CHROMOSOME-NUCLEAR MEMBRANE-CYTOPLASMIC INTERRELATIONS IN DROSOPHILA

The structural evidence for nucleocytoplasmic interrelationships observed in electron micrographs of salivary-gland cells of third instar larvae of Drosophila melanogaster has been reviewed. It has been found that the characteristic nuclear membrane outpocketings with their adjacent highly differentiated chromosomal materials occur at one stage of larval development at a time when a new cellular function is being initiated. Preliminary cytochemical studies to characterize the materials transferred from nucleus to cytoplasm indicate that deoxyribonucleic acid occurs within the blebs. Observations on chromosome and nuclear membrane structure are also presented.     

PATTERNS OF SPECIATION IN DROSOPHILA

To investigate the time course of speciation, we gathered literature data on 119 pairs of closely related Drosophila species with known genetic distances, mating discrimination, strength of hybrid sterility and inviability, and geographic ranges. Because genetic distance is correlated with divergence time, these data provide a cross-section of taxa at different stages of speciation. Mating discrimination and the sterility or inviability of hybrids increase gradually with time. Hybrid sterility and inviability evolve at similar rates. Among allopatric species, mating discrimination and postzygotic isolation evolve at comparable rates, but among sympatric species strong mating discrimination appears well before severe sterility or inviability. This suggests that prezygotic reproductive isolation may be reinforced when allopatric taxa become sympatric. Analysis of the evolution of postzygotic isolation shows that recently diverged taxa usually produce sterile or inviable male but not female hybrids. Moreover, there is a large temporal gap between the evolution of male-limited and female hybrid sterility or inviability. This gap, which is predicted by recent theories about the genetics of speciation, explains the overwhelming preponderance of hybridizations yielding male-limited hybrid sterility or inviability (Haldane's rule).     

CHEMICAL COMMUNICATION IN HAWAIIAN DROSOPHILA

We are interested in elucidating the extent to which lekking Hawaiian Drosophila species have diverged from their continental counterparts, which engage in sexual behavior at communal food sources, with regard to the chemical communication systems that the flies employ. Accordingly, we have analyzed flies from three closely related Hawaiian Drosophila species in the adiastola subgroup. These species are of interest because the males engage in a unique behavior: while courting, they raise their abdomens over their heads and emit anal droplets. Analysis of the flies' behavior, the hydrocarbons in males' anal droplets, and males' cuticular hydrocarbons suggest that females' responses to males may be mediated by cuticular pheromones and/or pheromones in males' extruded droplets that enable the females to distinguish conspecific from heterospeciflc males. Conversely, perception of cuticular hydrocarbons from conspecific females enables D. adiastola males to distinguish females from a closely related species from conspecific females. On the basis of these observations, we suggest that the adiastola subgroup species are unique among drosophilids in that they utilize an anal droplet-mediated pheromone communication system, some or all components of which are species specific. However, the lekking Hawaiian Drosophila species are similar to D. melanogaster and related continental species in that the Hawaiian flies employ a cuticular pheromone communication system, some components of which are sex and species-specific.     

CHROMOSOMAL DNA SYNTHESIS IN DROSOPHILA MELANOGASTER

Analysis of labeling patterns in three chromosome segments of Drosophila melanogaster has shown that the replicative activity within chromosomes is temporally ordered. Moreover, specific labeling patterns on one chromosome occur with specific patterns on another chromosome with a very high degree of correlation. This circumstance leads to the conclusion that DNA synthesis among all the regions in the three chromosome segments studied is coordinated. The various labeling patterns observed in any one chromosome and the combinations of labeling patterns observed in all three chromosome segments can be arranged in ordered arrays, if one assumes that the DNA synthesis in each chromosome region will go to completion without stopping once it has started. Such arrays can serve as models for the temporal order of DNA synthesis among chromosome regions. They predict that in any one chromosome DNA replication begins and ends at very few loci and that synthesis at a larger number of points occurs at an intermediate time.     

MALE EFFECTS ON FECUNDITY IN DROSOPHILA MELANOGASTER

Effects of male Drosophila melanogaster on female fecundity and productivity were examined, considering both females held in containers with males and females exposed to male effects not involving contact. Females were more fecund when male effects were present, and the largest increase was recorded when vials were previously conditioned by males. This effect was probably due to the growth of transmitted microorganisms, which were observed on the laying surface, as further experiments with vials conditioned by virgin females showed a similar increase in fecundity. A male-specific effect was isolated by conditioning bottles containing only agar with males and virgin females. The observation of a male factor that stimulates oviposition without mating is novel and suggests complicated fertility interactions between the sexes.     

ENVIRONMENTAL EFFECTS ON REMATING IN DROSOPHILA MELANOGASTER

The effects of density and food on remating were investigated using Drosophila melanogaster. The frequency of remating was unaffected by density for some combinations of fly strains but was reduced at low relative densities for other combinations. Until females had used most of their stored sperm, remating was less likely when food was absent or contact with food was prevented. Food availability had little effect on the incidence of remating once stored sperm were depleted and had no effect on initial virgin mating frequency. This study indicates that environmental factors can have a substantial direct influence on the frequency of remating in populations of Drosophila melanogaster.