O. Olvera

Population Genetics of Mexican Drosophila. V. A High Rate of Multiple Insemination in a Natural Population of Drosophila pseudoobscura

The frequency of multiple insemination was studied in female D. pseudoobscura from a Mexican population. This population was highly polymorphic for third chromosome gene arrangements, and simple inspection of the progeny of single females showed that at least 43% of them carried the sperm of two or more males. The number of distinct gene arrangements in the progeny of single females was used to estimate the total frequency of multiple inseminations according to a probabilistic model. The frequency of double insemination was estimated to lie between 60% and 100%, while triple and quadruple inseminations were estimated to be quite rare. Double inseminations were clearly the rule, and not the exception, in the population we studied. We discuss the possible adaptive role of multiple insemination in Drosophila populations and the implications that frequent multiple insemination has for interpreting ecological genetic studies with Drosophila.

POPULATION GENETICS OF MEXICAN DROSOPHILA VI. CYTOGENETIC ASPECTS OF THE INVERSION POLYMORPHISM IN DROSOPHILA PSEUDOOBSCURA

Natural populations of Drosophila are often polymorphic for inversions of chromosomal segments. Recombination is effectively suppressed over much of those chromosomes which are heterozygous for inversions. The inversions thus serve to organize genic variability into linked blocks whose effects on the viability and fertility of the flies may be profound. Natural selection has been shown to operate on many such chromosomal polymorphisms, and the selection is sometimes intense. Much of what we know about the population genetics of inversions comes from studies of Drosophila pseudoobscura. Some 40 years ago, Dobzhansky and Sturtevant (1938) showed that the sequence of bands seen in salivary chromosomes was variable in populations of this species, and that these different gene arrangements were the results of inversions. They pointed out that inversions carry more historical information than gene mutations, because the events which give rise to any specific inversion have such a low probability that the origin of the same inversion more than once is vanishingly small.

Evidence for the protective effect of ascorbic acid (vitamin C) in treatment with γ-rays and chromium (VI) oxide (CrO3) in somatic cells of Drosophila

Abstract Evidence is provided that ascorbic acid (vitamin C), when used as a pretreatment, protects against mutation/recombination induced by γ-rays and chromium (VI) oxide in mwh+/+flr3 larvae in the wing spot test in Drosophila

Chromosomal and Behavioral Studies of Mexican Drosophila. V. Frequencies of Multiple Insemination in Three Natural Populations

The detection and measurement of multiple insemination i natural populations contributes directly to our understanding ofmating systems and population structure. Multiply inseminated females in natural populations of Drosophila pseudoobscura have been found in a number of investigations, including those that have studied third-chromosome polymorphisms (Anderson 1974; Levine et al. 1980). The presence of sperm from two or more males in the reproductive tract of a female permits competition between the sperm, allowing selective fertilization to contribute to a males fitness. Selection experiments for reduced time between successive matings in D. melanogaster have been successfully conducted by Pyle and Gromko (1981), indicating the involvement of genetic f.actors in remating tendencies. As with any other genetically influenced trait, it is of interest o investigate whether the tendency of females in natural populations to mate with more than one male varies over space or time. At the spatial level, interpopulation differences in this tendency may reflect adaptations to different ecologies. Over time, such differences might represent behavioral reactions to some changing environmental variable. However, a constancy of remating frequency, especially if found in both spatial and temporal studies, would strongly suggest he existence of longterm evolutionary pressure toward some optimum level of multiple insemination, as it relates to the reproductive physiology of Drosophila. For these purposes, accurate estimates of the level of multiple insemination are desirable.

CHROMOSOMAL VARIATION IN NATURAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA INHABITING NORTHERN MEXICO

Abstract Samples of 8 natural populations of Drosophila pseudoobscura inhabiting northern Mexico were cytogenetically analyzed. Twelve distinct chromosomal arrangements of the third chromosome were identified. Number of inversions at each locality varied from 6 to 8. A total of 468 third chromosomes were examined in the area of study, and relative frequencies of inversions in each population were calculated. The most common inversions in decreasing order of relative frequency were: TL, SC, OL, and CU; the other 8 inversions had frequencies <10%. Each locality had 3–4 inversions that accounted for ≤80% of the total. All populations were heterogeneous. Relative frequency of each inversion did not form a gradient along the transect studied. Unexpected was the presence of the ST inversion, as well as the incidence, in some populations, at a moderate frequency, of the inversion OL. Another exception was the increase in frequency of the EP inversion compared with previous collections in the area. More information from the region is needed.

A Temporal Study of Desiccation Resistance of Sibling Drosophila Species from Laguna Verde, Veracruz, Mexico

-The sibling species, Drosophila melanogaster and D. simulans, were collected periodically from Laguna Verde, Veracruz, Mexico, for 3 yr and tested for desiccation resistance. The number of survivors was recorded after the flies had been kept for 4 h, 8 h and 12 h, at 0% RH, and 20 C. Significant temporal changes in relative survival were found as follows: after the 4 h exposure period for D. simulans and after both 8 h and 12 h for D. melanogaster, females from the first three collections were more resistant to desiccation than males but not from the last two. Comparisons of the species by sex, showed that, except for the 4 h exposure period, D. melanogaster was significantly more resistant to desiccation than D. simulans. There was no indication of seasonal variation in desiccation resistance of males or females of either species at any of the three experimental time periods. However, in the collection of July 1987, some of the flies showed significantly less resistance to desiccation than those of the same sex and species from other collections, as follows: at 4 h, D. simulans males; at 8 h, D. simulans males and females, and D. melanogaster males; at 12 h, D. simulans females and D. melanogaster males and females. These findings are discussed with relation to changes in the relative frequencies of the two species, climatological conditions of the area, and the start-up testing period of a nearby nuclear power plant.

Chromosomal and behavioral studies of MexicanDrosophila. I. Vagility characteristics of three populations ofD. pseudoobscura

Twelve isofemale lines from each of three MexicanDrosophila pseudoobscura populations, known to differ in their degrees of inversion polymorphism, were tested for vagility in the Sakai system. It was found that the flies from the most chromosomally polymorphic population (Zirahuen) had both a significantly lower vagility score and a higher level of expressed genotypic variation for vagility than the flies from the lesser chromosomally polymorphic populations (Amecameca and Tulancingo). The latter two populations could not be distinguished for either parameter of this behavioral trait. The possible role of inversion polymorphism, especially as related to the number of heterozygote combinations formed, in affecting vagility is discussed.

Antimutagenesis in Somatic Cells of Drosophila as Monitored in the Wing Spot Test

Tests of the antimutagenic effect of sodium copper chlorophyllin (SCC) in the wing spot test in Drosophila are reviewed. The mutagens include gamma rays (γ-rays), chromium (VI) oxide (CrO3), ethylnitrosourea (ENU), cyclophosphamide (CPA), and methyl methanesulfonate (MMS). SCC gives evidence of being antimutagenic against the DNA-damaging effect of all these agents with the exception of CPA. Results of tests with ascorbic acid (a.a.) in combination with γ-rays and CrO3 proved positive.

Chromosomal and Behavioral Studies of Mexican Drosophila. VI. Effects of Density on Larval Digging and Pupation in Drosophila pseudoobscura

Ten isofemale lines of Drosophila pseudoobscura from each of three collecting sites in Mexico were tested at three different densities for porportion of larvae pupating, pupation height, and digging depth. There were no significant differences among the populations for any of the characteristics tested. There were highly significant effects of density on proportion of larvae pupating and pupation height but not on digging depth. These results are discussed with reference to differences in the level of third chromosome inversion polymorphism of these populations and the previously tested vagility behavior.

Species Abundance, Viability, Desiccation Resistance and Dispersal of Sibling Drosophila Species from Laguna Verde, Veracruz, Mexico

-The sibling species, Drosophila melanogaster and D. simulans, were collected from a site at Laguna Verde, Veracruz, Mexico, in January 1987. Drosophila simulans was found in significantly greater abundance than D. melanogaster. Experiments with isofemale lines of each species indicated that there was no significant difference in egg-to-adult viability or in dispersal scores between the species. However, D. melanogaster surpassed D. simulans in desiccation resistance. These findings are discussed with reference to those obtained from flies collected in July 1986, at a site 1320 m away.