MEPS 406:161-171 (2010)  -  DOI: https://doi.org/10.3354/meps08522

ABSTRACT: Species in the blue mussel species-complex (Mytilus edulis, M. trossulus, and M. galloprovincialis) readily hybridize, but the level of introgression depends upon which species are sympatric. Hybridization between the native pan-Pacific mussel M. trossulus and the invasive Mediterranean mussel M. galloprovincialis typically results in limited introgression. We show that gender of hybrid mussels frequently cannot be determined in 2 hybrid zones formed between these species in Hokkaido, Japan. Hybrid individuals produce gametic tissue and have levels of reproductive effort similar to those of the pure species, but unlike the parental species, gametogenesis fails to produce mature gametes similar to those hybrid males or females. A majority of the gametic cells in hybrid individuals appear to be arrested in an early state of maturation, and rarely produce viable gametes. High levels of sterility afflict both F₁ and other hybrid individuals (putative F₂ and backcross individuals) forming a potent mechanism of reproductive isolation. While the incidence is greatly reduced, F₁ and other hybrids do produce mature gametes, resulting in low levels of introgression. Unsuccessful gametogenesis of hybrid mussels is likely a general phenomenon that limits introgression in the northern Pacific where the Mediterranean mussel has been introduced.

KEY WORDS: Hybrid zone · Reproductive isolation · Post-zygotic isolation · Mytilus · Blue mussel