Balancing selection and candidate loci for survival and growth during larval development in the Mediterranean mussel, Mytilus galloprovincialis

Abstract

Many marine bivalves have complex life histories with distinct developmental processes and genetic mechanisms. Larval development for most bivalves is often a prolonged and crucial physiological stage, where they suffer mass mortality due to early-acting genetic load. In this study, we describe genetic changes taking place within a single generation of families of the Mediterranean mussel Mytilus galloprovincialis over 23 days of larval development. Using replicated cultures and a pooled sequencing approach, we demonstrate that temporal balancing selection at the majority of loci preserve genetic variation in the early developmental stages of M. galloprovincialis. Balancing selection may be the mechanism which maintains standing genetic variation within the mussel genome and may improve the chances of survival and shield larvae from high levels of genetic load. Additionally, we used changes in allele frequencies to identify potential size-associated SNPs and viability-associated SNPs and found that patterns of genetic changes in directionally selected SNPs cannot be simply explained by traditional theories of genetic purging or directional selection without consideration of balancing selection. Finally, we observed a negative correlation between larval growth rates and survival, implying a potential trade-off relationship between the 2 commercially relevant phenotypes.

Integration of natural selection across the life cycle stabilizes a marine mussel hybrid zone

“…Hua et al. ( 2023 ) also found evidence that selection may occur across larval development in the mussel, Mytilus galloprovincialis .…”

“…and found different patterns of gene expression across larval development in oysters and found indications that selection varied among larval stages [however, also seeHedgecock (2022) andDurland et al (2022)]. Hua et al (2023) also found evidence that selection may occur across larval development in the mussel, Mytilus galloprovincialis.We also observed a large heterozygote deficiency among larvae, especially at the D-stage of development. There are at least two possible explanations for this observation; there may be positive assortative mating among gametes, or the larval population may result from the mixture of two differentiated adult populations.…”

Molecular circadian rhythms are robust in marine annelids lacking rhythmic behavior

“…Such coexistence of several different genotypes/phenotypes within the same population can allow for balancing evolution [ 63 ]. In fact, the persistence of multiple genotypes within a population seems to be especially common in broadcast spawners inhabiting heterogeneous environments [ 64 , 65 ], and even for clock genes that are typically under strong selection for optimal timing and fitness, multiple alleles can persist in the same population [ 23 , 66 , 67 ]. While at present we cannot distinguish to which extent the diversity described here for Platynereis is caused by genetic versus environmental factors, evolutionary selection acts on the phenotype, meaning that the same principles apply.…”