Crowding-induced phase transition in Oedaleus asiaticus as a developmental stage-gated, irreversible multi-gene switch

Solitary-phase Oedaleus asiaticus (Bei-Bienko) can switch to the far more destructive gregarious phase as population density rises, posing a serious threat to agriculture, animal husbandry and local economies. The current investigation assessed how developmental stages and population density impacted phase transitions and which genes served as key regulatory elements of this process. The results showed that the younger nymphs were, the higher proportion that shifted from solitary to gregarious phase, whereas adults never switched; a density of 60 insects produced the highest transition rate, whereas lower or higher densities resulted in fewer gregarious individuals. The gregarious locusts maintained their original physical characteristics when they were grown in solitary conditions. Transcriptome analysis showed that gregarious individuals exhibited strong gene expression increases which included the juvenile-hormone acid methyl-transferase gene ( JHAMT ) and the phenylalanine hydroxylase gene ( Hn ) and the DOPA decarboxylase gene ( DDC ) and a chemosensory protein gene ( CSP ). RNAi silencing of JHAMT , Hn , DDC or CSP resulted in a major decrease of phase transition occurrence while JHAMT , Hn and DDC knock-down caused partial albinism. The research demonstrates that population density functions as the key trigger for phase transition while gene interactions with their respective metabolites undergo central regulatory functions. Nonetheless, since the locusts used in this study were field-collected and reared for less than two generations, the results should be interpreted within the context of environmentally induced plasticity rather than genetically fixed phase differences.