Drosophila ananassae: a species characterized by spontaneous male recombination in appreciable frequency.

Mutation and recombination are primarily responsible for generating the genetic variability in natural populations of microorganisms, plant and animal species including humans. Upon such genetic variations, elemental forces of evolution such as natural selection, random genetic drift and migration operate to bring about micro-evolutionary changes. Recombination or crossing-over produces new combinations of genes due to interchange of corresponding segments between nonsister chromatids of homologous chromosomes, thus, it is an important evolutionary factor. Since the time of T. H. Morgan, Drosophila has been subjected to extensive investigations on crossing over while employing a number of markers, which were used for gene mapping. Interestingly, recombination occurs in females of D. melanogaster but not in males. Later on, male crossing over was investigated in various species and its occurrence was reported in D. melanogaster, D. ananassae, D. simulans, D. willistoni, D. littoralis and D. bipectinata. Recombination occurs at very low rate in all these species except for D. ananassae, which shows spontaneous male crossing over in appreciable frequency, which is meiotic in origin. This unusual phenomenon in D. ananassae is influenced by various genetic factors as well as it shows strain variation as far as frequency of male recombination is concerned. Further, the presence of chiasmata during meiosis in males at a frequency capable of accounting for the observed recombination frequency extends evidence for meiotic origin of recombination in males of D. ananassae.

Dobzhansky's concept of genetic coadaptation: Drosophila ananassae is an exception to this concept.

Dobzhansky was the first to show that the inversion polymorphism in Drosophila pseudoobscura is subject to natural selection and is a device to cope with the diversity of environments. His extensive work on D. pseudoobscura has revealed interesting phenomena of population genetics. In continuation of his work on this species, he constructed a number of homozygous lines for different gene arrangements in the third chromosome, and while employing these lines in intrapopulation and interpopulation crosses, he quantified the fitness of inversion homokaryotypes and heterokaryotypes. Interestingly, his results showed that heterokaryotypes formed by chromosomes originating from the same geographic area exhibited superiority over the corresponding homokaryotypes. However, superiority of heterokaryotypes was lost in the crosses when chromosomes were derived from different localities. Based on these results, Dobzhansky suggested the concept of genetic coadaptation. According to this concept, 'in each locality, the chromosomes with different gene arrangements aremutually adjusted or coadapted to yield highly fit inversion heterozygotes through long continued natural selection. However, this adaptive superiority of inversion heterozygotes breaks down in interracial hybridization experiments when two gene arrangements are derived from different localities'. This concept has received experimental evidence in its favour on the basis of work done in other species of Drosophila, such as D. willistoni, D. paulistorum, D. pavani and D. bipectinata. In all these species, interracial hybridization led to the loss of superiority of inversion heterozygotes. Further, it has been suggested that coadapted polygenic complexes contained in the chromosomes are disrupted as a result of recombination in interstrain crosses. This concept was also tested in D. ananassae, a cosmopolitan and domestic species commonly found in India, while employing three cosmopolitan inversions exhibiting heterotic buffering. In interstrain crosses involving monomorphic and polymorphic strains due to three cosmopolitan inversions, the persistence of heterosis was observed, which does not support the above-mentioned hypothesis of Dobzhansky. Thus, evidence for coadaptation is lacking in natural populations of D. ananassae, which is considered as an exception to the Dobzhansky's concept of genetic coadaptation. Thus, heterotic buffering associated with the three cosmopolitan inversions in D. ananassae is not populational heterosis; rather, it appears to be simple luxuriance.

The Drosophila bipectinata species complex: phylogenetic relationship among different members based on chromosomal variations.

Making interspecific hybridizations, where possible remains an unparalleled option for studying the intricacies of speciation. In the Drosophila bipectinata species complex comprising of four species, namely D. bipectinata, D. parabipectinata, D. malerkotliana and D. pseudoananassae, interspecific hybrids can be obtained in the laboratory, thus bequeathing an ideal opportunity for studying speciation and phylogeny. With the view of investigating the degree of divergence between each species pair, we planned to study the polytene chromosomes of the F1 hybrids, as it would mirror the level of compatibility between the genomes of the parental species. Two sets of crosses were made, one involving homozygous strains of all four species from India and the other including homozygous strains from different places across the globe. Polytene chromosomes of F1 larvae from both sets of crosses had similar configurations. In F1 larvae from crosses involving D. bipectinata, D. parabipectinata and D. malerkotliana, complex configurations (depicting overlapping inversions) could be detected in different arms. However, they were fairly synapsed, indicating that the differences are only at the level of gene arrangements. The polytene chromosomes of larvae obtained by crossing D. pseudoananassae with the other three species were very thin with gross asynapsis in all the arms, demonstrating that the genome of D. pseudoananassae is widely diverged from rest of the species. The overlapping inversions (reflected in complex configuration), are inferred in the light of earlier chromosomal studies performed in this complex.

The genus Drosophila is characterized by a large number of sibling species showing evolutionary significance.

Mayr (1942) defined sibling species as sympatric forms which are morphologically very similar or indistinguishable, but which possess specific biological characteristics and are reproductively isolated. Another term, cryptic species has also been used for such species. However, this concept changed later. Sibling species are as similar as twins. This category does not necessarily include phylogenetic siblings as members of a superspecies. Since the term sibling species was defined by Mayr, a large number of cases of sibling species pairs/groups have been reported and thus they are widespread in the animal kingdom. However, they seem to be more common in some groups such as insects. In insects, they have been reported in diptera, lepidoptera, coleoptera, orthoptera, hymenoptera and others. Sibling species are widespread among the dipteran insects and as such are well studied because some species are important medically (mosquitoes), genetically (Drosophila) and cytologically (Sciara and Chironomus). The well-studied classical pairs of sibling species in Drosophila are: D. pseudoobscura and D. persimilis, and D. melanogaster and D. simulans. Subsequently, a number of sibling species have been added to these pairs and a large number of other sibling species pairs/groups in different species groups of the genus Drosophila have been reported in literature. The present review briefly summarizes the cases of sibling species pairs/groups in the genus Drosophila with their evolutionary significance.

Species and genetic diversity in the genus Drosophila inhabiting the Indian subcontinent.

Biodiversity is the sum total of all living things on the earth with particular reference to the profound variety in structure,function and genetic constitution. It includes both number and frequency of species or genes in a given assemblage and the variety of resulting ecosystems in a region. It is usually considered at three different levels: genetic, species and ecological diversities. Genus Drosophila belongs to the family Drosophilidae (class Insecta, order Diptera), characterized by rich species diversity at global level and also in India, which is a megadiverse country. At global level, more than 1500 species have been described and several thousands estimated. Hawaiian Islands are particularly rich in species diversity with more than 500 species which provides a unique opportunity to study evolution in genus Drosophila. About 150 species of Drosophila have been reported from India. Certain species of Drosophila found in India have been investigated for genetic diversity within the species. In this regard, Drosophila ananassae is noteworthy. It is a cosmopolitan and domestic species with common occurrence in India and is endowed with many genetic peculiarities. Population genetics and evolutionary studies in this species have revealed as to how genetic diversity within a species play an important role in adaptation of populations to varying environments. In addition, the work carried on D. melanogaster, D. nasuta, D. bipectinata and certain other species in India has shown that these species vary in degree and pattern of genetic diversity, and have evolved different mechanisms for adjusting to their environments. The ecological adaptations to various kinds of stress studied in certain species of Drosophila inhabiting the Indian subcontinent are also discussed.

Sperm displacement in the Drosophila bipectinata species complex: evidence for interspecific variations.

We studied sperm displacement in two members of the Drosophila bipectinata species complex i.e., Drosophila parabipectinata and Drosophila malerkotliana by employing mutant and wild type strains. We found a significant increase in the productivity of remated females than that of once mated ones in all the crosses of the two species. The P2' values range from 0.67 to 0.70 in D. malerkotliana and 0.52 to 0.56 in D. parabipectinata. These results were compared with that of our earlier finding in D. bipectinata belonging to the same complex. We found that though the three species exhibit differences in the degree of sperm displacement, the pattern is somewhat similar in the three species. However, as far as its genetic variability and population size maintenance is considered, D. malerkotliana and D. bipectinata (P2' is 0.60-0.67) are at the forefront. Our finding is also supported by the fact that D. parabipectinata is derived from D. bipectinata and has not diverged much since its emergence as a separate species. The differences in the pattern of sperm displacement may be attributed to differences in remating latency, duration of copulation in first mating and sperm usage pattern among the members of this complex.

Mating latency, duration of copulation and fertility in four species of the Drosophila bipectinata complex.

Significant interspecific variations in mean duration of copulation and fertility were observed in four species of the Drosophila bipectinata species complex. However, D. bipectinata showed positive correlation between duration of copulation and fertility. Similarly, D. malerkotliana showed negative correlation between mating latency and duration of copulation. Likewise, D. pseudoananassae showed positive correlation between mating latency and fertility. These results suggest that D. pseudoananassae has distant relatedness from the other three species with respect to mating latency, duration of copulation and fertility which supports the previous findings.

Role of sexual selection in speciation in Drosophila.

The power of sexual selection to drive changes in the mate recognition system through divergence in sexually selected traits gives it the potential to be a potent force in speciation. To know how sexual selection can bring such type of divergence in the genus Drosophila, comparative studies based on intra- and inter-sexual selection are documented in this review. The studies provide evidence that both mate choice and male-male competition can cause selection of trait and preference which thereby leads to divergence among species. In the case of intrasexual selection, various kinds of signals play significant role in affecting the species mate recognition system and hence causing divergence between the species. However, intrasexual selection can bring the intraspecific divergence at the level of pre- and post-copulatory stage. This has been better explained through Hawaiian Drosophila which has been suggested a wonderful model system in explaining the events of speciation via sexual selection. This is due to their elaborate mating displays and some kind of ethological isolation persisting among them. Similarly, the genetic basis of sexually selected variations can provide yet another path in understanding the speciation genetics via sexual selection more closely.

Studies on remating behaviour in the Drosophila bipectinata species complex: intra- and interspecific variations.

Intra- and interspecific variations in female and male remating frequency, latency, and duration of copulation in first and second matings were analyzed in four species of the Drosophila bipectinata complex, employing four strains of each species i.e. D. bipectinata, D. parabipectinata, D. malerkotliana and D. pseudoananassae. Chi-square test revealed significant intraspecific variation in the number of remated females and males in D. malerkotliana and D. bipectinata, whereas D. parabipectinata showed insignificant intraspecific variations in number of remated females and males. D. pseudoananassae showed significant intraspecific variation in the number of remated females, but the frequency of remating was least. One way ANOVA depicted significant intraspecific variation in female and male remating time in D. bipectinata and D. parabipectinata. D. bipectinata took the shortest time to remate and the duration of copulation in first mating was longest. However, D. pseudoananassae exhibited the longest remating time and long duration of copulation in first mating. Results of t-test depicted that all four species exhibited shorter duration of copulation in second mating as compared to the first. Our study puts D. bipectinata and D. pseudoananassae at two extremes, and D. parabipectinata and D. malerkotliana at an intermediate position in a hierarchy of remating behaviours.

Experimental evidence for nutrition regulated stress resistance in Drosophila ananassae.

BACKGROUND: The amount and quality of nutrients consumed by organisms have a strong impact on stress resistance, life-history traits and reproduction. The balance between energy acquisition and expenditure is crucial to the survival and reproductive success of animals. The ability of organisms to adjust their development, physiology or behavior in response to environmental conditions, called phenotypic plasticity, is a defining property of life. One of the most familiar and important examples of phenotypic plasticity is the response of stress tolerance and reproduction to changes in developmental nutrition. Larval nutrition may affect a range of different life-history traits as well as responses to environmental stress in adult. PRINCIPAL FINDINGS: Here we investigate the effect of larval nutrition on desiccation, starvation, chill-coma recovery, heat resistance as well as egg to adult viability, egg production and ovariole number in Drosophila ananassae. We raised larvae on either protein rich diet or carbohydrate rich diet. We found that flies consuming protein rich diet have higher desiccation and heat shock resistance whereas flies developed on carbohydrate rich diet have higher starvation and cold resistance. Egg production was higher in females developed on protein rich diet and we also found trade-off between egg production and Egg to adult viability of the flies. Viability was higher in carbohydrate rich diet. However, sex specific viability was found in different nutritional regimes. Higher Egg production might be due to higher ovariole number in females of protein rich diet. CONCLUSION: Thus, Drosophila ananassae adapts different stress tolerance and life-history strategies according to the quality of the available diet, which are correlated with phenotypic adjustment at anatomical and physiological levels.

Interspecific sexual isolation and phylogeny among different members of the Drosophila bipectinata species complex.

The degree and pattern of sexual isolation among closely related species can be utilized for predicting the direction of evolution and deriving a phylogeny. The Drosophila bipectinata species complex is a group of four morphologically very similar species, belonging to the ananassae subgroup of the melanogaster species group. Sexual isolation among the members of this complex was studied by employing the male-choice technique. To test the difference between the homogamic and heterogamic matings, chi-square values were calculated under the assumption of random mating and the degree of sexual isolation was measured by estimating isolation index. To test the difference in proportions of heterogamic matings between reciprocal crosses, normal deviate (z) was calculated. The direction of evolution among these species has been discussed in the light of models proposed by different workers. Our results suggest that while D. bipectinata, Drosophila parabipectinata and Drosophila malerkotliana are closely related to each other, D. pseudoananassae is distantly related to these three species. The species pairs D. bipectinata and D. parabipectinata and D. parabipectinata and D. malerkotliana show asymmetrical sexual isolation. Based on this, it is suggested that D. bipectinata and D. malerkotliana share a common ancestor where as D. parabipectinata has been derived from D. bipectinata. Their phylogenetic relationship has been discussed in the light of phylogenies suggested by earlier workers.

Disruptive selection for sternopleural bristle phenotypes in Drosophila ananassae.

Disruptive selection is potentially critical in maintaining variation and initiating speciation and plays an important role in the organization of genetic variability in natural populations. It occurs when extreme phenotypes have a fitness advantage over intermediate phenotypes. Disruptive selection for high and low numbers of sternopleural bristles in Drosophila ananassae was applied for 12 generations to test its effect in induction of behavioural isolation. Pattern of mating between flies of high and low lines was tested in an Elens-Wattiaux mating chamber by using a multiple-choice technique after G(5) and G(12). Data was analyzed by calculating χ(2) under the assumption of random mating to test the difference between homoand heterogamic matings, and sexual isolation was tested by calculating the isolation estimate. The results show that there is no evidence for sexual isolation in G(5) and G(12). The realized heritability, standard error of regression coefficient, and t values suggest that disruptive selection for sternopleural bristle phenotypes was effective, but it does not lead to behavioral isolation in D. ananassae. Rather, it induces differences in mating propensity, which is influenced by sternopleural bristle phenotypes.

Origin of sexual isolation in Drosophila ananassae due to founder effects.

The origin of sexual isolation is the central event in the evolution of biological species and plays a key role in maintaining biological diversity. Three mass culture stocks of D. ananassae originating from different geographic localities showing no isolation with each other were subjected to different degrees of bottlenecks i.e. one pair, five pairs and ten pairs. These drift lines were passed through flush-crash cycle at every generation with same initial number of founders, and maintained for twenty-seven generations and then the pattern of matings was tested among these nine drift lines involving 36 crosses in total. In 23 of 36 crosses, the difference between homogamic and heterogamic matings was significant and isolation indices were significantly more than zero in one direction only providing evidence for asymmetrical sexual isolation. Further, when Bonferroni test for pair-wise analysis was employed, significant differences between homogamic and heterogamic matings were found in 25 crosses. These findings provide evidence for origin of sexual isolation by founder effects in D. ananassae.

Effect of chromosome arrangements on mate recognition system leading to behavioral isolation in Drosophila ananassae.

The mechanisms of speciation that appear in the early stages of reproductive isolation has been of recent interest to evolutionary biologists. Experiments were conducted to study behavioral isolation between karyotypically different homozygous strains derived from natural populations of Drosophila ananassae. Three mass cultures stocks established from flies collected from natural populations were employed and homozygous stocks (ST/ST and AL/AL) were made through selection for homozygosity. By employing male-choice technique, mating success was scored by direct observation in the Elens-Wattiaux mating chamber. There is preference for homogamic matings in all the three populations and the differences between homogamic and heterogamic matings are statistically significant in two populations (Lucknow and Varanasi). These findings provide evidence that there is incipient sexual isolation between karyotypically different strains of D. ananassae derived from natural populations which shows that chromosome arrangements may affect the mate recognition system in D. ananassae.

Drosophila ananassae: a good model species for genetical, behavioural and evolutionary studies.

Drosophila ananassae, a cosmopolitan and domestic species, was first described by Doleschall in 1858 from Indonesia. During 1930s, cytological and genetical investigations in D. ananassae were initiated in Japan and USA which showed that it is a genetically unique species. Since then a large number of studies have been carried out by researchers in Japan, U.S.A., India, France and Germany in this genetically unique species. Present review briefly summarizes the work done on genetical, behavioural and evolutionary aspects in D. ananassae which demonstrates that it is a ggod model species for such studies. Further, it is also discussed how the work on D. ananassae has enriched our understanding of basic phenomena like evolution and behaviour compared to similar studies on other model Drosophilds like D. melanogaster, D. pseudoobscura or D. subobsura.

Fluctuating asymmetry in hybrids of sibling species, Drosophila ananassae and Drosophila pallidosa, is trait and sex specific.

Due to inconsistent results of the empirical studies, the relationship between fluctuating asymmetry (FA, a measure of developmental stability) and interspecific hybridization has been the subject of intense debates. In the present study, we have assessed the impact of interspecific hybridization between 2 sibling species of Drosophila: Drosophila ananassae and Drosophila pallidosa on the levels of FA over 3 generations. Trait size of different morphological traits, namely, sternopleural bristle number, wing length (WL), wing to thorax (W/T) ratio, sex comb tooth number (SCTN), and ovariole number differed significantly among parental species and their hybrids of different generations in both the sexes. However, the levels of FA of different morphological traits were similar in parental species and their hybrids of different generations in males (except SCTN) and in females (except for WL and W/T ratio). These results are interpreted in terms of developmental stability as a function of a balance between the level of heterozygosity and the disruption of coadapted gene complexes.

Population genetics of Drosophila ananassae.

Drosophila ananassae Doleschall is a cosmopolitan and domestic species. It occupies a unique status among Drosophila species due to certain peculiarities in its genetic behaviour and is of common occurrence in India. Quantitative genetics of sexual and non-sexual traits provided evidence for genetic control of these traits. D. ananassae exhibits high level of chromosomal polymorphism in its natural populations. Indian natural populations of D. ananassae show geographic differentiation of inversion polymorphism due to their adaptation to varying environments and natural selection operates to maintain three cosmopolitan inversions. Populations do not show divergence on temporal scale, an evidence for rigid polymorphism. D. ananassae populations show substantial degree of sub-structuring and exist as semi-isolated populations. Gene flow is low despite co-transportation with human goods. There is persistence of cosmopolitan inversions when populations are transferred to laboratory conditions, which suggests that heterotic buffering is associated with these inversions in D. ananassae. Populations collected from similar environmental conditions that initially show high degree of genetic similarity have diverged to different degrees in laboratory environment. This randomness could be due to genetic drift. Interracial hybridization does not lead to breakdown of heterosis associated with cosmopolitan inversions, which shows that there is lack of genetic co-adaptation in D. ananassae. Linkage disequilibrium between independent inversions in laboratory populations has often been observed, which is likely to be due to suppression of crossing-over and random genetic drift. No evidence for chromosomal interactions has been found in natural and laboratory populations of D. ananassae. This strengthens the previous suggestion that there is lack of genetic co-adaptation in D. ananassae.

Effect of mutations on developmental stability and canalization in morphological traits in Drosophila ananassae.

The present study was designed to determine the effects of visible mutations of large effect on developmental stability and canalization in different morphological traits, namely, sternopleural bristle number, wing length, wing to thorax ratio, ovariole number, and sex comb tooth number (SCTN) in Drosophila ananassae. We have compared the mean trait size, fluctuating asymmetry (FA) (as an index of developmental stability), and morphological variation (as an index of canalization) of different mutant strains (yellow body color, y; claret eye color, ca; plexus wing, px; spread wing, spr; ebony body and sepia eye color, e se; yellow body and claret eye color, y ca; and cardinal eye color, curled wing, and ebony body color, cd cu e) with wild-type strain. The mean trait size of all morphological traits differs significantly among the wild-type and mutant strains. The wild-type and mutant strains vary significantly for the morphological variation and also for the levels of the FA in different morphological traits. However, we have found no increase in either the variance or in the degree of FA with the increase of the mutations (except in SCTN in y mutant). The plausible reasons for the variation in wild-type and mutant strains with particular reference to developmental stability and canalization have been discussed.

Coexistence of three different Drosophila species by rescheduling their life history traits in a natural population.

We present evidence for coexistence of three different Drosophila species by rescheduling their life history traits in a natural population using the same resource, at the same time and same place. D. ananassae has faster larval development time (DT) and faster DT(egg-fly) than other two species thus utilizing the resources at maximum at both larval and adult stages respectively. Therefore, D. ananassae skips the interspecific competition at pre-adult stage but suffers more from intraspecific competition. However, D. melanogaster and D. biarmipes have rescheduled their various life history traits to avoid interspecific competition. Differences of ranks tests for various life history traits suggest that except for DT(egg-pupa), the difference of ranks is highest for the combination of D. melanogaster and D. ananassae for all other life history traits. This difference is maintained by tradeoffs between larval development time and pupal period and between pupal period and DT(egg-pupa) in D. ananassae.