Evolution of P elements in natural populations of Drosophila willistoni and D. sturtevanti

To determine how population structure of the host species affects the spread of transposable elements and to assess the strength of selection acting on different structural regions, we sequenced P elements from strains of Drosophila willistoni and Drosophila sturtevanti sampled from across the distributions of these species. Elements from D. sturtevanti exhibited considerable sequence variation, and similarity among them was correlated to geographic distance between collection sites. By contrast, all D. willistoni elements sampled were essentially identical (pi < 0.2%) and exhibited patterns typical of a recent population expansion. While the canonical P elements sampled from D. sturtevanti appear to be long-time residents in that species, a rapid expansion of a very young canonical P-element lineage is suggested in D. willistoni, overcoming barriers such as large geographical distances and moderate levels of population subdivision. Between-species comparisons reveal selective constraints on P-element evolution, as indicated by significantly different substitution rates in noncoding, silent, and replacement sites. Most remarkably, in addition to replacement sites, selection pressure appears to be strong in the first and third introns and in the 3' and 5' flanking regions.     

Meiosis in Bombyx mori females.

Crossing over is absent in oocytes of the silkworm, Bombyx mori. Synaptonemal complexes are present during pachytene between the paired chromosomes. At leptotene, lateral components of the synaptonemal complex are attached in a bouquet to a limited region of the nuclear envelope. Before completion of lateral components, synaptonemal complex formation begins at the nuclear envelope. With synaptonemal complex formation proceeding from both ends bivalents occasionally become interlocked. After pairing is completed, the bouquet arrangement is dissolved possibly as a result of a flow of the inner membrane of the nuclear envelope thereby separating the telomeres. After the telomeres are released from the nuclear envelope, material is deposited onto the lateral components of the synaptonemal complex. The modified synaptonemal complexes are retained by the bivalents until metaphase I. It is suggested that these modified synaptonemal complexes substitute for chiasmata in order to ensure regular disjunction of homologous chromosomes in the absence of crossing over.     

Embryonic pole cells and mycoplasmalike symbionts in Drosophila paulistorum

Mycoplasmalike intracellular symbionts have been located in the pole cells of Drosophila paulistorum embryos. These cells are destined to form the germ cells of both sexes. The symbionts had been previously localized in larval and adult developing and mature ovaries and testes. It is via the egg cytoplasm that these microorganisms are transmitted between generations to apparently cause an infectious and hereditary hybrid male sterility.     

Meiosis in Drosophila melanogaster

Chromosome pairing during meiosis I in D. melanogaster males was investigated ultrastructurally by examining complete bivalents in electron micrographs of serial thin sections. The XY bivalent is characterized by the presence of unique material located between the two half-bivalents at the site of synapsis. The material has a fibrillar appearance and is less electron dense than the surrounding chromatin. YY bivalents in XYY males and XY bivalents containing the X chromosome, In(1)sc^4Lsc^8R, where the pairing sites of the X chromosome are inverted and partially deleted also possess this material. The material is not associated with autosomal bivalents and may represent a morphological manifestation of the hypothetical cohesive elements (collochores) which are thought to function in conjunction of the X and Y chromosomes (Cooper, 1964).     

Sexual Interactions Between Two Distantly Related Drosophila Species, D. melanogaster and D. willistoni (Diptera: Drosophilidae)

Molecular analysis suggests that the pomace fly Drosophila melanogaster acquired the P family of transposable elements from another Drosophila species, D. willistoni. Since the two species are distantly related, it has been assumed that transmission of P element DNA from D. willistoni to D. melanogaster was mediated by a vector. The possibility of an alternative mode of transmission was assessed by characterizing the sexual behaviors of D. willistoni males and females, then observing D. willistoni and D. melanogaster males and females to see whether males from one species interacted sexually with females from the other species in a laboratory setting. We observed that D. melanogaster males court D. willistoni females vigorously and, in some cases, stimulate the females to be receptive to copulation. However, D. willistoni males perform relatively little courtship in response to D. melanogaster females and do not attempt to copulate. Thus, it is unlikely that sexual interactions effected the transmission of P element DNA from D. willistoni to D. melanogaster in the flies' natural habitat.     

Meiosis in Drosophila males

The conjunctive mechanism of the XY bivalent is believed to differ from that of the autosomal bivalents in the achiasmate Drosophila melanogaster male. It has been proposed that hypothetical cohesive elements, termed collochores, hold the X and Y chromosomes together at or near their nucleolar organizing regions (NORs) and that collochores are not exhibited by autosomal bivalents. In electron micrographs, unique fibrillar material is observed between the X and Y chromosomes at the synaptic site. Recently, the 240 bp nontranscribed spacer associated with rRNA genes at the NOR has been implicated as the essential DNA sequence for XY pairing. To test whether this DNA sequence is always associated with XY pairing and to determine its relationship to the unique fibrillar material, we studied the XY bivalent in Drosophila simulans. The D. simulans Y chromosome has few, if any, rRNA genes, but does have a large block (3,000 kb or 12,500 copies) of the nontranscribed spacer repeat located at the distal end of its long arm. This is in contrast to the D. melanogaster Y, which has the repeat located among rRNA genes on its short arm. Using light and electron microscopy, we show that the X does indeed pair with the distal end of the long arm of the D. simulans Y. However, no fibrillar material is evident in serial thin sections of the D. simulans XY bivalent, suggesting that this material (in D. melanogaster) may be remnants of the NOR rather than a morphological manifestation of the hypothetical collochores. Indeed, in electron micrographs, the synaptic regions of the XY and autosomal bivalents appear similar with no obvious pairing structures, suggesting that the conjunctive mechanism holding homologous chromosomes together is the same for the XY and autosomal bivalents.     

Age-specific fitness components in hybrid females of Drosophila pseudoobscura and D. persimilis

Most models of hybridization assume that hybrids are less fit than their parental taxa. In contrast, some researchers have explored the possibility that hybrid individuals may actually have higher fitness and so play an important role in the generation of new species or adaptations. By estimating age-specific fitness components, we can determine not only how hybrid fitness differs from parental taxa, but also whether the fitness of hybrids relative to parental taxa changes with age. Here we describe an analysis of age-specific fitness traits in two species of Drosophila, D. pseudoobscura and D. persimilis, and their F1 hybrids. At early ages, hybrid females lay as many eggs as parental individuals, on average, but produce far fewer offspring. By late ages, in contrast, parental taxa show a steep decline in production not seen in hybrids, such that hybrids produce more offspring, on average, than parental taxa. Furthermore, egg-adult survival in hybrids is negatively correlated with egg density, whereas these traits are only weakly correlated in parental taxa. The results are limited somewhat by the fact that we analyze only two strains, and that these may be partially inbred. Nonetheless, the results are certainly illustrative, pointing out not only that at least some hybrid individuals may be as fit or fitter than parental taxa, but also that the difference between hybrids and parental taxa varies with age.     

Darwinian fitness and adaptedness in experimental populations of Drosophila willistoni

Fifteen second chromosomes were extracted from Drosophila willistoni flies collected in four natural populations. The adaptedness of populations homozygous for each chromosome was measured by average population size and productivity. Six control populations were established with mixtures of the wild second chromosomes. The Darwinian fitness of flies homozygous for each wild second ehromosome, and of flies carrying random combinations of these chromosomes, was measured relative to the fitness of flies heterozygous for a wild and a marker chromosome. The Darwinian fitness of homozygotes for each second chromosome relative to the fitness of flies carrying random combinations of the natural chromosomes was then inferred. The estimated loss of fitness on making the natural second chromosomes homozygous was substantial, ranging from 39 to 83 pereent, with an average reduction in fitness of 66 percent. These results with D. willistoni are consistent with those from similar experiments with other drosophila species, and they are compatible with a significant role for heterosis in the maintenance of genetic variability.Populations homozygous for wild chromosomes differ in their adaptedness to the experimental environment. Population size and productivity are correlated, although the correlation is far from complete. Some populations have high productivity and low population size, or vice versa. The control populations, with greater genetic variability, were superior in adaptedness to the average of the single-chromosome populations. The Darwinian fitness and the adaptedness of the genotypes in this experiment were not significantly correlated. It follows that certain measures used by population geneticists, such as genetic load and average Darwinian fitness, cannot be taken as general indices of how well adapted a population is to its environment.This work was supported by U.S. Public Health Service Grant RO1-HDO5055, NSF grant GB-20694 (International Biological Program). AEC contract AT-(30-1) 3096, and PHS Career Development Award K3 GM 37265. The collection of the flies was supported by the Fundacão de Amparo a Pesquisa do Estado de São Paulo, Brazil. The senior author's stay in New York, where the experiments were conducted, was financed in part by Research Fellowship 2-12861 from the Panamerican Union.     

Mating activity of yellow and sepia Drosophila willistoni mutants

By analyzing the mating activity of newly isolated yellow and sepia mutants of Drosophila willistoni no difference in behavior between sepia and wild-type flies were observed, whereas yellow males were less successful than wild-type males when competing for females. These results are in agreement with those reported for other Drosophila species. D. willistoni was different in the 'females-competing' crosses because wild-type males mated more frequently with wild-type females whereas yellow males mated successfully with both phenotypes. These results indicate the complexity of the courtship behavior in D. willistoni and provide data for comparative and evolutionary research into the genus.     

Transmission of Nosema kingi to offspring of Drosophila willistoni during copulation.

Transmission of the microsporidian Nosema kingi to offspring of Drosophila willistoni during copulation was investigated. Offspring from matings of infected females and non-infected males, infected males and non-infected females, and both infected females and males were parasitized with N. kingi. 28% of the offspring were infected when the parents mated immediately after exposure to the parasite. However, 43--93% of the offspring were infected with the microsporidian when copulation was delayed 2 weeks post-infection.     

Response to selection for increased hybridization between Drosophila melanogaster females and D. simulans males.

Females of Drosophila melanogaster and males of D. simulans hybridizing in a nonchoice condition were artificially selected for 12 generations. The frequency of hybridization increased from 10% to 79%. Response to selection occurred in both species, particularly in D. melanogaster. Female receptivity was the primary sexual trait that accounted for breaking up sexual isolation in these species, but it remained unclear which elements of the D. simulans male courtship were involved.     

Spindle Dynamics during Meiosis in Drosophila Oocytes

Mature oocytes of Drosophila are arrested in metaphase of meiosis I. Upon activation by ovulation or fertilization, oocytes undergo a series of rapid changes that have not been directly visualized previously. We report here the use of the Nonclaret disjunctional (Ncd) microtubule motor protein fused to the green fluorescent protein (GFP) to monitor changes in the meiotic spindle of live oocytes after activation in vitro. Meiotic spindles of metaphase-arrested oocytes are relatively stable, however, meiotic spindles of in vitro–activated oocytes are highly dynamic: the spindles elongate, rotate around their long axis, and undergo an acute pivoting movement to reorient perpendicular to the oocyte surface. Many oocytes spontaneously complete the meiotic divisions, permitting visualization of progression from meiosis I to II. The movements of the spindle after oocyte activation provide new information about the dynamic changes in the spindle that occur upon re-entry into meiosis and completion of the meiotic divisions. Spindles in live oocytes mutant for a lossof-function ncd allele fused to gfp were also imaged. The genesis of spindle defects in the live mutant oocytes provides new insights into the mechanism of Ncd function in the spindle during the meiotic divisions.