Genetic and ecological aspects of the hybrid zone between the mussels Mytilus edulis and Mytilus trossulus in the northwest Atlantic

Miranda, Marcelo B. B. (2004) Genetic and ecological aspects of the hybrid zone between the mussels Mytilus edulis and Mytilus trossulus in the northwest Atlantic. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Two mussel species (Mytilus edulis and Mytilus trossulus) form a hybrid zone in the northwest Atlantic that is complex in its structure and dynamics. The study of Mytilus hybrid zones can reveal important aspects of marine evolution and speciation. A patchy distribution of M. edulis and M trossulus around Newfoundland suggests that differential adaptation to environmental conditions may be important in determining the species relative frequency and fitness. The observation that M. trossulus is less tolerant to warm temperatures than M. edulis implies that physiological differentiation may contribute to reproductive isolation since discrete biogeographic distributions help to maintain the genetic identity of the species. Differences in the microgeographic distribution of the species indicate that habitat specialization may also contribute to reproductive isolation. The presence of pure M. trossulus populations in five sites sampled in Labrador likely result in a great larval input from this species into the hybrid zone through the Labrador Current. Poorer adaptation to the environmental conditions of Newfoundland would then result in a higher mortality of M trossulus in the area. Evidence for differential survival was suggested by the observed decrease in the relative frequency of M. trossulus in the larger size classes at most sites in Newfoundland and by the greater mortality observed in field grow-out and temperature experiments. Differences in life-history strategies may also influence the observed frequency pattern since M. trossulus reproduces earlier, spawns smaller eggs and possibly has a shorter generation time than M. edulis. Gamete recognition and incompatibility are probably important isolating mechanisms between M. edulis and M. trossulus. Intrinsic postzygotic mortality likely represents an additional isolating mechanism. Reduced hybrid viability was observed mainly in the early stages of development, but higher mortality of hybrids also occurred at later stages. F1 hybrids and advanced backcrosses were the groups that showed greatest variance in fitness, suggesting that although most hybrids have reduced fitness, some hybrid combinations may be as fit or even fitter than the parent species. The use of several DNA markers revealed that the distribution of Mytilus genotypes in the northwest Atlantic hybrid zone is strongly bimodal, showing that very few F I hybrids are present and that most hybrids are advanced backcrosses. Although low levels of introgression were observed, the presence of few backcross individuals that were homozygous for alleles from the opposite species suggests that introgression can be a potential mechanism for adaptive evolution in Mytilus. Despite the overall lower fitness of hybrids, some hybrid individuals can express hybrid vigour and may be important for the introgression of genes between the species. Other intrinsic differences were observed between the species such as settlement behaviour, growth, survival, shell morphology and shell breakage resistance. Such differences likely play a role in maintaining the coexistence of both species. Information on species differences together with the increased performance of some hybrids can be used to help improve aquaculture production. As in other hybrid zones, the Mytilus hybrid zone in the northwest Atlantic is probably maintained by a combination of several factors. Pre-zygotic mechanisms (habitat specialization, gamete recognition and incompatibility) as well as post- zygotic mechanisms (reduced hybrid fitness) are critical factors responsible for reproductive isolation that were identified in the present study.

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