Homeostatic drive counteracting selection for positive and negative phototaxis and geotaxis in Drosophila pseudoobscura.

Tested in Hirsch-Hadler mazes, Drosophila pseudoobscura is on the average geoneutral and photoneutral. Strongly geo- and photopositive and geo- and photonegative populations can be obtained by artificial selection, but upon relaxation of the selection they tend to relapse toward neutrality. This genetic homeostasis is due to natural selection favoring neutrality. Experiments are described in which artificial selection for positivity and for negativity was deliberately made so weak that it only counterbalanced the nautral selection or "homeostatic drive" and the effects of cross-migration. Under these conditions, the behavior of the population artificially selected for positivity diverges only slightly from that of the population selected for negativity, but, at least in females, both populations move closer to neutrality.

Genetics of natural populations, XLIII. Further studies on rates of dispersal of Drosophila pseudoobscura and its relatives.

The amount of gene flow among local populations of a species is determined by the dispersal capacity of that species. Population samples of Drosophila pseudoobscura, D. persimilis, D. azteca, and D. miranda were collected, marked with ultraviolet fluorescent dusts, and released as soon as possible after capture. One and two days after release, recaptures were made on baits placed at 40-meter intervals in straight lines intersecting the release point. On alternative days, the baits were placed in North-South or in East-West directions. The distribution of the recaptured flies about the release point is very nearly normal. No significant differences between the dispersal rates of the four species are observed; however, males disperse slightly further than females. The variances averaged 50,822 m2 on the first day and 80,048 m2 on the second day and the estimated mean distances from the release point averaged 263 m and 361 m respectively. The genetic implications of the results are discussed.

Distribution among the chromosomes of Drosophila pseudoobscura of the genes governing the response to light.

Most strains of Drosophila pseudoobscura are neutral to light when tested in phototactic mazes. However, clear-cut photopositive and photogenative populations are obtained by selection over a series of generations. The genetic nature of the differences between the positive and negative populations has been studied in crosses in which the three large autosomes carried mutant markers. All chromosomes contain genes which influence the response to light. The third chromosome has the strongest effect, followed by the second, the X, and the fourth chromosomes. This seriation is not in proportion to the relative lengths of the chromosomes. Either the effective genes are not very numerous, or some of them exert stronger influences than others.

Analysis of incipient reproductive isolation within a species of Drosophila.

Drosophila willistoni and its subspecies quechua are morphologically indistinguishable, but differ in relative frequencies of certain allozyme genes. The cross quechua female X willistoni male produces nearly or completely sterile males, while the reciprocal cross gives fertile males. Hybrid females are fertile. Analysis of the sterility with the aid of chromosomes marked with mutant genes shows that the second and X-chromosomes have major and third chromosomes minor effects. Backcross males of the same chromosomal constitution may be either fertile or sterile; the threshold effect may be due to environmental variations or to gene variants present in the strains crossed. Only a trace of ethological isolation between the subspecies is present. In contrast to a narrowly localized and geographically isolated subspecies bogotana of D. pseudoobscura, the subspecies quechua of D. willistoni is fairly widespread, although its geographic area is not accurately known. Neither with bogotana nor with quechua are there valid reasons to suppose that these subspecies are of very recent origin.

Population genetics of Mexican Drosophila. I. Chromosomal variation in natural populations of Drosophila pseudoobscura from Central Mexico.

Drosophila pseudoobscura populations of Central Mexico are chromosomally highly polymorphic. Five gene arangements in the third chromosome are endemic, including the two newly described in the present article. The phylogenetic tree of the gene arrangements known in the species is shown in Figure 1. The ones found in Central Mexico all belong to the Santa Cruz "phylad", while in the northern part of the species area both Santa Cruz and Standard phylads are widespread. Some inferences concerning the evolutionary history of the species are presented.

Behavior in different environments of populations of Drosophila, pseudoobscura selected for phototaxis and geotaxis.

When tested in Hirsch-Hadler mazes, Drosophila pseudoobscura descended from natural populations is phototactically and geotactically neutral on the average. Decidedly positive and negative populations have been obtained by artificial selection. The behavior is independent of temperature (15-27 degrees ) and age (2-17 days). The flies perceive light as weak as 0.2 lux; from 7 to 3000 lux the behavior difference between positive and negative strains increases little, and perhaps decreases at 8000 lux. A remarkable transformation occurs with red light; the population that is negatively phototactic with shorter wave lengths or with white light becomes photopositive.

Ethological isolation among three species of the planitibia subgroup of Hawaiian Drosophila.

Drosophila heteroneura and D. silvestris are sympatric species living on the island of Hawaii, while D. planitibia is allopatric on the nearby island of Maui. A pronounced ethological isolation is found between the sympatric species, and none between allopatric ones, except that D. planitibia females discriminate against D. heteroneura males. Male hybrids are sterile in allopatric crosses but fertile in sympatric ones. The ethological isolation and the hybrid sterility are uncorrelated. This is expected if premating isolation between closely related species is an ad hoc product of natural selection, while postmating isolation is an incidental result of genetic divergence. Some exceptions to the rule are discussed.

Temporal frequency changes of enzyme and chromosomal polymorphisms in natural populations of Drosophila.

A comparative study of chromosomal and enzyme polymorphisms in Drosophila pseudoobscura and in D. persimilis was conducted from April 1971 to October 1972 by taking samples at monthly intervals in two different localities. Seasonal cyclic changes occur in the chromosomal polymorphisms. Significant changes occur in the frequencies of the enzyme polymorphisms; preliminary evidence indicates that some of these changes may also be cyclic. Natural selection is the main process responsible for the changes in both kinds of polymorphisms.