Molecular Ecology and Systematics of BlueMussels (Genus Mytilus) (Mytilidae; Bivalvia;Mollusca) in the Southern Hemisphere

摘要

The Mytilus edulis species complex, comprised of M. edulis, M. galloprovincialisand M. trossulus, is antitropically distributed in temperate coastal regions of all oceans andmain seas of the world. This genus has been heavily studied in the Northern hemispherebut Southern hemisphere populations have received much less attention. This thesis aimsto place Southern hemisphere blue mussels into global evolutionary relationships amongMytilus species and investigate aspects of their molecular ecology, including, effects ofnon-native Northern hemisphere species introductions, biogeography across the Southernhemisphere, regional phylogeographic patterns and population genetics within NewZealand.Southern hemisphere blue mussel phylogenetic reconstruction resulted in thedetection of a monophyletic M. galloprovincialis lineage. Two new molecular markersdeveloped with specificity for this lineage and congruence among phylogeneticinvestigations indicates a panhemispheric distribution of this M. galloprovincialis lineagewith implications for naming a new sibling species of the M. edulis complex. Thisproposed new species, M. meridianus, is distributed in South America, the KerguelenIslands, New Zealand and Australia at latitudes between ~ 30°S and ~ 55°S.Non-native M. galloprovincialis introduced from the Northern hemisphere havebeen present in NZ, Australia and Chile for at least ten years and hybridise with native bluemussels. Introgression is observed in New Zealand and Australian but not Chilean hybridregions. The limited number of introduced mussels in Australia induces hybrid swampingof non-native alleles but an interlineage gender bias towards non-native maternal parentsmay 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 ahaplogroup shared by Northern hemisphere M. edulis and M. galloprovincialis. Althoughsingle gene histories are not congruent with respect to evolutionary relationships within theNorthern hemisphere due to introgressive hybridisation after speciation, it is clear thatSouthern hemisphere blue mussels arose from a species native to the northeast AtlanticOcean after speciation of the three ‘M. edulis complex’ members.Within the Southern hemisphere monophyletic clade lies three reciprocallymonophyletic clades restricted to the geographic regions South America/KerguelenIslands, New Zealand and Australia. Phylogeographic analysis indicates past gene flowbetween South American/Kerguelen Islands and New Zealand populations that has ceasedat present day and ongoing gene flow between South America and the Kerguelen Islandslikely via the West Wind Drift.Within NZ, population subdivision inferred from mtDNA indicates geneticvariation 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 thenorthwest North Island that has been narrowed down to a 15 km stretch of coastline in asheltered bay. The abrupt discontinuity within a small geographic area does not conformto classic population subdivision in this broad-cast spawning species, therefore, furtherinvestigation into the genomic content of northwest North Island mussels with respect tointrogressed non-native genes is warranted.Resolving complex phylogenetic patterns from interspecific introgression is key tounderstanding the evolutionary history of Southern hemisphere M. galloprovincialis.Further characterisation of hybrid introgression would increase accuracy of (1) inferencesof processes contributing to hybridisation dynamics and (2) population subdivision in NZ.Probing the basis for variation of hybridisation dynamics would help to predict theoutcomes of Northern hemisphere M. galloprovincialis introductions in other areas of theworld.