Molecular mechanisms underlying metamorphosis in the most-ancestral winged insect.

Abstract

Insects comprise over half of the described species, and the acquisition of metamorphosis must have contributed to their diversity and prosperity. The order Odonata (dragonflies and damselflies) is among the most-ancestral insects with drastic morphological changes upon metamorphosis, in which understanding of the molecular mechanisms will provide insight into the evolution of incomplete and complete metamorphosis in insects. In order to identify metamorphosis-related genes in Odonata, we performed comprehensive RNA-sequencing of the blue-tailed damselfly <i>Ischnura senegalensis</i> at different developmental stages. Comparative RNA-sequencing analyses between nymphs and adults identified eight nymph-specific and seven adult-specific transcripts. RNA interference (RNAi) of these candidate genes demonstrated that three transcription factors, <i>Krüppel homolog 1</i> (<i>Kr-h1</i>), <i>broad</i>, and <i>E93</i> play important roles in metamorphosis of both <i>I. senegalensis</i> and a phylogenetically distant dragonfly, <i>Pseudothemis zonata</i><i>E93</i> is essential for adult morphogenesis, and RNAi of <i>Kr-h1</i> induced precocious metamorphosis in epidermis via up-regulation of <i>E93</i> Precocious metamorphosis was also induced by RNAi of the juvenile hormone receptor <i>Methoprene-tolerant</i> (<i>Met</i>), confirming that the regulation of metamorphosis by the MEKRE93 (Met-Kr-h1-E93) pathway is conserved across diverse insects including the basal insect lineage Odonata. Notably, RNAi of <i>broad</i> produced unique grayish pigmentation on the nymphal abdominal epidermis. Survey of downstream genes for <i>Kr-h1</i>, <i>broad</i>, and <i>E93</i> uncovered that unlike other insects, <i>broad</i> regulates a substantial number of nymph-specific and adult-specific genes independently of <i>Kr-h1</i> and <i>E93</i> These findings highlight the importance of functional changes and rewiring of the transcription factors <i>Kr-h1</i>, <i>broad</i>, and <i>E93</i> in the evolution of insect metamorphosis.