Bumblebee worker castes show differences in allele-specific DNA methylation and allele-specific expression

Abstract

word count: 226. 2 1Allele-specific methylation and expression in bumblebees. Abstract 4Allele-specific expression is when one allele of a gene shows higher levels of expression compared 5 to the other allele, in a diploid organism. Genomic imprinting is an extreme example of this, 6 where some genes exhibit allele-specific expression in a parent-of-origin manner. Recent work 7 has identified potentially imprinted genes in species of Hymenoptera. However, the molecular 8 mechanism which drives this allelic expression bias remains unknown. In mammals DNA 9 methylation is often associated with imprinted genes. DNA methylation systems have been 10 described in species of Hymenoptera, providing a candidate imprinting mechanism. Using 11 previously generated RNA-Seq and whole genome bisulfite sequencing from reproductive and 12 sterile bumblebee (Bombus terrestris) workers we have identified genome-wide allele-specific 13 expression and allele-specific DNA methylation. The majority of genes displaying allele-specific 14 expression are common between reproductive castes and the proportion of allele-specific 15 expression bias generally varies between colonies. We have also identified genome-wide 16 allele-specific DNA methylation patterns in both castes. There is no significant overlap between 17 genes showing allele-specific expression and allele-specific methylation. These results indicate 18 that DNA methylation does not directly drive genome-wide allele-specific expression in this 19 species. Only a small number of the genes identified may be 'imprinted' and it may be these genes 20 which are associated with allele-specific DNA methylation. Future work utilising reciprocal 21 crosses to identify parent-of-origin DNA methylation will further clarify the role of DNA 22 methylation in parent-of-origin allele-specific expression. 23 42 species, see Standage et al. (2016). Additionally, allele-specific expression has been associated with 43 allele-specific methylation in two ant species, Camponotus floridanus and Harpegnathos saltator 44 (Bonasio et al., 2012). Another study did not find any relationship between allele-specific expression 45 and methylation in a hybrid cross of two non-social wasp species, Nasonia vitripennis and Nasonia 46 giraulti (Wang et al., 2016). 47Bumblebees provide an ideal system to further investigate the relationship between allele-specific 48 3 Allele-specific methylation and expression in bumblebees. methylation and allele-specific expression, specifically with a view of elucidating potential 49 mechanisms involved in genomic imprinting in social insects. Using a candidate gene approach, 50 previous research identified allele-specific expression in a gene (ecdysone 20-monooxygenase-like) 51 related to worker reproductive behaviour in B. terrestis (Amarasinghe et al., 2015). Additional 52 research has since used RNA-seq data to identify >500 loci showing allele-specific expression 53 throughout the B. terrestris genome (Lonsdale et al., 2017). This same study also...