Nuclear receptors in Bombyx mori: insights into genomic structure and developmental expression

Nuclear receptors (NRs) function as ligand-dependent transcription factors and are involved in diverse biological processes in different animals. The updated assembly of complete genome sequence of the Bombyx mori enabled a systematic analysis of the NRs in the five holometabolous insects including B. mori, Drosophila melanogaster, Anopheles gambiae, Apis mellifera, and Tribolium castaneum. As a result, nineteen NRs were identified in the B. mori genome, each of eighteen NRs has 1:1:1:1 ortholog in the other four insects. Interestingly, the average intron number of ligand-binding domain (LBD) of each NR gene in B. mori was 2.4, much higher than that in the other four insects; the genomic position of introns in LBDs of all orthologs for each NR presents more diversity. Phylogenetic trees of all NRs from the five insects were consistent or aberrant with classical phylogeny of these insect species. The characteristics in number, genomic structure and phylogeny of all NRs revealed their evolutionary conservation and divergence during insect evolution. The expression patterns of several NR genes displayed temporal specificity similar to that in D. melanogaster and may be associated with the key biological processes during silkworm metamorphosis. The RNAi of BmbetaFTZ-F1 resulted in abnormality in larva-pupa transition, further suggesting it is also crucial for silkworm metamorphosis. In conclusion, the present study provided new insights into the structure, evolution, expression, and functions of silkworm NRs.