DEVELOPMENTAL STABILITY IN SPECIES HYBRID AND BACKCROSS PROGENIES OF DROSOPHILA.

Drosophila pseudoobscura and D. persimilis are two sibling species, morphologically similar, but distinguishable from each other on the basis of their habitats, chromosomal inversions, electrophoretic variants (based on allelic frequencies; see Lewontin, 1974, for review) and various morphological characters by statistical analysis. However, there is no unmistakable way to distinguish them on the basis of individual morphology. Males can usually be distinguished by the shape of the penis and the penis index, but there is some overlap (Rizki, 1951). Lancefield in 1929 showed that F1 males obtained from the cross of D. pseudoobscura females and D. persimilis males or from the reciprocal cross are sterile, while both classes of F1 females are fertile. Dobzhansky (1936) demonstrated that the viability of backcross progeny obtained from the cross of D. pseudoobscura and D. persimilis is very low. Prakash (1972) discovered that when D. pseudoobscura females from Bogota, Colombia, are crossed to males from the main contiguous part of the species distribution, the F1 males are sterile, whereas the reciprocal cross yield fertile male and female progeny. This phenomenon appears to represent a first stage in species divergence. Even though D. pseudoobscura and D. persimilis are morphologically similar, they may differ genetically in their developmental mechanisms for morphological characters. Muller (1949) has pointed out in his discussion on "cryptic genetic change" that phenotypic characters which appear similar to one another in related species may have a different genetic basis, and such close similarity may only constitute a "phenotypic facade." However, the extent of such a phenotypic facade may differ from one group of organisms to another and even from one character to another within the same group of organisms. Furthermore, the degree to which hidden genetic differences exist between two related species may be a measure of the degree of divergence between the species. The present experiment was planned to observe possible cryptic genetic differences underlying morphologically similar characters between D. pseudoobscura and D. persimilis. The experimental design measured any increase in phenotypic variance in interspecific backcross progeny compared to parental and F1 classes. Such an increase in variance can be attributed to two sources: the segregation of chromosomes and a breakdown of developmental stability in species crosses and backcrosses. The segregation of chromosomes would lead to more extreme phenotypes even if the gene action is basically additive on the phenotype; while the breakdown occurs when genes are combined that are not coadapted and thus do not promote developmental stability. It is important to note that there is genetic differentiation of some kind between the species of a significant sort since not only are the F1 males of species crosses sterile, but backcross progeny have extremely low viability. There is no difference in mean bristle number between left and right proximal and between left and right distal sex combs on the tarsal segments or in genital arch tooth number between the left and right sides. Differences between left and 1 Dedicated to the memory of Prof. Th. Dobzhansky.