Genetic Variation Underlying Protein Expression in Eggs of the Marine Mussel Mytilus edulis

Abstract

Study of the genetic basis of gene expression variation is central to attempts to understand the causes of evolutionary change. Although there are many transcriptomics studies estimating genetic variance and heritability in model organisms such as humans there is a lack of equivalent proteomics studies. In the present study, the heritability underlying egg protein expression was estimated in the marine mussel Mytilus. We believe this to be the first such measurement of genetic variation for gene expression in eggs of any organism. The study of eggs is important in evolutionary theory and life history analysis because maternal effects might have profound effects on the rate of evolution of offspring traits. Evidence is presented that the egg proteome varies significantly between individual females and that heritability of protein expression in mussel eggs is moderate to high suggesting abundant genetic variation on which natural selection might act. The study of the mussel egg proteome is also important because of the unusual system of mitochondrial DNA inheritance in mussels whereby different mitochondrial genomes are transmitted independently through female and male lineages (doubly uniparental inheritance). It is likely that the mechanism underlying this system involves the interaction of specific egg factors with sperm mitochondria following fertilization, and its elucidation might be advanced by study of the proteome in females having different progeny sex ratios. Putative identifications are presented here for egg proteins using MS/MS in Mytilus lines differing in sex ratio. Ontology terms relating to stress response and protein folding occur more frequently for proteins showing large expression differences between the lines. The distribution of ontology terms in mussel eggs was compared with those for previous mussel proteomics studies (using other tissues) and with mammal eggs. Significant differences were observed between mussel eggs and mussel tissues but not between the two types of eggs. Molecular & Cellular Proteomics 8:132-144, 2009.

Proteomic Analysis of Eggs from Mytilus edulis Females Differing in Mitochondrial DNA Transmission Mode

“…The number of spots observed on the gels analyzed using SameSpots software was usually over 1000, in line with expectation for the technique (51) and a previous study on mussel eggs (47). After filtering (see supplemental Experimental Procedures), 244 spots were retained for analysis.…”

“…These followed closely those used previously for mussels (47). In brief, proteins were extracted from unfertilized eggs, solubilized by sonication on ice, and then quantified and cleaned using commercial kits.…”

“…Transformation and normalization procedures for spot volume followed those used in an earlier study (47). For each spot, spot volume (intensity) was first normalized by dividing by the total spot volume for the gel from which the spot came.…”

“…Significance at BH 50% for the eight spots is used as a further criterion for inclusion in Table III of additional identifications of these specific proteins. In addition, two identifications from an earlier study (47) are included. The multiple occurrences of four proteins, arginine kinase, enolase, ferritinlike protein, and proteasome subunits add confidence to the protein identifications.…”

Review of recent proteomic applications in aquatic toxicology

“…Of those published, many have focused on bivalve species, such as marine mussels of the genus Mytilus (M. edulis, M. galloprovincialis, M. trossulus) that reside off the European coast. Most of the studies published have utilized proteomic analysis to examine taxonomic differences [42][43][44][45][46][47], heritability [48], and evolution of the nacre [49]. One of the first environmental toxicology proteomic studies with aquatic invertebrates was conducted on Mytilus.…”