Interlineage Mytilus galloprovincialis Lmk. 1819 hybridization yields inconsistent genetic outcomes in the Southern hemisphere

A Southern hemisphere lineage of the blue mussel Mytilus galloprovincialis has been diverging in allopatry from Northern hemisphere conspecifics for 0.84–1.2 million years. Secondary contact between Southern and Northern hemisphere mussels in Chile, New Zealand and Australia provides an opportunity to better understand the extent and consequences of extensive range expansion. Non-native M. galloprovincialis and hybrids, as detected from RFLP assays of nuclear and mitochondrial DNA, are present in all three countries and significant cytonuclear disequilibria exist for native homozygotes in Chile and New Zealand, non-native homozygotes in Chile and non-native heterozygotes in New Zealand. Introductions into Australia are rare events given that no pure non-native mussels were detected. Immigration from one or both taxa into the hybrid zone may underlie disequilibria in New Zealand, whilst gender-directional crossing with limited ongoing hybridization contributes to disequilibria in Chile. Hybridization dynamics do not pose a threat to the Southern lineage in Chile and Australia, but in New Zealand, introgression, continued immigration and slight hybridization gender bias towards non-native maternal parents could lead to the regional extirpation of the native lineage.