Myriam Budnik

Quantitative analysis of genetic differentiation among European and Chilean strains of Drosophila subobscura.

An analysis of the interpopulational differences observed between four European and four Chilean stocks of Drosophila subobscura, with regard to wing size, egg-to-adult development time and preadult viability, shows highly significant differences between the eight stocks irrespective of the continent from which the flies are derived. The divergence of the local populations with regard to the quantitative traits described here, supplements other evidence of interpopulational differences in mating activity, chromosomal and enzyme polymorphisms. These together indicate that the evolutionary process that promotes race formation is relatively rapid, bearing in mind that D. subobscura was detected for the first time in South America in 1978.

Preadult competition between colonizing populations of Drosophila subobscura and established populations of Drosophila simulans in Chile

SummaryThe palearctic species Drosophila subobscura was detected for the first time in Chile and probably in the Americas in 1978. At present, its distribution area extends between 29°54′S and 53°40′S, where it has become abundant in both domestic and wild environments, depending on the latitude and the season of the year (appearing at maximum frequency in early spring). The seasonal fluctuations of D. subobscura are the inverse of those of D. simulans, which shares the same habitat in certain regions. Under laboratory conditions D. subobscura proved to be a poor competitor during the larval developmental period in relation to D. simulans, which on the contrary seemed to be “facilitated” by the larvae of the other species. However, the existence of D. subobscura in Chile is most probably due to its population flush at a time when D. simulans is at low density.

Phylogeny of the Drosophila mesophragmatica Group (Diptera, Drosophilidae): An Example of Andean Evolution

Abstract The mesophragmatica group of Drosophila belongs to the virilis-repleta radiation of the Drosophila subgenus. This group comprises 13 Neotropical species that are endemic to the South-American continent and seem to be fundamentally Andean in their distribution. The mesophragmatica-group phylogeny has been inferred previously by other authors based on morphological, cytological, and isozyme analyses. However, the relationships within the group have not yet been completely resolved, although its monophyletic origin has already been confirmed by molecular data. This work attempts to enhance the molecular approach to the relationships among the species of the mesophragmatica group, using both nuclear and mitochondrial markers. Phylogenetic analyses were performed using fragments of the nuclear alcohol dehydrogenase (Adh; 631 bp), alpha-methyldopa (Amd; 1211 bp), dopa-decarboxylase (Ddc; 1105 bp), and hunchback (Hb; 687 bp) genes and the mitochondrial cytochrome oxidase subunit II (COII; 672 bp) gene, and included a total of 4306 bp. The sequences obtained for eight representatives of the mesophragmatica group were analyzed both individually and in combination by distance methods, maximum parsimony, and maximum likelihood. Our results support subdivision of the mesophragmatica group into three main lineages: the first is composed of D. viracochi; the second comprises a clade grouping the sibling species D. pavani and D. gaucha; and the third encompasses D. gasici, D. brncici, and D. mesophragmatica. The best supported scenario suggests that D. viracochi is an early offshoot in the mesophragmatica group, with this and other early branchings occuring in the Pliocene/Pleistocene Epochs, possibly associated with Andean glacial refuges. Also based on the phylogenies obtained, we present a genealogical view of the evolution of previously described characters within the group.

Response ofDrosophila pavani, Drosophila gaucha and their hybrids to larval biotic residues

Los productos de desecho larval de las especies neotropicalesDrosophila pavani, D. gaucha y sus hibridos estériles, inhiben el desarrollo de las larvas deD. pavani y los hibridos. El desarrollo deD. gaucha es afectado solo por sus propios residuos metabólicos. La viabilidad más baja se obtuvo cuando los preadultos se crian en medios alimenticios contaminados por sus propios desechos metabólicos.

Experiments on sexual isolation between Chilean and European strains ofDrosophila subobscura

Males and females of Chilean strains ofDrosophila subobscura exhibit a pronounced tendency toward homogamic mating. This tendency shows a clear relation to the distance between the geographic localities from which the respective strains came. Nevertheless, when the Chilean flies are confronted with European strains, the ethological isolation is observed in some cases but not in others, depending on the geographic origin of the strains.

THE EFFECTS OF CROWDING ON CHROMOSOMAL POLYMORPHISM OF DROSOPHILA PAVANI

There are a number of examples in the literature which show that the adaptive values of certain genotypes depend on the population density. Evidence of this type of interaction is derived chiefly from experiments in which individuals of different genetic constitutions or polymorphic populations were maintained under crowded conditions with a limited amount of food and space (rev. in Bakker, 1961; Ayala, 1970). In the Drosophila genus, TimofeeffRessovsky (1934), Dobzhansky and Spassky (1944), Lewontin (1955), Birch (1955), and Bakker (1961, 1969), among others, demonstrated differential survival of certain genotypes under different conditions of crowding during larval development and adult life. Similar situations have been observed in other insects such as Musca domestica (Sokal and Sullivan, 1963; Sullivan and Sokal, 1963; Bhalla, 1964; Bhalla and Sokal, 1964), TriboUum castaneum (Sokal and Huber, 1962), Dacus tryoni (Birch, 1961), and Copepods (Battaglia, 1958). The ability to survive and reproduce under crowded conditions represents an important adaptive character; and, for this reason, the present study of the relationships between survival under high densities and chromosomal polymorphism in the Neotropical species Drosophila pavani was undertaken. Natural populations of D. pavani are polymorphic for gene arrangements in the chromosomes due to the presence of inversions (Brncic, 1957). In the second

Colonization of Drosophila Subobscura in Chile: Experimental Preadult Competition With Local Species Under Bi- and Tri-Specific Conditions

Preadult competition of Drosophila subobscura, D. simulans and D. pavani under mono, bi-and tri-specific conditions, was studied experimentally. The parameters analysed were egg-to-adult development time and preadult viability. The results showed that 1) the competitive ability of each species is modified when in bi- or tri-specific condition, 2) the outcome of competition in a 3 species system cannot be predicted from the pairwise interactions between species, 3) D. subobscura is a bad competitor. We discuss these findings in terms of the colonization of Chile by D. subobscura.

EFFECTS OF LARVAL BIOTIC RESIDUES ON CHROMOSOMAL POLYMORPHISM OF DROSOPHILA PAVANI

There are many observations which suggest that in the Drosophila genus individuals carrying diverse genetic arrangements in their chromosomes display a differential capacity to survive under crowded conditions (Birch, 1955; Lewontin, 1955; Spiess, 1958; Ayala, 1970; Budnik et al., 1971; Parsons, 1973). In the Chilean endemic species D. pavani, Budnik et al. (1971, 1972) have demonstrated that among the flies which developed under crowded conditions there are significantly higher frequencies of heterokaryotypes in respect to homokaryotypes for the inversions found in the right and left arms of the fourth chromosome. As there is a decrease of the egg-to-adult survival and a lengthening of the developmental period under these circumstances, it was inferred that the superiority of the heterokaryotypes was related to a better competitive ability. During the preadult stages, species or populations can experience two aspects of competition: (1) exploitation, that refers to the use by competing individuals of resources in short supply, mainly food and space, and (2) interference, that occurs if the resource is exploited less efficiently when two species or genotypes compete than when only one is present. In a series of experiments, Budnik and Brncic (1974, 1975) have demonstrated that the decrease in egg-to-adult viability of D. pavani under crowding conditions cannot be attribhited to food and space restrictions only. There are strong indications that the viability of the species is deeply affected also by the biotic waste products of the growing larvae. In view of the above fact, it was considered of interest to study the effect of the accumulation of the larval metabolic wastes on the survival and development of the individuals carrying different gene arrangements in the fourth chromosome of D. pavani.