Colonization of the Americas by Drosophila subobscura: lethal-gene allelism and association with chromosomal arrangements.

Drosophila subobscura is a Palearctic species that has recently colonized the Americas. It was first found in 1978 in Puerto Montt, Chile, and in 1982 in Port Townsend, WA. The colonization and rapid expansion of the species in western South and North America provides distinctive opportunities for investigating the process of evolution in action. The inversion polymorphism in the O chromosome from populations of central California and northern Washington, separated by 1300 km, corresponds to a previously observed latitudinal cline, also observed in Europe. Recessive lethal genes are not randomly distributed among the chromosomal arrangements. The incidence of lethal allelism is high, yielding unrealistically low estimates of the effective size of these populations (on the order of 1000 individuals). The high incidence of lethal allelism is likely to be a consequence of the low number of the American colonizers (on the order of 10-100 individuals), but the persistence of the allelism over several years suggests that some lethal-carrying chromosomes may be heterotic owing to shared associations between lethal and other genes.

A grand experiment in evolution: the Drosophila subobscura colonization of the Americas

Drosophila subobscura is a Palearctic species that has been extensively studied by population and evolutionary geneticists for nearly half a century. In 1978, it appeared in Puerto Montt, Chile; within a few years it extended over much of Chile and into Argentina and became the most common drosophilid in many places. In 1982, it appeared in the American northwest; shortly thereafter it was found extensively distributed from southern British Columbia, through Washington and Oregon, into southern California, west of Sierra Nevada. In North America also it has become a common drosophilid in many places. The source of the colonizers has been sought with four lines of research: sequence arrangement of the polytene chromosomes, allozyme polymorphisms, mitochondrial DNA restriction patterns, and frequency of lethal alleles. The origin of the colonizers remains uncertain, although all evidence indicates that both the North American and the South American colonizers derive from the same Palearctic population. The overall configuration of the chromosomal and allozyme frequencies suggests a western Mediterranean origin, which is consistent with the mtDNA data. The presence of a particular chromosome arrangement, O5, suggests a northern European origin. Lethal allelism has opened up the possibility of discovering the precise origin of the colonizers: all O5 chromosomes in the Americas carry a particular recessive lethal gene. There is strong evidence that the number of founders was not very small and not very large, perhaps between 10 individuals and several score. The chromosomal polymorphisms of D. subobscura exhibit well-defined latitudinal clines in the Old World. In the few years since the colonization, clines in every chromosome have evolved in the Americas that have identical latitudinal polarity with those in the Old World. This would seem strong evidence that the polymorphisms and the clines are adaptive.Key words: chromosomal polymorphism, mitochondrial DNA evolution, allozyme polymorphism, lethal allelism, adaptation, geographic clines.

GENE FLOW IN NATURAL POPULATIONS OF DROSOPHILA MELANOGASTER WITH SPECIAL REFERENCE TO LETHAL ALLELISM RATES AND PROTEIN VARIATION

ABSTRACT A simultaneous survey of 14 protein loci, together with frequencies and within- and between-population allelism rates of lethal chromosomes, was carried out in five (four Japanese and one Korean) natural populations and one cage population of Drosophila melanogaster. It was found that lethal allelism rates decrease rapidly as geographic distance between two populations increases, while variation at protein loci shows a remarkable similarity over all populations examined. These findings suggest that there are very high levels of gene flow in these natural populations and that selection at protein loci which can maintain substantial geographic variation, if present, is overshadowed by gene flow. There is no indication that invasion of D. melanogaster to the Far East occurred so recently that the frequencies of lethal chromosomes are still in nonequilibrium.

CLAN LETHALS AND INTER-YEAR ALLELISM IN DEATH VALLEY DROSOPHILA PSEUDOOBSCURA

ABSTRACT A chromosome 2 lethal allelism rate of about 3% was found in the 1974 population of D. pseudoobscura in Death Valley, California. This rate was significantly higher than allelism rates in other Southern California populations. The Death Valley population was sampled again in 1975 and 1977, with allelism rates of 1% and 0.5%, respectively. In 1974, several lethals were in high frequencies (about 1%), a pattern that reappeared in 1975 and 1977. However, none of the lethals in high frequency one year were in high frequency another year; the particular lethal alleles present in this ephemeral population appear to be due to their random presence in the flies which refound the population every winter. The results for the Death Valley population are compared with a Japanese population of D. melanogaster in which lethals in high frequency one year are also in high frequency in succeeding years and with earlier work on chromosome 3 of D. pseudoobscura, which showed a lower lethal frequency and higher allelism rate.