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.
Highlights
Study of the genetic basis of gene expression variation is central to attempts to understand the causes of evolutionary change
Knowledge of the genetic and environmental components of such variation is of central importance for assessment of the roles of selection, mutation, and genetic drift in causing evolutionary change in gene expression profiles (4 – 6)
Of 8131 transcripts studied in two strains of mice and their reciprocal F1 crosses, 18% showed a heritability of gene expression of Ͼ50%, and 20% showed evidence of dominance effects
Summary
Animals and Egg Collection—M. edulis female mussels belonging to two lines from a breeding program were used. Twenty outlying values (of 12 gels ϫ 261 ϭ 3132 values) identified using Grubb’s test (49) were removed from the data set and replaced with values imputed using the regression method, implemented in SPSS 13.0 for Windows (Statistical Package for the Social Sciences (SPSS Inc.)), with added random residuals This was followed by renormalization by subtraction (resulting variable ϭ log2final). The correlation tG measures the relative amount of genetic variation; tB measures the relative amount of biological variation Both variables log2final and log2std show a good visual fit to the normal distribution over the whole data set, an acceptable fit to a straight line in the Q-Q plot, and a non-significant Kolmogorov-Smirnov test result, the Levene test for homogeneity of variance is significant. To further validate protein identifications, peptide sequences were used to search a local Mytilus cDNA sequence database and Mytilus sequences in NCBI databases using TBlast
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