The wild side of life: Drosophila reproduction in nature

Drosophila species vary in the rates at which females remate and the number of sperm they receive in the laboratory. In species such as D. melanogaster and D. pseudoobscura, in which females receive thousands of sperm and remate infrequently compared with species such as D. hydei and D. nigrospiracula, where females receive only a few hundred sperm and remate many times in a day, wild caught females should produce far more progeny. We tested this prediction by collecting, directly from nature, females of six species whose remating rates and number of sperm received vary from high to low and assessing the proportion of females with sperm and the number of progeny females produce. Over 95% of D. pseudoobscura and D. melanogaster females were inseminated while far fewer of the other species contained any sperm. In addition, D, pseudoobscura females produced progeny for over two weeks, D. melanogaster for over a week, while D. hydei and D. nigrospiracula females ran out of sperm after 1-2 d. These observations suggest extreme sperm limitation in these latter species.     

Mating system evolution in sperm-heteromorphic Drosophila

In Drosophila species of the obscura group, males exhibit sperm-heteromorphism, simultaneously producing both long sperm, capable of fertilization, and short sperm that are not. The production of multiple sperm types calls into question whether mating system correlates, such as sperm length and number trade-offs and female remating behavior, are the same as previously described in sperm-monomorphic systems. We examine three obscura group species, D. pseudoobscura, D. persimilis, and D. affinis that differ significantly in the lengths of their long fertilizing sperm, to test predictions about the relationship between sperm length and four mating system characters: male age at sexual maturity; sperm number; female remating; and male reproductive output. In D. affinis, where males produce the longest fertilizing sperm, their sexual maturity is delayed and they produce fewer long sperm compared to the other two species, as predicted if long sperm are costly to produce. Female D. affinis, although they receive fewer sperm than females of the other two species, do not remate more frequently or produce fewer progeny from a single mating. Different responses between sperm-heteromorphic and sperm-monomorphic systems underscore the complex nature of the coevolution between male and female mating system characters.     

Mutual interest between the sexes and reproductive success in Drosophila pseudoobscura

Abstract.— The pre-mating behavior of female Drosophila pseudoobscura has been considered passive and "coy" relative to more active, "ardent," and indiscriminate male behavior. To test whether this long-held view–the "received wisdom" about mating behavior in Drosophila –is really true we carried out observations on how often D. pseudoobscura females approached males prior to courtship and copulation. By including only virgin females and males in the experiments, we eliminated the possibility that males are "coy" due to sperm limitation and females flexibly "coy" due to male manipulations that may affect the duration of remating inhibition. We observed the movements of females and males in vials during the first five minutes of exposure to one another. Video records revealed females went toward males as frequently as males toward females; we inferred that females were as interested in males as males in females. The total number of offspring emerging as adults correlated significantly with mutual, precourtship interest of both males and females in their vial-mates and latency to copulation. Thus, we hypothesize that females in nature approach males, perhaps actively soliciting male courtship simply by remaining close to them.     

Indiscriminate females and choosy males: within- and between-species variation in Drosophila

Abstract The classic view of choosy, passive females and indiscriminate, competitive males gained theoretical foundations with parental investment theory. When females invest more in offspring than males, parental investment theory says that selection operates so that females discriminate among males for mates (i.e., females are choosy and passive) and males are indiscriminate (i.e., males are profligate and competitive). Here we report tests of predictions using Drosophila pseudoobscura and D. melanogaster , with typical asymmetry in gamete sizes (females > males), and in D. hydei with far less asymmetry in gamete size. Experimental observations revealed that the labels "choosy, passive females" and "profligate, indiscriminate males" did not capture the variation within and between species in premating behavior. In each of the species some females were as active in approaching males (or more so) than males in approaching females, and some males were as discriminating (or more so) than females. In pairs focal males and females responded differently to opposite-sex than to same-sex conspecifics. Drosophila hydei were less sex-role stereotyped than the other two species consistent with parental investment theory. However, D. pseudoobscura females approached males more often than did D. melanogaster females, and male D. hydei approached females as often as males of the other two species, both results inconsistent with parental investment theory. Male D. pseudoobscura and D. hydei were more likely to approach males in same-sex pairs than male D. melanogaster , inconsistent with parental investment theory.     

Female Sterile Mutations on the Second Chromosome of Drosophila Melanogaster. II. Mutations Blocking Oogenesis or Altering Egg Morphology

In mutagenesis screens for recessive female sterile mutations on the second chromosome of Drosophila melanogaster 528 lines were isolated which allow the homozygous females to survive but cause sterility. In 62 of these lines early stages of oogenesis are affected, and these females usually do not lay any eggs. In 333 lines oogenesis proceeds apparently normally to stage 8 of oogenesis, but morphological defects become often apparent during later stages of oogenesis, and are visible in the defective eggs produced by these females whereas 133 lay eggs that appear morphologically normal, but do not support normal embryonic development. Of the lines 341 have been genetically characterized and define a total of 140 loci on the second chromosome. Not all the loci are specific for oogenesis. From the numbers obtained we estimate that the second chromosome of Drosophila contains about 13 loci that are relatively specific for early oogenesis, 70 loci that are specifically required in mid to late oogenesis, and around 30 maternal-effect lethals.     

Desiccation resistance in two species of Drosophila

Three populations of Drosophila pseudoobscura, each one represented by 12 isofemale lines, and one laboratory strain of D. melanogaster were tested for desiccation resistance at two time periods. Except in the case of one population of D. pseudoobscura, the ability to withstand drying was significantly greater in females than in the corresponding males. The males of the three populations of D. pseudoobscura differed significantly among themselves in their resistance to desiccation, as did the females. The females of D. melanogaster exhibited a consistently higher survival rate than those of D. pseudoobscura, but not the males. These results are discussed with reference to the third chromosome inversion polymorphism of D. pseudoobscura and the cosmopolitan distribution of D. melanogaster.     

Effect of Microwave Exposure on the Ovarian Development of Drosophila melanogaster

In the present experiments the effect of GSM radiation on ovarian development of virgin Drosophila melanogaster female insects was studied. Newly emerged adult female flies were collected and divided into separate identical groups. After the a lapse of certain number of hours-different for each group-the insects (exposed and sham-exposed) were dissected and their intact ovaries were collected and photographed under an optical microscope with the same magnification. The size of the ovaries was compared between exposed and sham-exposed virgin female insects, during the time needed for the completion of oogenesis and maturation of the first eggs in the ovarioles. Immediately after the intact ovaries were photographed, they were further dissected into individual ovarioles and treated for TUNEL and acridine-orange assays to determine the degree of DNA damage in the egg chamber cells. The study showed that the ovarian size of the exposed insects is significantly smaller than that of the corresponding sham-exposed insects, due to destruction of egg chambers by the GSM radiation, after DNA damage and consequent cell death induction in the egg chamber cells of the virgin females as shown in previous experiments on inseminated females. The difference in ovarian size between sham-exposed and exposed virgin female flies becomes most evident 39-45 h after eclosion when the first eggs within the ovaries are at the late vitellogenic and post-vitellogenic stages (mid-late oogenesis). More than 45 h after eclosion, the difference in ovarian size decreases, as the first mature eggs of the sham-exposed insects are leaving the ovaries and are laid.     

Female remating, sperm competition and sexual selection in Drosophila

Female remating is fundamental to evolutionary biology as it determines the pattern of sexual selection and sexual conflict. Remating in females is an important component of Drosophila mating systems because it affects sperm usage patterns and sexual selection. Remating is common in females of many species of Drosophila in both natural and laboratory populations. It has been reported in many insect species and also in vertebrates. Female remating is a prerequisite for sperm competition between males, and the consequences of this competition, such as sperm precedence or sperm displacement, have been reported for many species of Drosophila. Female remating is dependent on the amount of sperm stored, the male seminal fluid components, nutrition, the quantity of eggs laid, experimental design and density of flies in laboratory. Remating by a female is an insurance against male sterility and sub-fertility and increases genetic heterogeneity of female offspring. Remating gives greater female productivity in many species of Drosophila. We examined female remating with respect to sperm competition and sexual selection in Drosophila and addressed the possible benefits for females. We also reviewed the role of accessory gland fluid in remating, costs associated with remating, the genetic basis of female remating and some possible mechanisms of sperm competition in the light of last male sperm priority and paternity assurance in Drosophila and other insects. We also suggest future areas of research.     

Evolutionary rearrangement of the amylase genomic regions between Drosophila melanogaster and Drosophila pseudoobscura

Two Drosophila pseudoobscura genomic clones have sequence similarity to the Drosophila melanogaster amylase region that maps to the 53CD region on the D. melanogaster cytogenetic map. The two clones with similarity to amylase map to sections 73A and 78C of the D. pseudoobscura third chromosome cytogenetic map. The complete sequences of both the 73A and 78C regions were compared to the D. melanogaster genome to determine if the coding region for amylase is present in both regions and to determine the evolutionary mechanism responsible for the observed distribution of the amylase gene or genes. The D. pseudoobscura 73A and 78C linkage groups are conserved with the D. melanogaster 41E and 53CD regions, respectively. The amylase gene, however, has not maintained its conserved linkage between the two species. These data indicate that amylase has moved via a transposition event in the D. melanogaster or D. pseudoobscura lineage. The predicted genes within the 73A and 78C regions show patterns of molecular evolution in synonymous and nonsynonymous sites that are consistent with previous studies of these two species.     

No accelerated rate of protein evolution in male-biased Drosophila pseudoobscura genes

Sexually dimorphic traits are often subject to diversifying selection. Genes with a male-biased gene expression also are probably affected by sexual selection and have a high rate of protein evolution. We used SAGE to measure sex-biased gene expression in Drosophila pseudoobscura. Consistent with previous results from D. melanogaster, a larger number of genes were male biased (402 genes) than female biased (138 genes). About 34% of the genes changed the sex-related expression pattern between D. melanogaster and D. pseudoobscura. Combining gene expression with protein divergence between both species, we observed a striking difference in the rate of evolution for genes with a male-biased gene expression in one species only. Contrary to expectations, D. pseudoobscura genes in this category showed no accelerated rate of protein evolution, while D. melanogaster genes did. If sexual selection is driving molecular evolution of male-biased genes, our data imply a radically different selection regime in D. pseudoobscura.     

A novel system of fertility rescue in Drosophila hybrids reveals a link between hybrid lethality and female sterility

Hybrid daughters of crosses between Drosophila melanogaster females and males from the D. simulans species clade are fully viable at low temperature but have agametic ovaries and are thus sterile. We report here that mutations in the D. melanogaster gene Hybrid male rescue (Hmr), along with unidentified polymorphic factors, rescue this agametic phenotype in both D. melanogaster/D. simulans and D. melanogaster/D. mauritiana F(1) female hybrids. These hybrids produced small numbers of progeny in backcrosses, their low fecundity being caused by incomplete rescue of oogenesis as well as by zygotic lethality. F(1) hybrid males from these crosses remained fully sterile. Hmr(+) is the first Drosophila gene shown to cause hybrid female sterility. These results also suggest that, while there is some common genetic basis to hybrid lethality and female sterility in D. melanogaster, hybrid females are more sensitive to fertility defects than to lethality.     

Effects of octopamine on reproduction, juvenile hormone metabolism, dopamine, and 20-hydroxyecdysone contents in Drosophila

The effect of an experimentally increased octopamine content (feeding flies with OA) on the levels of juvenile hormone (JH) degradation, dopamine (DA), and 20-hydroxyecdysone (20E) contents, oogenesis, and fecundity of wild type Drosophila flies has been studied. OA feeding of the flies was found to (1) cause a considerable decrease in JH degradation in females, but not males, of D. melanogaster and D. virilis; (2) have no effect on DA content in D. melanogaster and D. virilis; (3) increase 20E contents in D. virilis females; (4) decrease to a large extent the number of vitellogenic (stages 8-10) and mature (stage 14) oocytes in D. virilis; and (5) decrease the fecundity of D. melanogaster and D. virilis. A possible mechanism of action of OA as a neurohormone on the reproductive function of Drosophila is discussed.     

Perspective: female remating,operational sex ratio,and the arena of sexual selection in Drosophila species

Abstract.— As commonly observed among closely related species within a variety of taxa, Drosophila species differ considerably in whether they exhibit sexual dimorphism in coloration or morphology. Those Drosophila species in which male external sexual characters are minimal or absent tend, instead, to have exaggerated ejaculate traits such as sperm gigantism or seminal nutrient donations. Underlying explanations for the interspecific differences in the presence of external morphological sexual dimorphism versus exaggerated ejaculate traits are addressed here by examining the opportunity for sexual selection on males to occur before versus after mating in 21 species of Drosophila . Female remating frequency, an important component of the operational sex ratio, differs widely among Drosophila species and appears to dictate whether the arena of sexual selection is prior to, as opposed to after, copulation. Infrequent female mating results in fewer mating opportunities for males and thus stronger competition for receptive females that favors the evolution of male characters that maximize mating success. On the other hand, rapid female remating results in overlapping ejaculates in the female reproductive tract, such that ejaculate traits which enhance fertilization success are favored. The strong association between female remating frequency in a given species and the presence of sexually selected external versus internal male characters indicates that the relationship be examined in other taxa as well.     

A Comparison of Female Postcopulatory Behavior in Drosophila melanogaster and Drosophila biarmipes

The postcopulatory behavior of Drosophila biarmipes and Drosophila melanogaster females was analyzed and compared. Females from both species were shown to undergo a series of behavioral changes following mating, including significant reductions in both sexual attractiveness and receptivity. However, while both attractiveness and receptivity returned to “virgin-like” levels within a few days in D. melanogaster, D. biarmipes females, which regained their sexual attractiveness within a few days, remained unreceptive to copulation for at least 2 weeks. With respect to fecundity, D. melanogaster females produced more offspring when given opportunities to remate, while D. biarmipes females did not benefit from remating opportunities. These observations suggest that D. biarmipes females may have the ability to store sperm and produce offspring from a single mating over longer periods of time than other drosophilids.     

Seminal influences: Drosophila Acps and the molecular interplay between males and females during reproduction

Successful reproduction requires contributions from both the male and the female. In Drosophila, contributions from the male include accessory gland proteins (Acps) that are components of the seminal fluid. Upon their transfer to the female, Acps affect the female's physiology and behavior. Although primary sequences of Acp genes exhibit variation among species and genera, the conservation of protein biochemical classes in the seminal fluid suggests a conservation of functions. Bioinformatics coupled with molecular and genetic tools available for Drosophila melanogaster has expanded the functional analysis of Acps in recent years to the genomic/proteomic scale. Molecular interplay between Acps and the female enhances her egg production, reduces her receptivity to remating, alters her immune response and feeding behavior, facilitates storage and utilization of sperm in the female and affects her longevity. Here, we provide an overview of the D. melanogaster Acps and integrate the results from several studies that bring the current number of known D. melanogaster Acps to 112. We then discuss several examples of how the female's physiological processes and behaviors are mediated by interactions between Acps and the female. Understanding how Acps elicit particular female responses will provide insights into reproductive biology and chemical communication, tools for analyzing models of sexual cooperation and/or sexual conflict, and information potentially useful for strategies for managing insect pests.     

Characterization of defects in adult germline development and oogenesis of sterile and rescued female hybrids in crosses between Drosophila simulans and Drosophila melanogaster

Crosses between Drosophila melanogaster and D. simulans normally result in progeny that are either inviable or sterile. Recent discovery of strains that rescue these inviability and sterility phenotypes has made it possible to study the developmental basis of reproductive isolation between these two species in greater detail. By producing both rescued and unrescued hybrids and examining the protein product staining patterns of genes known to be involved in early germline development and gametogenesis, we have found that in crosses between D. simulans and D. melanogaster, hybrid female sterility results from the improper control of primordial germline proliferation, germline stem cell maintenance, and cystoblast formation and differentiation during early oogenesis. Rescued hybrid females are fertile, yet they generally have lower amounts of adult germline from the outset and show a premature degeneration of adult germline cells with age. In addition, older rescued hybrid females also exhibit mutant egg phenotypes associated with defects in dorso-ventral patterning which may result from the improper partitioning of cytoplasmic factors during early oogenesis that could stem from the early defect. Although a variety of germline and oogenic defects are described for the hybrid females, all of them can potentially result from the same underlying primary defect. Hybrid males from these same crosses, on the other hand, have no detectable germline in adult reproductive tissues, even when hybrid sterility rescue strains are used, indicating that male sterility and female sterility stem from distinctly different developmental defects.     

The Use of Yolk Protein Variations in Drosophila Species to Analyse the Control of Vitellogenesis

The yolk proteins of many insects, including Drosophila , are synthesised in the fat body of adult females and are transported through the haemolymph to be accumulated in the oocytes. We have used differences in the size and number of yolk polypeptides in different species of Drosophila to investigate the role of the ovary and of juvenile hormone in vitellogenesis.
The yolk proteins of eight species of Drosophila were compared with those of Drosophila melanogaster . Only Drosophila simulans had three yolk polypeptides of similar molecular weight to the three polypeptides in D. melanogaster and gave a high degree of cross reactivity with antibody raised against the yolk proteins of D. melanogaster . All other species had one to three bands on a sodium dodecyl sulphate gel representing the yolk polypeptides; they are between 44,000 and 49,500 daltons in molecular weight, showing weak cross reactivity with anti- D. melanogaster yolk antibody. Interspecies ovary transplants established that males of D. arizonensis and D.pseudoobscura which supported vitellogenesis of D. melanogaster ovaries, did so by permitting the implanted ovaries to synthesise their own yolk proteins. The synthetic juvenile hormone, ZR515, was unable to induce ovaries, which failed to develop in other species of males, to undergo vitellogenesis. In females, however, ZR515 was able to induce uptake of the yolk proteins of some of the species into the D. melanogaster donor ovaries, which had failed to develop in the absence of hormone. These interspecies differences in the yolk proteins have therefore been used to investigate the control of vitellogenesis and the role of juvenile hormone in this process in Drosophila .     

Molecular evolution of X-linked accessory gland proteins in Drosophila pseudoobscura

In Drosophila melanogaster and Drosophila simulans, positive Darwinian selection drives high rates of evolution of male reproductive genes, and accessory gland proteins (Acps) in particular. Here, we tested whether 13 X-linked male-specific genes, 4 Acps and 9 non-Acps, are under selective forces in the Drosophila pseudoobscura species group, much as those in the D. melanogaster group. We observed a statistically significant correlation in relative rates of nonsynonymous evolution between the two species groups tested. One Acp examined had a higher rate of nonsynonymous substitution than predicted by a neutral model in both species groups, suggesting its divergence was driven by positive Darwinian selection. To further test for the signature of selection, we examined polymorphism of three Acps within D. pseudoobscura. From this test, no Acp individually bore the signature of positive selection, but the 3 Acps together possessed an excess of nonsynonymous differences between species, relative to polymorphism within species. We conclude that faster evolution of Acps in the D. pseudoobscura group appears to be driven by positive selection, as previously suggested in the D. melanogaster group.     

Adaptive evolution of recently duplicated accessory gland protein genes in desert Drosophila

The relationship between animal mating system variation and patterns of protein polymorphism and divergence is poorly understood. Drosophila provides an excellent system for addressing this issue, as there is abundant interspecific mating system variation. For example, compared to D. melanogaster subgroup species, repleta group species have higher remating rates, delayed sexual maturity, and several other interesting differences. We previously showed that accessory gland protein genes (Acp's) of Drosophila mojavensis and D. arizonae evolve more rapidly than Acp's in the D. melanogaster subgroup and that adaptive Acp protein evolution is likely more common in D. mojavensis/D. arizonae than in D. melanogaster/D. simulans. These findings are consistent with the idea that greater postcopulatory selection results in more adaptive evolution of seminal fluid proteins in the repleta group flies. Here we report another interesting evolutionary difference between the repleta group and the D. melanogaster subgroup Acp's. Acp gene duplications are present in D. melanogaster, but their high sequence divergence indicates that the fixation rate of duplicated Acp's has been low in this lineage. Here we report that D. mojavensis and D. arizonae genomes contain several very young duplicated Acp's and that these Acp's have experienced very rapid, adaptive protein divergence. We propose that rapid remating of female desert Drosophila generates selection for continuous diversification of the male Acp complement to improve male fertilization potential. Thus, mating system variation may be associated with adaptive protein divergence as well as with duplication of Acp's in Drosophila.