In response to crowding, locusts develop characteristic black patterns that are well discernible in the gregarious phase at outbreaks. We report here a dark-colorinducing neuropeptide (dark-pigmentotropin) from the corpora cardiaca of two plague locusts, Schistocerca gregaria and Locusta migratoria. The chromatographic isolation of this neuropeptide was monitored by using a bioassay with an albino mutant of L. migratoria. Body-color polymorphism is widespread among animals. Two locust species, Schistocerca gregaria and Locusta migratoria, display conspicuous differences in body color, particularly during the nymphal stage. A major extrinsic factor influencing locust body color is phase polymorphism, a term used to describe continuous polymorphism in response to population density: locusts at a low density (solitary phase) are often green or brown, whereas those at outbreaks (gregarious phase) develop black patterns (1-4). Although the role of juvenile hormone in the induction of the green color is well established (2-4), little information is available about the hormonal factor that induces dark color in locusts. It has long been known that some factor present in the brain and the corp cardiaca (CC) promotes darkening in locusts (2, 3), but progress in identifying its chemical nature has been hampered by the lack of a convenient bioassay.Recently, we discovered an albino mutant, originating from a laboratory colony of an Okinawa (Japan) strain of L. migratoria (5). Albinism in this mutant is controlled by a single recessive Mendelian unite (5), as described also for other albino mutants of this species (6, 7), as well as of S. gregaria (8) and the grasshopper Melanoplus sanguinipes (under the name Melanoplus bilituratus) (9). The albinism in the Okinawa strain of L. migratoria is caused by the deficiency of a peptide(s) present in the central nervous system and the CC. Implantation of a brain or CC taken from normal (pigmented) individuals or injection of their methanolic extract induces dark color in albino locusts (10-12), but injection of such methanolic extract made from albino individuals has no dark-colorinducing effect in albino locusts (11). Of interest, implantation of brains or CC taken from other taxa, including S. gregaria and other acridids, cockroaches, katydids, crickets, and moths also are effective in inducing dark color in albino L. migratoria (10,12,13). This result indicates that similar substances inducing dark color in L. migratoria may exist in diverse groups of insects. Because whitish albino locusts can be obtained easily by mass rearing, they provide an excellent bioassay system for the characterization of this dark-color inducing peptide. Its role in body color polymorphism and phase polymorphism in locusts can then be determined by means of the synthetic analog.
MATERIALS AND METHODSInsects and Tissue Extraction. The colony of the desert locust S. gregaria was maintained according to Ashby's method (14) and that of the migratory locust L. migratoria migratorioides as described...
