Life history and genetic variation in Mytilus edulis (Linnaeus, 1758) and M. trossulus (Gould, 1850) in a hybrid zone on the east coast of Newfoundland

Toro, Jorge Eduardo (1999) Life history and genetic variation in Mytilus edulis (Linnaeus, 1758) and M. trossulus (Gould, 1850) in a hybrid zone on the east coast of Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Two mussel species (Mytilus edulis L. and Mytilus trossulus Gould) were found sympatrically in two locations (Bellevue and Chance Cove) in eastern Newfoundland. There was genetic evidence for naturally occurring hybrids between M. edulis and M. trossulus. Mussels were classified as pure forms or hybrids (FI, F2 and from backcrosses) based on four diagnostic markers, two allozyme loci (Mpi and Est-D) and two nuclear PCR-based DNA markers (ITS and Glu-5). In addition, a PCR-based mtDNA marker (COW) was used to characterize the distribution of mtDNA mitotypes among pure and hybrid individuals. All females and males from pure species were found to be homoplasmic and heteroplasmic, respectively, for the mtDNA genomic combination. Two individuals showed heterospecific combination of the F edulis genome and the M. trossulus genome (both classified as FI hybrids using four nuclear diagnostic markers). There were differences in the proportions of pure M. edulis and M. trossulus and hybrids between sites within each location. M. edulis and hybrids were associated with protected sites, M. trossulus with exposed sites. Life history variation between species was also found, whereby M. trossulus was the predominant form among small individuals (larvae, spat and juveniles) and M. edulis among the large individuals, suggesting a strong viability selection against pure M. trossulus and its hybrid backcrosses. The overall low frequency of hybrids at the adult stage (7.28 %) and the results of artificial hybridization in the laboratory, which showed an increased proportion of abnormal larvae among interspecific crosses, indicate strong selection against hybrids, and clearly showed them to be at selective disadvantage, supporting the tension-zone model for the early stages of the mussel life-history. M. trossulus and hybrids spawned over a prolonged period of time (from late spring to early autumn), while most M. edulis individuals spawned simultaneously in late July. Hybrid mussels exhibited spawning activity intermediate between that of the parental species. M. trossulus showed a higher reproductive output than M. edulis of similar shell length, while hybrids showed intermediate fecundity values between M. edulis and M. trossulus. Post-spawning mortality in M. trossulus seems to be associated with reproductive stress, but further studies should be carried out to establish the cause of the summer mortality which affects mainly M. trossulus.

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