Further Studies on Factor Interaction in Drosophila Virilis.

A character is a result of the cooperative action of all genes, although some genes alone, and in case of “factor interaction,” more than one gene, are chiefly concerned in the development of a particular character. Interacting genes, as well as modifiers, may be specific in their effect, since they can act only in the presence of a particular genetical environment (BRIDGES 1919), and non-specific, when their action is not confined to a particular gene. The present paper deals with the inheritance of thorax protrusions, curling of bristles and other characteristics which appear in various phenotypical expressions as a result of the interaction of certain genes with nonspecific modifiers. The following five genes are involved in the interaction here studied: Ruffled (ru), a fifth chromosome recessive gene. In ruffled flies the tips of the dorso-central bristles and hairs near them are curled forward, and the thorax may be slightly telescoped (figure la). Rounded (R), a second chromosome dominant gene. Rounded flies have truncate wings, short and bushy arista (figure Id). Clipped (CZ), a fourth chromosome dominant gene, lethal when homozygous. In clipped flies the posterior end of the wings are cut and the majority have two dorso-central bristles slightly curled (figure le). Shaggy (sh), a fifth chromosome recessive gene. In shaggy flies the abdominal hairs are directed irregularly. Beaded (Bd), a fifth chromosome dominant, lethal when homozygous. In beaded flies the wing margins are scalloped and one or two of the dorsocentral bristles may become slightly curled, depending upon condition of the food and age of the flies. The following are new characters which have resulted from the interaction between some of the above described genes: Vortex1, two well-developed horn-like protrusions on the thorax, located lateral to and midway between the anterior and posterior dorsocentral bristles. The microchaetae or small hairs around these protrusions are arranged in a whorl. Vortex-2, protrusions are only slightly developed, and in extreme cases leaving only two brown pigmented spots surrounded by whorled hairs.