A singular cockroach insulin-like peptide specific for a male reproductive gland
The insulin-IGF-signalling (IIS) pathway plays crucial roles in animal physiology. In insects, this pathway works using multiple ligands, insulin-like peptides (ILPs), and a lower number of insulin receptors (InRs). Consequently, much of the regulation of IIS pathway activity is driven by the expression of ILPs, its timing, tissue, and mode of action: endocrine, paracrine, or autocrine. In the adult male of the cockroach Blattella germanica, tissue expression and the regulation in relation to food restriction of the BgILP1-7 genes is similar to that observed in females. However, we identified the expression of BgILP8, an ILP absent in females, in the conglobate gland of males, and only in this organ. BgILP8 expression in the conglobate gland is high during the initial days of adulthood and is reduced under starvation. The conglobate gland is a male sexual accessory gland specific to cockroaches, involved in spermatophore formation. It is composed of twisted tubules that ramify into different branches and converge in a general collecting tubule that opens into the ejaculatory duct. The tubules are lined with a single layer of secretory cells, each of the cells crossed by a small ductule formed by cuticle and actin fibres through which the secretory products will be released. Decreasing BgILP8 levels through RNAi (dsILP8) resulted in a reduction in the size of secretory cells, although the total protein extracted remained unaffected. A comparison of the transcriptomes of control and dsILP8 glands revealed only small differences. Further, comparing genes expressed in the conglobate gland to those expressed in adult females provided a list of putative conglobate gland-specific genes. Our results suggest that BgILP8 plays a role in the development of the conglobate gland. However, a potential function in other male organs or its possible transfer to the female in the copulation cannot be ruled out. Overall design: Two micrograms of dsRNA (dsILP8 or dsMock) were injected into the abdomen of Blattella germanica males on the day 7 of the sixth instar nymph. Dissections were performed at adult day 7.