Chromosomal Monomorphism in Brazilian and Argentine Populations of Drosophila Simulans and Drosophila Repleta.

ATURAL populations of Drosophila are generally composed of complex arrays of different gene arrangements, but show a remarkable uniformity regarding the outward appearance of the flies. Therefore, color polymorphism is rare and chromosome polymorphism is common in this genus. Only a few species are either polymorphic with respect to externally visible traits or monomorphic with respect to gene arrangements (cf. FREIRE-MAIA 1964 and STONE 1955 for reviews). Table 1 presents some data on the analysis made of the salivary gland chromosomes of samples of D. simulans and D. repleta obtained in Brazilian and Argentine domestic populations. The technique was the usual one (FREIRE-MAIA 1961). No chromosome rearrangement has been found in any one of the 540 strains of D. simulans and 56 of D. repleta which have been examined. This chromosome monomorphism is not the rule among the domestic species whose samples we have analysed ( FREIRE-MAIA, ZANARDINI and FREIRE-MAIA 1953; FREIRE-MAIA 1955, 1961; FREIRE-MAIA and FREIRE-MAIA 1964). In D. ananassae, D. pararepleta, D. melanogaster and D. immigrans, for instance, respectively 1.42, 1.12, 0.64 and 0.28 inversions per larva have been verified. It is interesting to observe that D. simulans and D. melanogaster are sibling species, but show very clear differences regarding adaptation to climate. At least in the populations we have studied, D. simulans is commoner in the colder regions (a type of “preference” also found in D. immigrans) , while D. melanogaster is more frequent in the warm areas. It deserves to be mentioned, however, that D. simulans seems to be just as successful in the cold regions (in spite of its monomorphism) as is D. melanogaster (with its 13 different paracentric inversions) in the warm ones.