Susi Koref-Santibanez

Effects of age and experience on mating activity in the sibling species Drosophila pavani and Drosophila gaucha.

The effects of age and experience on sexual activity and on intra- and interspecific discrimination were studied in two sibling species of the mesophragmatica group, Drosophila pavani and Drosophila gaucha. Sexual activity of a total of 2970 individual couples of the same or of both species was observed at two ages: 10 days, (“young inexperienced”) and 18-20 days (“old,” either “inexperienced” or “experienced,” if either the male or the female had copulated previously). In the 1186 (39.97%) pairs that mated, the latency to copula and duration of copula were registered. Age has a different effect in both species: “young” Drosophila pavani and “old” Drosophila gaucha females are less receptive to males of either species of the corresponding age. The receptivity of females is also reflected in heterospecific matings, as Drosophila gaucha males increase their mating activity with age. In both species, female receptivity decreases with experience, whereas mating activity of males increases with experience, especially that of Drosophila gaucha toward heterospecific females. Drosophila pavani females take longer to mate than those of Drosophila gaucha. In both species “old” males tend to mate faster, whereas experience increases the latency to mating in females and decreases it in males. Both species differ significantly in the duration of copula. It is longer in Drosophila pavani than in Drosophila gaucha and is determined mainly by the male. The duration of copula increases with age, especially in Drosophila pavani females, whereas it is reduced in males of the same species.

COURTSHIP BEHAVIOR IN THE SEMISPECIES OF THE SUPERSPECIES DROSOPHILA PAULISTORUM

Drosophila paulistorum, a member of the willistoni group of the subgenus Sophophora occurs in the American tropics from Guatemala and Trinidad to southern Brazil. Strains of this species can be divided into six incipient species or semispecies: Centroamerican, Orinocan, Amazonian, Transitional, Interior, and Andean-Brazilian (Dobzhansky and Spassky, 1959; Dobzhansky et al., 1969; Spassky, et al., 1971). The semi-species show strong sexual (ethological) isolation from one another, and when crosses between them do occur, male hybrids are completely sterile (Ehrman, 1960, 1962; Dobzhansky et al., 1969). The semispecies differ in their geographic distribution (Spassky et al., 1971), in the gene arrangements in their chromosomes (Kastritsis, 1967, 1969), in statistical averages of some morphological traits (Pasteur, 1970), and in allozyme frequencies (Richmond, 1971). Each semispecies has a distinct geographic distribution, but in some places two or more overlap; where they do, they apparently never cross. Ethological isolation is the primary mechanism that keeps the semispecies genetically separate in nature (Ehrman, 1961). Sympatric strains of two semispecies show greater isolation coefficients than do allopatric strains of the same semispecies. One of the important components of ethological isolation must be courtship behavior. A study of the courtship behavior of the six semispecies of the Drosophila paulistorum complex was undertake in order to elucidate some of the mechanisms resulting in sexual isolation.

COURTSHIP AND SEXUAL ISOLATION IN DROSOPHILA PAVANI BRNCIC AND DROSOPHILA GAUCHA JAEGER AND SALZANO

Within populations of a species, reproductive isolating mechanisms, including sexual isolation, may arise and be favored by natural selection if the populations have diverged sufficiently to make compromise genotypes unfavorable (Dobzhansky, 1944). Reproductive isolation, which may first be expressed at the sensorial level, may precede morphological differentiation (Spieth, 1952), and may manifest itself already at the racial or subspecific level. In an effort to determine the mechanisms of the establishment of reproductive isolation, several authors have studied incipient sexual isolation within species (Rendel, 1944; Dobzhansky, 1945; Patterson, Wharton and Stone, 1947; Smith, 1958; Koref-Santibafiez and Waddington, 1958; Hoenigsberg and Koref-Santibafiez, 1960, and others). In general, it has been shown that the rudiments of isolation within a species are genetically the same as those between species. Courtship is the ritual which precedes mating, and the components of this ritual are so specific that they may be used as taxonomic characteristics (Spieth, 1952). Differences between groups which do not yet manifest themselves at the copulation level tend to appear as differential utilization of certain courtship elements, which may be explained by different stimuli thresholds on the part of either the male or the female (Hoenigsberg and Koref-Santibafiez, 1959 a and b, 1960). Most of these studies were made on laboratory populations of cosmo-

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

Geographical variation of chromosomal structure in Drosophila gasici

SummaryDrosophila gasiciBrncic 1957, is a neotropical species found in several parts of the Andes Mountain System. By means of the analysis of their external characteristics, chromosomes and hybridization test they have been included in the mesophragmatica group of species of the sub-genus Drosophila.The present paper describes the mitotic and polytene chromosomes of D. gasici from samples of natural populations collected at Bogotá (Colombia), Cochabamba (Bolivia), Arica (Chile) and San Luis (Argentina). The comparative study of all these populations has disclosed that the species has split in well defined geographic races. The Colombian and Chilean flies differ from those living in Bolivia and Argentina by three independent inversions in chromosome I (the sexual pair). The only polymorphic populations seem to be the Chilean ones which exhibit two inversions in the second chromosome, besides the Standard gene arrangement. All the other populations are homozygous for all their chromosomal sequences. Studies on reproductive isolation have demonstrated that there is some sexual discrimination between the Colombian and Chilean flies in respect to the Bolivian and Argentinean ones.The populational structure of D. gasici is in contrast to that observed in the other six species belonging to the mesophragmatica group in which there are no good evidences of geographical variations at the chromosomal level.