Max Levitan

Climate change and recent genetic flux in populations of Drosophila robusta

BackgroundStudied since the early 1940s, chromosomal polymorphisms in the deciduous woods species Drosophila robusta have been characterized by well-defined latitudinal, longitudinal, and elevational clines, but – until at least ten years ago – stable, local population frequencies. Recent biogeographical analyses indicate that D. robusta invaded North America from southeast Asia and has persisted in eastern temperate forests for at least 20–25 my without speciating. The abundant chromosome polymorphisms found across the range of D. robusta are thus likely to be relatively ancient, having accumulated over many well known climatic cycles in North America. Sufficient long-term data are now available such that we can now gauge the rate of these evolutionary changes in natural populations due to environmental change.ResultsRecent local collections have revealed significant changes in the frequencies of several chromosomal forms. New data presented here extend the range of these changes to six states, three in the northeastern United States and three west of the Mississippi River. These data reinforce recent directional changes in which the frequencies of three gene arrangements have reached percentage levels typical of distant southern populations consistent with regional climatic changes. Another gene arrangement has been steadily decreasing in frequency at a number of the sites studied. Meteorological records from 1945 to 2003 indicate temperature increases at all study sites, particularly average minimum air temperatures.ConclusionsObservation of parallel genetic flux suggests that these long-term temporal frequency shifts in widely disparate populations of D. robusta are evolutionary responses to environmental change. Since these chromosomes are known to be sensitive to ambient temperature, regional climatic shifts associated with global warming are likely to be responsible.

STUDIES OF LINKAGE IN POPULATIONS. I. ASSOCIATIONS OF SECOND CHROMOSOME INVERSIONS IN DROSOPHILA ROBUSTA

Despite recent progress in the development of population genetics in relation to evolution, studies of actual populations continue to lag far behind the theoretical work. This is particularly evident in problems concerning linkage. Mathematical studies of the disposition of linked loci under randon mating appeared early in the history of modern genetics (Weinberg, 1909, and Robbins, 1918), but few data bearing on their predictions have appeared in the literature. The present paper reports a study of linked gene arrangements in natural populations of Drosophila robusta Sturtevant. Certain of the arrangements are found to be present in non-random combinations. The deviations from independence apparently depend on selection which differentiates not only between different combinations of the arrangements but also between similar combinations in which the components occupy different positions in the cell. If this interpretation of the data is correct, these are the first position effects to be noted in natural populations and the data point to a new method for discovering the phenomenon.

STUDIES OF LINKAGE IN POPULATIONS. XIV. HISTORICAL CHANGES IN FREQUENCIES OF GENE ARRANGEMENTS AND ARRANGEMENT COMBINATIONS IN NATURAL POPULATIONS OF DROSOPHILA ROBUSTA

Abstract Data are reported showing large directional changes in the frequencies of some gene arrangements and arrangement combinations in certain natural populations of Drosophila robusta in the eastern United States. The changes involve the same X‐chromosomal inversion differences in two of the three localities studied and similar autosomal inversions in all three. These genetic changes provide a rare opportunity to observe evolutionary forces at work in nature. They are interpreted as being due to natural selection.

Rapid response to perturbation of chromosome frequencies in natural populations of Drosophila robusta

Perturbation of gene or chromosome frequencies in natural populations is one of the most powerful ways of demonstrating whether natural selection maintains genetic polymorphism or if other evolutionary forces are at work. Gene arrangement frequencies in two natural populations of Drosophila robusta were perturbed multiple times by releasing adult flies with contrasting karyotypes and carefully monitoring post-perturbation presence of hybrids and chromosome frequencies. In all cases, frequencies quickly returned to pre-perturbation levels, and in the following sampling periods, no evidence of the introduced chromosomes was apparent. Analysis of post-perturbation frequency changes included tests for heterogeneity among chromosome arrangements in rates of return to equilibrium values using population admixture analysis. In several cases, significant heterogeneity was detected indicating some form of natural selection was operating. Technical challenges to carrying out perturbation experiments in the wild are also discussed.

Chromosomal breaks with attachments to the nucleolus

Among over 2,000 chromosomal aberrations produced by a cytoplasmic chromosome-breakage factor in Drosophila robusta, 28 have been unique in that the broken chromosome has become attached to the nucleolus. These have taken three forms according to the origin of the broken pieces. Particularly interesting are three aberrations of form 3 in which the chromosome attached to the nucleolus is an interstitial deletion. The data suggest that the nucleolus contains material with chromosome-like properties, particularly in possessing a centromere.