Hampton L. Carson

Niche Separation in a Pair of Homosequential Drosophila Species from the Island of Hawaii

Two exceedingly closely related species of Drosophila coexist in a sparsely vegetated xeric area on the island of Hawaii. Of the two major trees in the area, Myoporum sandwicense alone appears to support the existence of both species. Whereas both feed as adults on Myoporum flux, D. silvarentis oviposits only on fluxes which wet the trunk well above the ground surface. Drosophila heedi larvae are found exclusively in caked soil moistened by flux dripping from above. Implications of this type of niche separation for the evolution of sibling or near-sibling species are discussed.

A Widespread Chromosomal Polymorphism in a Widespread Species, Drosophila buzzatii

1) New collections of D. buzzatii are reported from Australia, East Africa and Israel. In each instance, the species has been captured only near plants of the cactus genus Opuntia. In Australia, the species has been shown to breed on the fruits of the giant Opuntia, O. Ficus-indica. 2) In each new population, including one very old laboratory strain from Trapani, Italy, a single chromosomal polymorphism is present in chromosome 2; this is due to the coexistence of the standard chromosome 2 and the inversion 2j. 3) These data support the previous suggestion that D. buzzatii originated in Argentina where populations show polymorphism for two inversions in addition to 2j. 4) In its spread over five continents, D. buzzatii appears to have remained specifically associated with Opuntia; thus it appears to have retained niche-specificity. The spread seems to have involved a subspecies polymorphic for the 2j inversion. The factors which may operate in the maintenance of this ubiquitous polymorphism are discussed.

SPATIAL DISTRIBUTION AND DISPERSAL IN A POPULATION OF DROSOPHILA

Spatial distribution in a Drosophila engyochracea population in Kipuka Ki (island of Hawaii) is nonuniform. Density of flies was highest close to a known breeding site, and sex ratio is also different in the two sides of the road that crosses Kipuka Ki. Females are more attracted to banana baits than males. Flies disperse easily all over the experimental area. The road acts as a small barrier to dispersal, especially for males. Data suggest that females move more freely than males, but the evidence is poor. Nevertheless, it is safe to say at least that males do not disperse farther and easier than females. Migration rates and population sizes have been estimated using a modified Lincoln-Pearson mathematical model and allowing several restrictive conditions. The study area was calculated to have an actual population size (N) of approximately of 40,000, or about 5.4 flies per square meter. The spatial distribution and dispersal of D. engyochracea have important implications in population parameters. Namely, effective population size (Ne) is not much influenced by difference in sex numbers, but it is presumably influenced by the unequal distribution of sexes in the population area.

Parallel Polymorphisms in Different Species of Hawaiian Drosophila

Six very closely related species of the Drosophila grimshawi subgroup on the islands of Maui, Molokai, Lanai, and Hawaii show polymorphism in chromosome 4. In four of these species, it has been shown that very similar but not identical chromosome inversions have occurred and have been perpetuated independently as polymorphisms within the different species. These appear to be cases of parallel evolution at the microevolutionary level. A possible model for their mode of origin has been advanced.

Rare Parthenogenesis in Drosophila robusta

1. By screening large quantities of unfertilized eggs laid by unmated females in the laboratory, 14 parthenogenetically-produced individuals of Drosophila robusta were found. All were females. It is suggested that they arose through automictic fusion of haploid products of meiosis in the unfertilized egg. 2. Each of the initial seven females came from a different wild strain, five from Nebraska and two from Missouri. The rate of parthenogenesis in these strains was .93 adult females per million unfertilized eggs laid. 3. Virgin females with one or more parthenogenetic females in their ancestry show a rate of parthenogenesis about 21/2 times that of the original strains. 4. A majority of the parthenogenetic females show reduced fertility, complete sterility and/or morphological abnormalities. 5. The evidence indicates that rare parthenogenesis such as is found in D. robusta, D. parthenogenetica, and D. polymorpha is genetically based and may serve as an example of an evolutionary stage through which the obligatory parthenogenetic species, D. mangabeirai, may have passed during the evolution of its parthenogenesis.

Development of Balanced Polymorphism in Laboratory Populations of Drosophila melanogaster

Introduction of a single set of wild type autosomes into experimental populations of D. melanogaster homozygous for sepia, spineless and rough resulted in the quick assumption of a condition in which rough and sepia are balanced in the population at gene frequencies which have not declined progressively thereafter for a period of two and a half years (70 generations). The gene spineless, on the other hand, declined several times to new low levels but has not been eliminated. Its final frequency, about 12 per cent, is sufficiently low that slow elimination cannot be ruled out. These results are explained by the hypothesis that the heterozygotes, at least for rough and sepia, show superior fitness under the conditions of the experiment. As the condition was established almost immediately, it is likely that simple heterosis at certain loci (or possibly adjacent chromosome sections) functions in the immediate establishment of equilibria without the necessity of prior coadaptation.

Linkage, Interference and Semilethals in the White Group of Habrobracon

1. Broken, bk, white wh, (or its allele carrot, whc) and stumpy, st, are linked in that order by 25 + or - and 9 + or - crossover units respectively. 2. Three-point crosses involving these factors were made; map distances and ratios of coincidence were determined in each case. 3. Deviations from the expected ratios are due to semilethal factors to the left of bk. Frequency of the class containing bk relative to the alternative class, is highest for the straights and region 2 crossovers as might be expected if region 1 crossovers interfere with crossing over between the semilethal and bk. 4. Shiny, gzs, shows no linkage with the white group.