servlet/TabSelector) . Visited on 12 May 2021.
Lesser, MP (2016), Climate change stressors cause metabolic depression in the blue mussel, Mytilus edulis, from the Gulf of Maine. Limnol . Oceanogr ., 61: 1705-1717. https://doi.org/10.1002/lno.10326
Melzner F, Stange P, Trübenbach K, Thomsen J, Casties I, et al. (2011) Food Supply and Seawater pCO2 Impact Calcification and Internal Shell Dissolution in the Blue Mussel Mytilus edulis. PLOS ONE 6(9): e24223.
Norling P & N Kautsky N (2007) . Structural and functional effects of Mytilus edulis on diversity of associated species and ecosystem functioning. Marine Ecology-progress Series - MAR ECOL-PROGR SER. 351. 163-175. 10.3354/meps07033.
Riisgård , H.U., Lüskow , F., Pleissner , D. et al. Effect of salinity on filtration rates of mussels Mytilus edulis with special emphasis on dwarfed mussels from the low-saline Central Baltic Sea. Helgol Mar Res 67, 591–598 (2013). https://doi.org/10.1007/s10152-013-0347-2