Determining the cue for metamorphosis in the solitary bee Megachile rotundata

Metamorphosis in holometabolous insects begins at the transition from the larval stage when the majority of feeding and growth occurs. The physiological regulation of growth and metamorphosis has been well studied in the model organisms Manduca sexta and Drosophila melanogaster, but few studies have determined the developmental basis of size or the cue for metamorphosis in other insect species. Our goal was to determine the regulation and timing of metamorphosis in the solitary bee Megachile rotundata. We measured the critical weight, which is considered a proxy for the physiological commitment to metamorphosis. To confirm critical weight initiates the start of metamorphosis, we measured Juvenile Hormone (JH) titers and genes involved in JH synthesis and response. We also measured genes involved in ecdysone synthesis and response. We found that M. rotundata has a critical weight and that this critical weight initiates the start of metamorphosis. This metamorphic commitment was demonstrated by declines in JH titers, and the upregulation of genes involved in ecdysone synthesis, and genes that are involved in pupal commitment.