Population density effects on gamete traits and fertilisation dynamics under varying sperm environments in mussels

Gamete traits can vary widely among species, populations and individuals, influencing fertilisation dynamics and overall reproductive fitness. Sexual selection can play an important role in determining the evolution of gamete traits with local environmental conditions determining the strength and direction of sexual selection. Here, we test for signatures of post-mating selection on gamete traits in relation to population density, and possible interactive effects of population density and sperm concentration on sperm motility and fertilisation rates among natural populations of mussels. Our study shows that males from high-density populations produce smaller sperm compared with males from low-density populations, but we detected no effect of population density on egg size. Our results also reveal that females from low-density populations tended to exhibit lower fertilisation rates across a range of sperm concentrations, although this became less important as sperm concentration increased. Variances in fertilisation success were higher for females than males and the effect of gamete compatibility between males and females increases as sperm concentrations increase. These results suggest that local population density can influence gamete traits and fertilisation dynamics but also highlight the importance of phenotypic plasticity in governing sperm-egg interactions in a highly dynamic selective environment.

The effect of mitochondrial recombination on fertilization success in blue mussels

The presence of doubly uniparental inheritance (DUI) in bivalves represents a unique mode of mitochondrial transmission, whereby paternal (male-transmitted M-type) and maternal (female-transmitted F-type) haplotypes are transmitted to offspring separately. Male embryos retain both haplotypes, but the M-type is selectively removed from females. Due to the presence of heteroplasmy in males, mtDNA can recombine resulting in a 'masculinized' haplotype referred to as Mf-type. While mtDNA recombination is usually rare, it has been recorded in multiple mussel species across the Northern Hemisphere. Given that mitochondria are the powerhouse of the cell, different mtDNA haplotypes may have different selective advantages under diverse environmental conditions. This may be particularly important for sperm fitness and fertilization success. In this study we aimed to i) determine the presence, prevalence of the Mf-type in Australian blue mussels (Mytilus sp.) and ii) investigate the effect of Mf-mtDNA on sperm performance (a fitness correlate). We found a high prevalence of recombined mtDNA (≈35 %) located within the control region of the mitochondrial genome, which occurred only in specimens that contained Southern Hemisphere mtDNA. The presence of two female mitotypes were identified in the studied mussels, one likely originating from the Northern Hemisphere, and the other either representing the endemic M. planulatus species or introduced genotypes from the Southern Hemisphere. Despite having recombination events present in a third of the studied population, analysis of sperm performance indicated no difference in fertilization success related to mitotype.

SNP genotyping revealed a hybrid zone between Mytilus chilensis and M. platensis in southern South America (the Strait of Magellan, Isla Grande de Tierra del Fuego and the Falkland Islands)

Hybrid zones among mussel species have been extensively studied in the northern hemisphere. In South America, it has only recently become possible to study the natural hybrid zones, due to the clarification of the taxonomy of native mussels of the Mytilus genus. Analysing 54 SNP markers, we show the genetic species composition and admixture in the hybrid zone between M. chilensis and M. platensis in the southern end of South America. Bayesian, non-Bayesian clustering and re-assignment algorithms showed that the natural hybrid zone between M. chilensis and M. platensis in the Strait of Magellan, Isla Grande de Tierra del Fuego and the Falkland Islands shows clinal architecture. The hybrid zone can be divided into three different areas: the first one is on the Atlantic coast where only pure M. platensis and hybrid were found. In the second one, inside the Strait of Magellan, pure individuals of both species and mussels with variable degrees of hybridisation coexist. In the last area at the Strait in front of Punta Arenas City, fjords on the Isla Grande de Tierra del Fuego, and at the Beagle Channel, only M. chilensis and a low number of hybrids were found. According to the proportion of hybrids, bays with protected conditions away from strong currents would give better conditions for hybridisation. We do not find evidence of any other mussel species such as M. edulis, M. galloprovincialis, M. planulatus or M. trossulus in the zone.