Hatchlings of the desert locust, Schistocerca gregaria, display phase polyphenism in body coloration and size. This phenomenon has been found to be maternally controlled and two different mechanisms have been proposed to explain the underlying process. One mechanism claims that a water-soluble pheromonal factor secreted by gregarious female adults into the foam plugs of egg pods induces darkening in their progeny. The other mechanism states that hatchling body coloration is pre-determined in the ovaries and that no foam factor is involved in this phenomenon. This mechanism was supported by the observation that hatchling melanization was not prevented by the early washing or separation of eggs, which should have removed the pheromonal factor from the eggs and produced green hatchlings according to the other mechanism. This paper reviews the latest findings related to this phenomenon with special reference to reproductive cycles and genetic differences. The close relationships between egg size and the degree of melanization in hatchlings may provide strong evidence against the possible involvement of the pheromonal factor, because egg size is determined in the ovarioles. Furthermore, the absence of "solitarizing" effects of early washing and separation on hatchling melanization was also confirmed in different genetic strains. A hypothesis proposed by others that such effects occur only in eggs from the first reproductive cycle was tested and rejected. Based on these and other results, a model to explain the mechanisms underlying the maternal control of progeny characteristics and embryonic control of melanization in the hatchling was proposed.
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