Methylation and Gene Expression Differences Between Reproductive Castes of Bumblebee Workers

Abstract

word count: 244. 4 Document work count: 6353. 5 Abstract 7Phenotypic plasticity is the production of multiple phenotypes from a single genome and is 8 notably observed in social insects. Multiple epigenetic mechanisms have been associated with 9 social insect plasticity, with DNA methylation being explored to the greatest extent. DNA 10 methylation is thought to play a role in caste determination in Apis mellifera, and other social 11 insects, but there is limited knowledge on it's role in other bee species. In this study we analysed 12 whole genome bisulfite sequencing and RNA-seq data sets from head tissue of reproductive and 13 sterile castes of the eusocial bumblebee Bombus terrestris. We found genome-wide methylation 14 in B. terrestris is similar to other social insects and does not differ between reproductive castes. 15 We did, however, find differentially methylated genes between castes, which are enriched for 16 multiple biological processes including reproduction. However we found no relationship between 17 differential methylation and differential gene expression or differential exon usage between castes. 18 Our results also indicate high inter-colony variation in methylation. These findings suggest 19 methylation is associated with caste differences but may serve an alternate function, other than 20 direct caste determination in this species. This study provides the first insights into the nature 21 of a bumblebee caste specific methylome as well as it's interaction with gene expression and 22 caste specific alternative splicing, providing greater understanding of the role of methylation in 23 phenotypic plasticity within social bee species. Future experimental work is needed to determine 24 the function of methylation and other epigenetic mechanisms in social insects. 25 1 Methylation Differences in Bumblebee Workers Impact Summary 26 Social insects, such as ants, termites, bees and wasps, can produce individuals with extreme physical 27 and behavioural differences within the same colony known as castes (e.g. workers/soldiers/queens). 28 These individuals have similar genomes and many studies have associated epigenetic mechanisms 29 with the differences observed. Epigenetic modifications are changes that affect how genes are 30 expressed without changing the underlying DNA code. Here we investigated differences in DNA 31 methylation (a well researched modified base) between different reproductive castes of the bumblebee, 32Bombus terrestris, an economically and environmentally important pollinator species. We found B. 33terrestris has a similar methylation profile to other social insect species in terms of the distribution of 34 methylation throughout the genome and the relationship between methylation and gene expression. 35Genes that have differences in methylation between reproductive castes are involved in multiple 36 biological processes, including reproduction, suggesting methylation may hold multiple functions 37 in this species. These differentially methylated genes...

The effect of DNA methylation on bumblebee colony development

“…Experiments with Nasonia have also indicated stable inheritance of methylation status through generations [30]. This also agrees with recent findings that have shown high-inter-colony variation in methylation [15].…”

“…In Bombus terrestris, we found an average CpG methylation of 0.5% in the control group, and similar methylation rates for the other methylation types. These results largely correspond with recent findings in Bombus terrestris audax, where non-CpG methylation rates were 0.4 (CHG) and 0.5% (CHH) [15]. Compared to CpG methylation, however, CHG and CHH methylation were not particularly enriched in coding regions, which indicates that they have a minor regulatory role.…”

“…While DNA methylation at gene promoter regions suggests gene silencing as the main function, as proposed for mammals [12], methylation within gene-coding regions suggests a role in alternative splicing [13]. However, in the particular case of insects such as bumblebees, differentially expressed genes contain lower levels of methylation compared to non-differentially expressed genes, which further indicates that DNA methylation in bumblebees is more related to gene expression than to an alteration of gene function [14,15].…”

“…( [27]). Recently, [15] found differences in methylation levels between reproductive castes of bumblebee workers, with some differentially methylated genes involved in behavior and reproductive processes. Their results also showed high inter-colony variance in methylation levels, suggesting that different couples of queens and males transmit different methylomes to their progeny [28][29][30], which in turn will lead to developmental differences at the colony level [31].…”

A Role of Histone Acetylation in the Regulation of Circadian Rhythm in Ants

“…Future studies should confirm these findings in other social insects and contrast the mechanistic regulation of rhythmicity across phenotypic levels (individual and colony level) to better understand circadian rhythm in a social context. Importantly, our study adds to the growing body of evidence for the implications of epigenetic processes in regulating social life in insects (Herb et al, 2012;Kucharski et al, 2008;Libbrecht et al, 2013Libbrecht et al, , 2016Marshall et al, 2019;Morandin et al, 2019; and more generally circadian rhythm in animals (Mendoza-Viveros et al, 2017;Stevenson, 2018).…”