Molecular Ecology and Systematics of Blue Mussels (Genus Mytilus) (Mytilidae; Bivalvia; Mollusca) in the Southern Hemisphere
Authors:   , Westfall Kristen Marie 
Published: November 2021

<p>The Mytilus edulis species complex, comprised of M. edulis, M. galloprovincialis and M. trossulus, is antitropically distributed in temperate coastal regions of all oceans and main seas of the world. This genus has been heavily studied in the Northern hemisphere but Southern hemisphere populations have received much less attention. This thesis aims to place Southern hemisphere blue mussels into global evolutionary relationships among Mytilus species and investigate aspects of their molecular ecology, including, effects of non-native Northern hemisphere species introductions, biogeography across the Southern hemisphere, regional phylogeographic patterns and population genetics within New Zealand. Southern hemisphere blue mussel phylogenetic reconstruction resulted in the detection of a monophyletic M. galloprovincialis lineage. Two new molecular markers developed with specificity for this lineage and congruence among phylogenetic investigations indicates a panhemispheric distribution of this M. galloprovincialis lineage with implications for naming a new sibling species of the M. edulis complex. This proposed new species, M. meridianus, is distributed in South America, the Kerguelen Islands, New Zealand and Australia at latitudes between ~ 30°S and ~ 55°S. Non-native M. galloprovincialis introduced from the Northern hemisphere have been present in NZ, Australia and Chile for at least ten years and hybridise with native blue mussels. Introgression is observed in New Zealand and Australian but not Chilean hybrid regions. The limited number of introduced mussels in Australia induces hybrid swamping of non-native alleles but an interlineage gender bias towards non-native maternal parents may result in eventual loss of the unique genomic content of native blue mussels in NZ. Southern hemisphere blue mussels form a monophyletic sister clade to a haplogroup shared by Northern hemisphere M. edulis and M. galloprovincialis. Although single gene histories are not congruent with respect to evolutionary relationships within the Northern hemisphere due to introgressive hybridisation after speciation, it is clear that Southern hemisphere blue mussels arose from a species native to the northeast Atlantic Ocean after speciation of the three ‘M. edulis complex’ members. Within the Southern hemisphere monophyletic clade lies three reciprocally monophyletic clades restricted to the geographic regions South America/Kerguelen Islands, New Zealand and Australia. Phylogeographic analysis indicates past gene flow between South American/Kerguelen Islands and New Zealand populations that has ceased at present day and ongoing gene flow between South America and the Kerguelen Islands likely via the West Wind Drift. Within NZ, population subdivision inferred from mtDNA indicates genetic variation is distributed within an east-west phylogeographic split on the North Island. These populations experienced gene flow in the past that has ceased at present day. Microsatellite allele frequencies indicate a different population subdivision within the northwest North Island that has been narrowed down to a 15 km stretch of coastline in a sheltered bay. The abrupt discontinuity within a small geographic area does not conform to classic population subdivision in this broad-cast spawning species, therefore, further investigation into the genomic content of northwest North Island mussels with respect to introgressed non-native genes is warranted. Resolving complex phylogenetic patterns from interspecific introgression is key to understanding the evolutionary history of Southern hemisphere M. galloprovincialis. Further characterisation of hybrid introgression would increase accuracy of (1) inferences of processes contributing to hybridisation dynamics and (2) population subdivision in NZ. Probing the basis for variation of hybridisation dynamics would help to predict the outcomes of Northern hemisphere M. galloprovincialis introductions in other areas of the world.</p>