A neuropeptide controlling the dark pigmentation in color polymorphism of the migratory locust, Locusta migratoria

The neuroendocrine control of the dark body color in hoppers of Locusta migratoria was studied using an albino strain as a bioassay system. Methanol or saline extracts of corpora cardiaca originating from 5th instar hoppers of a normal (non-albino) strain were dried, dissolved in 10% aqueous ethanol or mixed with peanut oil, and injected into albino hoppers. No dark color was induced by samples of the methanol extracts dissolved in 10% aqueous ethanol. However, samples of the extracts mixed with peanut oil induced dose-dependent darkening in the albino hoppers within a few days after injection; the higher the dose, the darker the color induced. Methanol extracts dissolved in saline and heated at 95°C for 10 min retained the dark-color inducing effect after being redried, mixed with peanut oil and injected, but those incubated with a proteinase for 2 h lost it completely. Therefore, this dark-color inducing factor seems to be a heat-stable neuropeptide. Organ implantation into albino hoppers revealed that the corpora cardiaca and brain of normal solitary hoppers contained this neurohormone as did those of normal gregarious hoppers. Injection of a high dose (2 pairs of corpora cardiaca equivalent) of extract mixed with peanut oil, or daily repeated injections on the first 4 days of the 4th instar, induced very dark color after the following ecdysis, but black patterns characteristic of gregarious hoppers were not induced. However, some albino hoppers developed such patterns after receiving 3 daily repeated injections of 1 pair of corpora cardiaca equivalent during the second half of the 4th instar. The role of this neurohormone in locust polychromatism and in locust phase polymorphism is discussed.