Two insulin-like peptides may regulate egg production in opposite directions via juvenile hormone signaling in the queenless ant Pristomyrmex punctatus

Abstract

In the major eusocial species of Hymenoptera, the regulatory mechanisms controlling queen/worker differentiation and exclusive reproduction by queens have been studied extensively. These studies have shown that insulin/insulin-like growth factors and juvenile hormones (JHs) act as key endocrine factors. However, although considerable knowledge has accumulated in this area, large disparities in the regulatory mechanisms governing caste differentiation have been observed in different hymenopteran taxa to date. We focused on the queenless ant Pristomyrmex punctatus (Hymenoptera: Formicidae), which exhibits the simplest type of sociality and in which reproductive tasks (egg production) are distributed among morphologically and genetically identical workers. To elucidate the molecular mechanisms underlying reproduction in P. punctatus, we analyzed the correlations between the gene expression profiles of a reproductive marker gene, vitellogenin (PripuVTG1), and candidate regulatory genes comprising the major components of the JH and insulin/insulin-like growth factor signaling pathways that are involved in the regulation of reproduction upstream of JH signaling. Expression of insulin-like peptide 1 (PripuILP1) and JH signaling-related genes was negatively correlated with PripuVTG1 expression. On the contrary, insulin-like peptide 2 (PripuILP2a) was positively correlated with PripuVTG1. These findings suggest that an equilibrium perhaps controlled by switches in JH signaling exists between these two ILP paralogs, and that these interactions are important for regulating reproduction. Our findings are expected to be useful for understanding how various modes of sociality have evolved in insects.

Keywords: distribution of reproductive tasks among colony members; insulin; juvenile hormone; queenless ant; vitellogenin.