Clonal analysis in hybrids between Drosophila melanogaster and Drosophila simulans

We have analysed the viability of cellular clones induced by mitotic recombination in Drosophila melanogaster/D. simulans hybrid females during larval growth. These clones contain a portion of either melanogaster or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome arm. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a significant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome that is homozygous, the less viable is the recombinant melano-gaster/simulans hybrid clone. The few clones that appeared may represent cases in which mitotic recombination took place in distal chromosome intervals, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster/simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe more) that cause hybrid lethality. Therefore, the two species, D. melanogaster and D. simulans, have diverged to such a degree that the absence of part of the genome of one species cannot be substituted by the corresponding part of the genome of the other, probably due to the formation of co-adapted gene complexes in both species following their divergent evolution after speciation. The disruption of those coadapted gene complexes would cause the lethality of the recombinant hybrid clones.     

Developmental stability in hybrids between the sibling species pair,Drosophila melanogaster and Drosophila simulans

Drosophila melanogaster and its sibling species D. simulans were hybridized in the laboratory to test the hypothesis that developmental homeostasis in hybrids between two species having no prior gene flow would be significantly reduced. Developmental stability was assessed by measuring fluctuating asymmetry for three bilateral traits: sternopleural chaetae, wing length, and fronto-orbital plus frontal chaetae. Male F₁ hybrids showed no decrease in developmental stability compared to males of parental species. Female hybrids showed significant fluctuating asymmetry compared to other flies. The results are discussed with respect to ideas about coadaptation and gene flow based upon previous studies of hybrid developmental stability.     

A meristic trait in Drosophila melanogaster x simulans hybrids

A quantitative trait, abdominal bristle number, is described in a number of stocks of the two sibling species Drosophila melanogaster and Drosophila simulans and in interspecific hybrids obtained from these stocks. Coefficients of variation for the trait in the parental species and the hybrids are compared, and in the case of the latter are found to fall in the same range as those of the pure species. The results are discussed in the light of previous findings on other bristle systems.     

Variable modes of inheritance of morphometrical traits in hybrids between Drosophila melanogaster and Drosophila simulans.

We investigated body-size inheritance in interspecific sterile hybrids by crossing a Drosophila simulans strain with 13 strains of Drosophila melanogaster, which were of various origins and chosen for their broad range of genetic variation. A highly significant parent-offspring correlation was observed, showing that the D. melanogaster genes for size are still expressed in a hybrid background. Superimposed on to this additive inheritance, the size of hybrids was always less than the mid-parent value. This phenomenon, which at first sight might be described as dominance or overdominance, is more precisely interpreted as a consequence of a hybrid breakdown, that is, a dysfunction of the parental genes for size when put to work together. This interpretation is enforced by the fact that phenotypic variability was much more prevalent in hybrids than in parents. We also analysed body pigmentation inheritance in the same crosses and got a very different picture. There was no increase in the phenotypic variance of F(1) hybrids and only a low parent-offspring correlation. Apparent overdominance could be observed but in opposite directions, with no evidence of hybrid breakdown. Our data point to the possibility of analysing a diversity of quantitative traits in interspecific hybrids, and indicate that breakdown might be restricted to some traits only.     

Interspecific competition between Drosophila melanogaster and Drosophila simulans: Effects of adult density on adult viability

Survival (viability) of newly eclosed adults of D. simulans st and D. melanogaster Or R-C, which had excess quantities of dead yeast available throughout life, was measured daily for 7 days in an experiment where adult density (6 levels), species frequency (6 levels) and ³²P tissue content (2 levels) were varied factorially. A separate experiment comparing viability in different types of experimental unit also was done. Similar experiments were done for D. simulans st competing against D. melanogaster y w. The entire data were subjected to a least-squares (unbalanced, missing-plot) analysis of variance. D. melanogaster Or R-C had a higher average viability than D. melanogaster y w or D. simulans st, which were equal. The competing strain of D. melanogaster influenced D. simulans st viability — viability being higher when y w was the competitor. Viability decreased over the seven day period but at different rates for the three strains. Increasing density reduced viability for all three strains, but species frequency effects, although significant, were generally not consistent. Females had higher viability than males in both D. melanogaster strains, but the reverse was true in D. simulans st. ³²P lowered viability and experimental unit type altered viability. Numerous interactions were significant.Adult density was shown to have a delayed effect on viability — the delay before the appearance of the effect (an increase in death rate) being decreased as density rose. The term variably delayed density dependent has been adopted to describe the fitness component, adult viability. Some high density populations showed a readjustment (a decrease) in their death rate as a reaction to the effect of reduced density caused by high early mortality.     

Associations between female remating behavior, oogenesis and oviposition in Drosophila melanogaster and Drosophila pseudoobscura

An association between female remating behavior, oogenesis and oviposition was examined in Drosophila melanogaster and Drosophila pseudoobscura to investigate mechanisms that elicit remating. Females receptive to remating oviposited more eggs in both species; however, the species differed in the association between remating behavior and the number and distribution of oocyte stages. We found no differences in the number of either developing eggs of different stages or mature eggs between female D. pseudoobscura that were either receptive or nonreceptive to remating. In contrast, D. melanogaster females that are receptive to remating had significantly more mature eggs in the ovaries than nonreceptive females. Nonremating females had a significantly greater number of immature, vitellogenic oocytes. These results suggest that factors associated with oogenesis are related to female remating behavior in D. melanogaster but not in D. pseudoobscura. We discuss these results in conjunction with other evidence on the role male ejaculatory components play in mediating female remating behavior.     

Genetics of Drosophila simulans male mating discrimination in crosses with D. melanogaster

The genetic bases of sexual isolation between Drosophila melanogaster and D. simulans have been mainly studied in females, and there is little information about the role of the males in interspecific mating discrimination. Using D. simulans synthetic lines with compound chromosomes from a population of the Seychelles Islands (high frequency of interspecific mating) and a multimarker strain (low frequency), we show that D. simulans males play an important role in discriminating D. melanogaster females. The genetics of male discrimination fits well with the inheritance mode of a single locus, dominant for sexual isolation, located in chromosome II near the net mutation (2L-0.0). The heterospecific mating success of the male was not related to his sexual vigor. The specific load of male cuticular hydrocarbons was counted as a possible source of discrimination used by the D. melanogaster female.     

Drosophila female sterile mutation spoonbill interferes with multiple pathways in oogenesis

spoonbill is a Drosophila female-sterile mutation, which displays a range of eggshell and egg chamber patterning defects. Previous analysis has shown that the mutation interfered with the function of two major signaling pathways, GRK/EGFR and DPP. In this report, the nature of spoonbill was further investigated to examine whether it was associated with additional pathways in oogenesis. Clonal analysis, presented here, demonstrated that most of the aberrant phenotypes associated with spoonbill were dependent on a mutant germline. Nevertheless, SPOONBILL may function also in the soma to ensure proper polarization and migration of the border-cell-cluster. Further, genetic interaction studies implicated spoonbill in additional unrelated pathways such as the one(s) involved in actin polymerization/depolymerization. Based on the previous data and the results presented here, it is anticipated that spoonbill may encode a multifunctional protein that perhaps coordinately regulated the activity of multiple signaling pathways during oogenesis.     

Interspecific transfer of P elements by crosses between Drosophila simulans and Drosophila mauritiana

Interspecific crosses were carried out between P element-transformed strains of D. simulans and a strain of D. mauritiana, a species devoid of this transposable element family. Four lines were established from hybrid females backcrossed with D. mauritiana males for four generations, and then maintained by intra-line mass mating. In situ hybridization of polytene chromosomes and southern blots showed that full-length and deleted P elements were present in all of the lines after 15 generations. We conclude that at least some of the P elements observed in two lines result from their transposition into D. mauritiana genome. Gonadal sterility, induced at 29°C in D. melanogaster by P elements also occurred with these two latter lines.     

Enhanced cost of mating in female sterile mutants of Drosophila melanogaster

In Drosophila females, mating is known to cause a reduction in life span, which is referred to as 'the cost of mating'. Since mating enhances oogenesis and oviposition, the cost of mating may be regarded as a trade-off between reproduction and longevity. We examined whether the cost of mating exists in mutant females that are unable to produce eggs. Three different mutant alleles of ovarian tumors (otu) and an allele of dunce (dnc(M11)) of Drosophila melanogaster were used to sterilize females. For all the female sterile mutants tested, mating dramatically decreased the life span of homozygous sterile females. Even more extreme shortening of life spans were observed when the sex peptide gene (Acp70A) was expressed in homozygous otu females, though they were virgin, indicating that the shortening in life span is due to seminal factors. These results indicate that the cost of mating is greater in females defective in oogenesis than that in normally fertile females.     

Female sterile mutations and egg chamber development in Drosophila melanogaster

Drosophila oogenesis provides an excellent opportunity to study fundamental aspects of developmental biology and to learn the importance of multiple signalling pathways in the regulation of cellular morphogenesis. Taking advantage of the genetic and molecular approaches extremely powerful in this organism, over the years an enormous collection of data has accumulated on the genes involved in important steps of egg chamber development, such as germline and somatic stem cell maintenance, division and differentiation; oocyte determination and positioning; establishment of follicle cell fate and axes formation. These different processes are mediated by a reciprocal cross-talk between germline and somatic follicle cells. Here, in a schematic and simplified form, we point out what we believe are the main recent results on the molecular and cellular mechanisms underlying ovarian development and outline our recent contribution to this field.     

Effects of female behavior on wing display and courtship of male Drosophila simulans and Drosophila melanogaster

Corrolations between female rejection behaviors and male wing display were calculated for both Drosophila simulans and Drosophila melanogaster intraspicific pair-matings. No significant correlations were found for D. melanogaster, but in D. simulans flicking by the female appeared to be associated with a shift in male wing display pattern resulting in higher levels of vibration. Flicking did not appear to discourage courtship by males in either species.     

Heterosis in crosses between geographically separated populations of Drosophila melanogaster

Summary An experiment was performed to test the hypothesis that the genetic distance between populations estimated from enzyme loci could be used to predict the amount of heterosis that would occur in crosses between these populations. A partial diallel cross using 11 populations of Drosophila melanogaster from the AustralianPacific region and from England was carried out. Heterosis for larval viability, fecundity, cold shock mortality, and an index of these three traits was recorded. When two populations originating from the same location were crossed, no heterosis occurred, but otherwise heterosis was significant for all traits. For larval viability, a similar low level of heterosis occurred in all crosses. For cold shock mortality, the level of heterosis varied widely and fecundity showed a pattern intermediate between these two. The geographic distance between the sites from which populations originated was not correlated with the amount of heterosis in their crosses. There was a tendency for populations from ecologically different environments to show heterosis in crosses. Genetic distance based on ten enzyme loci was correlated with heterosis for cold shock mortality and the combined trait index. These results can be explained by the hypothesis that genes affecting larval viability are subject to strong, uniform selection in all populations, which limits the extent to which gene frequencies can drift apart. However, genes affecting cold shock mortality and the enzyme loci are subject to different selection pressures in different environments. This divergent selection combined with genetic drift causes divergence in gene frequency and heterosis.