A Comparative Study of Chromosomal Variation in Species of the Mesophragmatica Group of Drosophila.

HE adaptive importance of chromosomal polymorphism in Drosophila has T been demonstrated through analysis of natural populations as well as through experiments in the laboratory (c j . DOBZHANSKY 1951; PATTERSON and STONE 1952; DA CUNHA 1955). However, in different species the genetic variability takes different forms, and chromosomal inversions are characteristic of the adaptive polymorphism in some species of Drosophila but not in others. Even within the groups of species which exhibit variatlons in gene order, some of them have more inversions than others. A comparative study of closely related members included in various species groups of Drosophila has permitted DOBZHANSKY et al. (1950) and DA CUNHA et al. (1953) to establish very interesting relationships between the degree of chromosomal polymorphism and the capacity of a species to exploit the ecological opportunities offered by the environment The greater the ecological versatility of a population or a species, the greater the chr’omosomal polymorphism, and vice versa. Nevertheless some very widely distributed and well adapted species exhibit only few inversions or they have none at all (cf. PATTERSON and STONE 1952). This is obviously due to the fact that each species has its own means of adjusting itself to the environment. Variations in gene arrangements are not the only index for the adaptive capacity of a species. Each species has its own genetic system which not always can be compared to those exhibited by others. For an understanding of the origin of such contrasting types of chromosomal adaptive mechanisms, one might logically study the chromosomal structure of different closely related species of which something is known about their evolutionary relationships. In the present paper a preliminary report of the contrasting types of chromosomal variation exhibited by six members included in the mesophragmatica group of species of Drosophila is given.