Biological parameters and the segregation distortion (SD) phenomenon in Drosophila melanogaster

The relationship between some biological parameters (mortality, longevity, fertility, fecundity and sex ratio) and segregation of second chromosomes in heterozygous and homozygous SD males has been analyzed. The results obtained in SD/SD+ heterozygous males show: (1) their reduced fertility with respect to that of control males, (2) an alteration in the sex ratio in the SD+ progeny only, and (3) inversely related sex-ratio and segregation distortion values. In SDi/SDj combinations: (1) surprisingly, fertility is intermediate between that of SD/SD+ heterozygous males and that of control males, (2) the segregation ratios of the second chromosomes are normal (0.50), and (3) the sex ratio = 0.50 in both classes of SD progeny. The relationship between mortality (and therefore longevity) and fertility of the different genotypes and fecundity per male indicates that the total productivity of heterozygous males is less than that so far claimed. Indeed, their productivity depends not only on the mechanism of nonformation of the SD+ sperm, but also on their reduced longevity. The k = 0.50 and the high fecundity of SDi/SDj combinations indicated that in these males the SD phenomenon is partially suppressed, the SD chromosomes being insensitive to each other, thus implying that particular Rsp alleles are sensitive to given Sd alleles. The complementation pattern for male fertility of SD homozygous males again supports previous evidence that Sd factors from natural populations are, in effect, different Sd genes.