The Genetics O F Melanism in Malacosoma Dzsstrza Hubner (lepidoptf23a: Lasiocampidae)

The forest tent caterpillar is polymorphic far two melanic genes affecting wing color of moths. These are the first genetically determined morphological traits reported for the genus. Dark (D) is a sex-limited, autosomal dominant with a phenotype of dark brown males. Frequencies in population samples varied from 8 to 44%. Characteristics of Dark and nonmelanic males were compared. Larval development times, larval survival and pupal weights were not significantly different, but mean fecundity was higher for females with Dark progeny. Band (b), a darkened area across the forewings, occurred in low frequency in both sexes. M O T H S of the genus Malacosoma are hardwood defoliators sharing several characteristics such as egg clumping, silk structures, communal feeding and wing color variability ( STEHR and COOK 1968). Adult wing spans are 25 to 35 mm, coloring is light tan to dark brown. I studied the inheritance of melanic forms in M . disstria, the forest tent caterpillar, as part of a preliminary investigation of the population genetics of this species that also included quantitative characters (LORIMER 1979a) and isozymes (LORIMER 1979b). This information will ultimately be used to relate genetic organization and fluctuations in population size. The evolutionary importance of melanism in the Lepidoptera is well illustrated by the phenomenon of industrial melanism. In the classic example, dark moths of Biston betularia L. (Selidosemidae) were protected from bird predation while resting on soot-blackened trees (reviewed by KETTLEWELL 1973). The frequency of melanic forms increased with proximity to industrial centers. But not all melanic polymorphisms are related to air polllutioin. Melanism appears to be a “basic and recurring necessity” that may come about for very different reasons (KETTLEWELL 1961). KETTLEWELL (1973) estimates that 10% of the 800 species of macrolepidoptera in Britain maintain melanic forms in other than industrial conditions. In-depth studies of melanic polymorphisms have revealed fascinating ecological, physiological and genetic interactions. Nevertheless, melanism has been examined in few American species of Lepidoptera, and records of genetic analyses are rare ( GINEVAN 197 1 ) . In this paper I report two polymorphic genes for melanism in M . disstria. One is sex-limited to males, the other autosomal and occurs in both sexes. Genetics 9 2 : 555-561 June, 1979.