Reproductive Biology of Bivalves

 

The aim of this project is a better understanding of the reproductive biology of bivalve molluscs. Bivalves show different modes of sexual reproduction, ranging from simultaneous hermaphroditism, to sequential hermaphroditism, to strict gonochorism, even if the vast majority of species are reported to be strictly gonochoric, with hermaphroditic species thought to be rather uncommon (Gosling 2007, Yusa 2007, Yusa et al 2013, Breton et al 2017). The sex determination system in bivalves is still unknown, but according to several studies it appears to be both genetic and environmental. In sequential hermaphrodites, a role of body size and resource allocation has been proposed: below a certain size, being female can result in a lower reproductive success compared to being male, because spermatogenesis is less energy- demanding compared with oogenesis. Above a certain size, the reproductive success for being female increases. Temperature, food availability, and pollution have been proposed as intervening factors as well (Breton et al 2017). We use comparative analyses of genomes and transcriptomes (mostly from developing and mature gonads) to identify candidate genes involved in gametogenesis, germ line formation, and sex determination/differentiation.

As every other broadcast spawning organisms, bivalves release millions of gametes per individual in the water, at each spawning event. Spermatozoa swim to reach eggs guided by egg- produced chemoattractants and not simply carried by water currents (Ghiselli et al. 2018). In Mytilus galloprovincialis, Evans et al. (2012) showed that egg chemoattractants may play a role in sexual selection by enabling sperm to ‘choose’ between the eggs of different conspecific females. If sperm differentially ‘selects’ eggs on the basis of chemical cues, egg chemoattractants could mediate mate choice for genetically compatible partners. This observation opens unforeseen perspectives about sexual selection and mate choice for broadcast spawning animals. We therefore are going to analyze and compare genes involved in sperm-egg recognition, and in the production of egg chemoattractants, in order to investigate sperm competition and sexual selection in bivalves.

This project focused on reproductive biology lies halfway between basic and applied research: most of the produced data will be useful for both evolutionary studies and aquaculture. Indeed, we will likely obtain information for developing sex-specific genetic markers, which will be a precious tool, because in bivalves sex is detectable only during periods of sexual maturity, and only with invasive techniques.
COLLABORATIONS: this project is carried out in collaboration with the Breton Lab (Université de Montréal, Canada), and with the Cellular and Developmental Biology unit (University of Bologna).