Genome-wide search for parent-of-origin allele specific expression in Bombus terrestris.

Genomic imprinting is the expression bias of one allele in a diploid organism, expression being dependent upon which parent the allele was inherited from. Haig9s kinship theory predicts that genomic imprinting occurs due to an evolutionary conflict-of-interest between the maternal alleles and paternal alleles of an individual. In social insects, it has been suggested that genomic imprinting should be widespread. One recent study identified parent-of-origin gene expression in honeybees and found evidence supporting one prediction of Haig9s kinship theory. However, very little is known about genomic imprinting in insects and multiple theoretical predictions must be tested to avoid single-study confirmation bias. We therefore decided to test if parent-of-origin gene expression also occurs in another social bee using reciprocal crosses. We found equal numbers of maternally and paternally expressed alleles in both reproductive and sterile workers with the majority of genes showing the same expression bias in both castes. The most highly biased genes were maternally expressed. There was very little overlap of differentially expressed genes and genes showing parent-of-origin expression. We also found low evolutionary conservation of potentially imprinted genes with the honeybee. Our results offer some support for the kinship theory of genomic imprinting. However, this theory does not fully explain the patterns we have obtained showing the need to consider alternative models. Additionally, the patterns we see differ from previous work in honeybees. This highlights the importance of using multiple species to test theory. Finally, the lack of conservation between species suggests rapid evolution of imprinted genes in Hymenoptera.