B. Spassky

Geography of the sibling species related to Drosophila willistoni, and of the semispecies of the Drosophila paulistorum complex.

The six sibling species of Drosophila willistoni group are among the most favorable, and at the same time challenging materials for studies on the genetics of speciation processes. The species are D. willistoni Sturtevant, D. paulistorum Dobzhansky & Pavan, D. pavlovskiana Kastritsis & Dobzhansky, D. equinoxialis Dobzhansky, D. tropicalis Burla & da Cunha, and D. insularis Dobzhansky. Burla et al. (1949) found slight morphological differences that were insufficient for identification of single individuals, but Spassky (1957) noted differences in the male genitalia which do permit such identification. Unambiguous discrimination is also possible through examination of the gene arrangements in the chromosomes of larval salivary glands (Burla et al., 1949; Dobzhansky et al., 1950), and of the variant enzymes detected by electrophoresis (Ayala et al., 1970). Owing to ethological isolation, crossmating of the species occurs rarely, although under laboratory conditions some cross-inseminations can be obtained (Burla et al., 1949). Intercrosses of D. insularis females with D. tropicalis and D. willistoni males, and occasionally with D. paulistorum

SEXUAL SELECTION, GEOTAXIS, AND CHROMOSOMAL POLYMORPHISM IN EXPERIMENTAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA

Hirsch and his students (Hirsch and Erlenmeyer-Kimling, 1961; ErlenmeyerKimling, et al., 1962; and other publications) have shown that Drosophila melanogaster responds readily to selection for positive and for negative geotactic behavior. So does D. pseudoobscura (Dobzhansky and Spassky, 1962). In both species, the geotactic behavior is under polygenic control. Also in both species, the genetic basis of the geotaxis displays the phenomenon called by Lerner (19 S4) genetic homeostasis; when the selection is relaxed, the selected strains relapse partly towards the original, preselection states. The loss of the selection gains is even more rapid if the direction of the selection is reversed. Using the apparatus devised by Hirsch (a classification maze), the geotactic behavior becomes a trait lending itself readily and conveniently to selection and other genetic experiments. After the exploratory experiments referred to above, Dobzhanskyand Spassky have initiated (in 1962) a study of the effects of selection for positive, or for negative, geotaxis in pairs of populations of very unequal size (one population 10 times larger than the other) , which in every generation exchange a fixed number of migrants. It is hoped that the diversifying (disruptive) selection, combined with migration between the experimental populations of Drosophila, may to some extent serve as a model of certain genetic processes which take place also in human populations having certain types of social structures. Be that as it may, these experiments have yielded some completely unanticipated adventitious results

Responses of Various Strains of Drosophila pseudoobscura and Drosophila persimilis to Light and to Gravity

Strains of Drosopbila pseudoobscura and D. persimilis vary in their reactions to light and to gravity. Some are positive, others neutral, and still others negative. D. persimilis is, on the average, more photopositive than D. pseudoobscura; but there is a considerable overlap. Males tend to be more photopositive than females, but there are again some exceptions. The modal behavior with respect to gravity is in both species geotactic neutrality.

Comparative genetics of Drosophila willistoni

Drosophila willistoni is one of the species which were used in the pioneer studies on comparative genetics of Drosophila. Lancefield and Metz (1922) and Ferry, Lancefield and Metz (1923) reported a number of mutants, published a linkage map of the X-chromosome, and suggested that only about half of the X of this species corresponds to the X-chromosome of D. melanogaster, while the other half of the X of D. willistoni carries genes which are autosomal in D. melanogaster. The work on D. willistoni was then abandoned. Sturtevant and Novitski (1941) re-analysed the published information and concluded that one of the two limbs of the X-chromosome of D. willistoni corresponds to the X, and the other limb to the left part of the third chromosome of D. melanogaster. The homologies of the other chromosomes remained obscure. The mutant strains of Metz and his collaborators are no longer in existence. Because D. willistoni is very favourable for investigations on the population genetics of tropical species, we initiated iii 1944 a new collection of mutants and the construction of linkage maps. The results are reported in the present article.

EVOLUTIONARY CHANGES IN LABORATORY CULTURES OF DROSOPHILA PSEUDOOBSCURA

Tt is commonplace that the evolutionary changes which organisms underwent during the immense time span of the earths history cannot be reproduced in laboratory experiments. However, too much stress has often been laid on this obvious limitation of the experimental method. Drosophila mutants described by classical geneticists certainly suggest degradation and decay rather than adaptation and evolution. Misled by this superficial appearance, some critics have questioned the validity of all interpretations of evolution based on inference from experimental data. Although such sceptical voices are now heard less and less often, it remains desirable to devise experiments which could serve as models of evolutionary changes that occur in natural populations. Mutations that arise in normal strains of species living in their habitual environments are deleterious, or at best neutral, to their carriers. This is expected on theoretical grounds, because almost any mutation that one can observe in experiments had arisen repeatedly in the natural populations of the species, and, consequently, had been exposed to the action

Comparative genetics of Drosophila prosaltans

THE architecture of the germ plasm undergoes changes in the process of evolution. The genes become altered by mutation, and their arrangement in the chromosomes is modified by structural change. Comparison of the genetic maps of chromosomes in related species is one of the methods of studying the evolution of the germ plasm. Species of Drosophila are favourable for such studies, and about ten of them have been investigated in more or less detail. It appears that the degree of divergence in systematic characters is not very strongly correlated either with the amount of gene differentiation, or with divergence in the gene arrangement. The information now available is, however, insufficient to give a clear idea about the mutual relationships of the various classes of changes that are observed. More data is obviously necessary. No species of the saltans group of the subgenus Sophophora has been studied. Since 1944, we have collected mutants and examined the linkage relationships in Drosophila prosaltans Duda, which is a member of this group. The results are reported in the present article.