The role of small RNAs in caste determination and differentiation in the bumble bee, Bombus terrestris
Investigating the molecular basis of queen-worker caste determination and differentiation in eusocial insects allows researchers to address fundamental aspects of evolution and development. In this regard, little is known of the role of gene regulation by microRNAs (miRNAs), non-coding RNA molecules that regulate development in animals and plants. In this thesis I therefore investigated the role of miRNAs in caste determination and differentiation in the bumble bee Bombus terrestris. In Chapter 2, I used deep sequencing (miRNA-seq) and bioinformatics to annotate miRNAs in B. terrestris and a second Bombus species, B. impatiens. I found that B. terrestris yielded 131 miRNAs and B. impatiens yielded 114 miRNAs and that, of these, 17 were new miRNAs that had not previously been sequenced in any species. In Chapter 3, using miRNA-seq and Northern blot analysis of female B. terrestris larvae, I isolated a miRNA (Bte-miR-6001) that was more highly expressed in queen- than in worker-destined larvae. This miRNA comprised an entire intron of the gene Very high density lipoprotein (Vhdl), which is homologous to the gene for the key storage protein vitellogenin. In Chapter 4, using miRNA-seq and Northern blot analysis of adult females, I isolated some miRNAs (e.g. Bte-miR-279b, Bte-miR-279c) that were more highly expressed in queens and reproductive workers and others (e.g. Bte-miR-184, Bte-miR-133) that were highly expressed in non-reproductive workers. Finally, in Chapter 5, using RNAi (RNA interference), I tested whether expression at the gene locus foraging (for) is associated with locomotory behaviour of B. terrestris foundress queens. Although RNAi did not induce gene knockdown, there was a positive association between for expression and queen reproduction. Overall, by isolating miRNAs associated with caste and reproduction in B. terrestris, the work reported in this thesis contributes substantially to our understanding of the molecular basis of caste in eusocial in insects.
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