Viability of homokaryotypes in the tumorous-head strain of Drosophila melanogaster.

HE tumorous-head trait in Drosophila melanogaster is caused by a third chromosome gene (tu-?) showing reduced penetrance. The penetrance can be increased by a sex-linked recessive gene ( tuI ) controlling a maternal effect (GARDNER and WOOLF 1949), by polygenic modifiers (GARDNER, STOTT and DEARDEN 1952), and by high temperature during an early developmental period (GARDNER and WOOLF 1950). When tu-1 and tu-? are homozygous, the frequency of flies showing an amorphous head growth (defined as penetrance in the tumorous-head literature) in a strain maintained at 25°C will vary from about 20% to over 80% depending on the polygenic background. Most tumorous-head strains are characterized by chromosomal polymorphism (WOOLF and PHELPS 1960; WOOLF 1965). Two types'of third chromosomes are present, symbolized 3A and 3B. They differ because of the presence of the recessive gene st (scarlet eyes) and the Payne inversion in the left arm of chromosome 3B. Since chromosome 3B is homozygous lethal, two types of adult flies occur: homokaryotypes (3A/3A) and heterokaryotypes (3A/3B). Chromosome 3B is maintained at a high frequency (over 80%) in laboratory stocks because female heterokaryotypes are more productive (number of adult offspring produced) than female homokaryotypes and because of the greater viability (probability of development from zygote to adult) of female and male heterokaryotypes ( WOOLF and CHURCH 1963). The reduced productivity of female homokaryotypes results from a complex genetic system involving chromosome 2 and the left arm of chromosome 3A (WOOLF and KNOWLES 1964; WOOLF, KNOWLES and JARVIS 1964). KNOWLES (1 967) has demonstrated that the reduced productivity of homokaryotype females is a function of reduced fecundity, fertility and longevity. Equilibrium formulae were derived by LI (1 963) for this situation existing in tumoroushead strains where selection favors the heterokaryotypes but acts differentially in male and female homokaryotypes. Penetrance is higher in females in tumorous-head strains ( GARDNER and WOOLF 1949), females are more apt to show an extreme expression of the abnormality, and heterokaryotypes of both sexes show a higher penetrance ( WOOLF and LOTT 1965). Males outnumber females and at high temperature the sex ratio (M/F) increases (GARDNER and WOOLF 1950). It has been proposed (WOOLF and LOTT 1965) that heterosis brought about by heterozygosity for the Payne in-