Variation of wing shape in the Drosophila virilis species group (Diptera: Drosophilidae)

Wing shape variation was analysed with geometric morphometric methods in 17 laboratory strains, representing 11 closely related species (including two subspecies) of the Drosophila virilis group: D. virilis, D. lummei, D. novamexicana, D. americana americana, D. americana texana, D. montana, D. lacicola, D. flavomontana, D. borealis, D. littoralis, D. ezoana and D. kanekoi. Overall shape estimated using Procrustes coordinates of 14 landmarks was highly variable among strains and very similar in females and males. The landmarks in the distal part of the wing showed higher variation across strains than those in the proximal part. Procrustes distances between species were not consistent with phylogenetic distances previously suggested for the virilis group. Moreover, Procrustes distances between strains within species and within two major phylads (virilis and montana) were comparable with those between species and between phylads, respectively. The most different from other members of the group was the endemic D. kanekoi species, currently viewed as separate subphylad within the montana phylad. Allometric effects were found to be partly responsible for shape differences between the strains. Three most significant shape transformations were considered using the relative warp analysis and the strains were ordinated in accordance with transformation values. The pattern of relative warp scores could be easily interpreted only for the third warp explaining about 13% of shape variation. It separated the largest species, D. montana, D. ezoana and D. kanekoi, from other ones and was mainly associated with shape changes in the proximal region of the wing. The results of the present work suggest that wing shape in the virilis species group is not related to the speciation process. The observed proximal‐distal contrasts and allometric effects are in agreement with data of other studies, in which wing shape variation was analysed within Drosophila species.     

Isolation and characterization of microsatellites in Drosophila montana and their cross‐species amplification in D. virilis

We report the isolation of 20 microsatellite loci from Drosophila montana and their cross amplification in the relative D. virilis. All microsatellite loci were polymorphic in the focal species D. montana, with gene diversities ranging from 0.23 to 0.93. In D. virilis only eight loci (40%) amplified and two loci were polymorphic (10%). These markers represent the first report of microsatellites isolated in D. montana. They could be applied for studying population structure and phylogeography. The largest benefit, however, will be their use in studies of quantitative trait loci, such as the mapping of behavioural quantitative trait loci.     

Morphological analysis of male mating organ in the Drosophila virilis species group: a multivariate approach

The shape of the male mating organ differs among 11 closely related species of the Drosophila virilis species group. Multivariate analyses of variation of a suite of 35 morphological traits (indices) describing the phallus shape were carried out in order to characterize interspecies variability of the traits. An overwhelming majority of the traits displayed species‐specific variability. The main result of the investigation was the revelation of the differences involved in the traits studied in the evolution of the D. virilis species group. The structure of species‐specific variability of some traits was discovered to be in accordance with the generally accepted taxonomy of the species group, while that of other traits required isolation of D. virilisper se from lummei phylad (former virilis phylad), and confirmed separation of montana phylad into three subphylads: montana proper, littoralis and kanekoi. Several subsets of traits having separate variability were determined in different parts of the male mating organ which correspond to spots of evolutionarily significant variability.     

The DAIBAM MITE element is involved in the origin of one fixed and two polymorphic Drosophila virilis phylad inversions

Chromosomal inversions can originate from breakage and repair by non-homologous end-joining. Nevertheless, they can also originate from ectopic recombination between transposable elements located on the same chromosome inserted in opposite orientations. Here, we show that a MITE element (DAIBAM), previously involved in the origin of one Drosophila americana polymorphic inversion, is also involved in the origin of one fixed inversion between D. virilis and D. americana and another D. americana polymorphic inversion. Therefore, DAIBAM is responsible for at least 20% of the chromosomal rearrangements that are observed within and between species of the virilis phylad (D. virilis, D. lummei, D. novamexicana and D. americana), having thus played a significant role in the chromosomal evolution of this group of closely related species.     

Exploration of potential microbial control agents for the invasive crayfish,Orconectes virilis

The introduced crayfish, Orconectes virilis (Cambarinae; Hagen 1870), has become a serious invasive species in Arizona (USA), altering stream ecosystems and contributing to the decline of native species. But because it is native to the eastern US, and related crayfish including endangered species inhabit nearby states, introduction of a biological control agent presents a unique challenge. This represents a feasibility study, in which we explored bacteria, nematodes, and a virus as potential biological control agents for O. virilis while avoiding harm to native species. White Spot Syndrome Virus (WSSV) from shrimp was found to be highly pathogenic and readily passed by cannibalistic behavior but not by water transmission. Potential activity of WSSV against representative non-target arthropods that may be found in the same ecosystems was also explored.     

Ecdysteroids in Stress Responsive and Nonresponsive Drosophila virilis Lines under Stress Conditions

After exposure to thermal stress or a control temperature, the relative abundance of ecdysone (E) and 20-hydroxyecdysone (20E) was measured in a wild-type line of Drosophila virilis (101) that is stress responsive and in a mutant line (147) that is not stress responsive. In line 101, the 20E content was higher and E content lower in females than in males. The abundance of E and 20E in females of line 147 was significantly higher than that in females of line 101. Females of line 101 were found to respond to 60 min of heat stress (38°C) by an increase in the abundance of both E and 20E, while in males of this line the amount of 20E increased and that of E declined. A role of the ecdysteroids in the control of reproduction of D. virilis under stress is discussed.     

Evolution of 5S ribosomal RNA genes in the chromosomes of the virilis group of Drosophila

Drosophila melanogaster 5S ribosomal RNA labeled with ¹²⁵I was used as an in situ hybridization probe to localize complementary sequences in chromosomes of species in the Drosophila virilis group. Whereas virilis, the ancestral species, has two different 5S gene loci, the derived species show only one of these loci; in the two lines that have evolved from virilis it is the opposite locus that is conserved. The possible events leading to such an arrangement are discussed.The author was a Predoctoral Fellow supported by Grant GM 1974 from the National Institute of General Medical Sciences, National Institutes of Health.Contribution from Oak Ridge National Laboratory, operated by the Union Carbide Corporation for the U.S. Energy Research and Development Administration.     

Analysis of DNAs from two species of the virilis group of Drosophila and implications for satellite DNA evolution

The DNAs from two virilis group species of Drosophila, D. lummei and D. kanekoi, have been analyzed. D. lummei DNA has a major satellite which, on the basis of CsCl equilibrium centrifugation, thermal denaturation, renaturation and in situ hybridization is identical to D. virilis satellite I. D. kanekoi DNA has a major satellite at the same buoyant density in neutral CsCl gradients as satellite III of D. virilis. However, on the basis of alkaline CsCl gradients, the satellite contains a major and a minor component, neither one of which is identical to D. virilis satellite III. By in situ hybridization experiments, sequences complementary to the major component of the D. kanekoi satellite are detected in only some species and in a way not consistent with the phylogeny of the group. However, by filter hybridization experiments using nick-translated D. kanekoi satellite as well as D. lummei satellite I and D. virilis satellite III DNAs as probes, homologous sequences are detected in the DNAs of all virilis group species. Surprisingly, sequences homologous to these satellite DNAs are detected in DNAs from non-virilis group Drosophila species as well as from yeast, sea urchin, Xenopus and mouse.     

Distribution and evolution of mobile elements in the virilis species group of Drosophila

The distributions of Penelope and Ulysses, two transposable elements that can induce hybrid dysgenesis, were studied in several species groups of Drosophila. No significant hybridization to Penelope and Ulysses probes was detected by Southern blot analyses of species outside the virilis group. In contrast, both element families have had a long residence in all species of the virilis species group, as indicated by their strong presence in the heterochromatic chromocenter. Except for D. kanekoi, D. lummei, and some strains of D. virilis, species of the group carry full-sized, and at least potentially functional, copies of both element families. Consistent with the occurrence of recent transposition, Penelope and Ulysses elements are located at different chromosomal sites in different geographical strains of the same species. A total of 79 Penelope and 47 Ulysses euchromatic insertion sites were localized to chromosomal subsections in species of the virilis group. Highly significant deviations from independence of the distributions of Penelope and Ulysses and previously established inversion breakpoints were documented, suggesting that these transposable elements may have played an important role in genomic reorganization and evolution of the virilis species group, which is especially rich in karyotypic variation. Received: 13 April 1999; in revised form: 20 July 1999 / Accepted: 27 July 1999     

Detection and location of three simple sequence DNAs in polytene chromosomes from virilis group species of Drosophila

In vitro synthesized RNAs complementary to the three satellite DNAs of Drosophila virilis have been used in a series of in situ hybridization experiments with polytene chromosomes from virilis group species. Gall and Atherton (1974) demonstrated that each of the satellites of D. virilis is comprised of many repeats of a distinct, seven base pair long, simple sequence. With few exceptions, copies of each of these simple sequences are detected in the chromocenters of all virilis group species. This is true even in species which do not possess satellite DNAs at buoyant densities corresponding to those of the satellite DNAs of D. virilis. Small quantities of the three simple sequences are also detected in euchromatic arms of several different species. The same euchromatic location may contain detectable copies of one, two, or all three simple sequence DNAs. The amounts of simple sequences at each location in the euchromatin may vary between species, between different stocks of the same species, and even between individuals of the same stock. The simple sequences located in the euchromatin appear to undergo DNA replication during formation of polytene chromosomes unlike those in heterochromatin. The locations of the euchromatic sequences are not the results of single chromosomal inversion events involving heterochromatic and euchromatic breakpoints.     

Interspecies comparison of a gene pair with partially redundant function: the rst and kirre genes in D. virilis and D. melanogaster

Abstract The D. melanogaster rst and kirre genes encode two highly related immunoglobulin-like cell adhesion molecules that function redundantly during embryonic muscle development. The two genes appear to be derived from a common ancestor by gene duplication. Gene duplications have been proposed to be of major evolutionary significance since duplicated redundant sequences can accumulate mutations without detrimental effects for the organism and leave the duplicated genes free to assume novel functions. To address the issue of conservation of the duplicated sequences and their putative redundancy, as well as to identify putative functional divergence of the paralogs during drosophilid evolution, we performed an interspecies comparison of the rst and kirre genes from D. virilis and D. melanogaster. The D. virilis genome contains orthologues of both rst and kirre and hence the duplication took place before the split of the two lineages and has subsequently been conserved. However, whilst the Rst orthologues show a high degree of sequence similarity, this similarity is lower in Kirre orthologues. Especially the intracellular domains of D. virilis and D. melanogaster Kirre sequences are highly divergent: the D. virilis kirre gene lacks the 3′-most exon present in D. melanogaster, which contains motifs conserved between kirre and rst in D. melanogaster. Hence, while each of the two genes is highly conserved at the level of its exon-intron organization, the selection forces acting on the rst and kirre coding sequences are different. These findings are discussed in the light of general evolutionary mechanisms.     

Morphometric Analysis of Male Genitalia in Sibling Species of Drosophila virilis Sturt.

The structure of male genital organs in sibling species of the virilis group of Drosophila was examined using methods of multivariate statistics. The differences among these species were estimated using 33 indices and 2 angle parameters. The intraspecific and interspecific correlation structure of the examined characters and the order of their divergence were established. The key characters with respect to forming interspecific differences in the virilis species group were identified. Based on these results, the relative systematic positions of the sibling species are discussed as well as similarities and differences of the pattern of relationships among the species from that generally accepted for the virilis group.     

Nucleotide and repeat length variation at the nonA gene of the Drosophila virilis group species and its effects on male courtship song

Genes found to affect male courtship song characters in Drosophila melanogaster are good candidates when tracing genes responsible for species-specific songs in other Drosophila species. It has previously been shown that Thr–Gly repeat length variation at the period gene affects song traits in D. melanogaster, which gives the repetitive regions a special interest. In this work, we have characterised the patterns of nucleotide variation for gene regions containing two Gly and one Gln–Ala repeat in another D. melanogaster song gene, no-on-transient A, in D. virilis group species. The levels of nucleotide variability in D. virilis nonA were similar to those found for other genes of the species, and the gene sequences showed no signs of deviation from neutrality. The Gly 2 repeat preceding the central domain of the gene exhibited length variation, which did not, however, correlate with song variation either within D. virilis or between the species of D. virilis group. The Gly 3 repeat located on the other side of the central domain showed amino acid divergence parallel to the consensus phylogeny of the D. virilis group species. The species of the virilis subgroup having Asn after the first three glycines in this repeat have simple songs with no species-specificity, while the species of the montana subgroup having two Gly or Asn–Ser in this site have unique courtship songs. Amino acid differences between the species in this repeat may, however, reflect species phylogeny rather than have an effect on song divergence per se.     

Vitellogenetic defects in hybrids of the species pair Drosophila virilis and Drosophila texana

In interspecific matings between the species Drosophila virilis and Drosophila texana, female sterility can be observed in F₂ backcross females and in F₂ hybrid females. The results presented in this report show that the female sterility, whenever it exists, is due to prevention of vitellogenin synthesis in the fat body, but other abnormalities such as defects with the hybrid ovaries are not excluded. The observation that sterility appears among females from backcrosses suggests that incompatibilities between interspecific genes may cause female sterility even in the presence of a complete habloid genome from one or the other species. Yet, the parallel observation that female sterility appears only in hybrid females with recombinant chromosomes indicates that sterility results when conspecific combinations of genes on the same chromosome are broken by interspecific recombination. © 1996 Wiley-Liss, Inc.     

Strategic differences in thermal adaptation between two Drosophila species,D. virilis and D. immigrans

Summary Preadult viability and developmental time at four different temperatures, heat and cold resistances of adult flies, effects of acclimatization on heat resistance, and preferred temperature of adult flies were compared between two species of Drosophila, D. virilis and D. immigrans. Four Japanese local populations were surveyed for each species. As compared with immigrans, virilis was higher in its ability to tolerate both heat and cold stresses and was viable over a broader temperature range. On the other hand, immigrans revealed a superior ability to acclimatize and a rigid preference for gradually changing thermal environment. Differences between geographical populations are remarkable for heat tolerance in virilis and cold tolerance in immigrans. In conclusion, thermal adaptation of virilis seems to be based on the high tolerance to extreme temperatures and that of immigrans mainly on the behavioural preference for viable temperatures.     

Interspecies variability of number of bristles on dorsal surface of aedeagus in D. virilis species group and its genetic mapping with interspecies hybrids of D. virilis and D. lummei

The results of morphologic and hybrid analyses of the feature of the male reproductive system of sibling species in the virilis group were presented. Bristles appeared on the surfaces of male genitals (aedeagus). The occurrence of a specific expression of the examined feature in the phyllades of D. virilis group, the correspondence of both the number and distribution pattern of the bristles on surfaces of the aedeagus and developmental temperature in D. virilis and D. lummei, as well as the link between feature and sexual behavior, have been shown. Dominance of D. lummei phenotype in the interspecies D. virilis × D. lummei was found. The interspecies hybrids D. virilis and D. lummei were used for a genetic analysis of the variability of the examined feature. The significant influence of chromosomes 2 and 6 on the number of bristles on the aedeagus in hybrid males was shown. Furthermore, the correspondence between the effects of the autosomes 2 and 6 on the variability of the examined feature and the genetic status of the other chromosomes (the effect of interaction between genetic factors, chromosomes here) was revealed. The adaptive value of the examined feature related to the involvement in the formation of isolating barriers at the copulation stage is under discussion.     

The Importance of Acoustic Signals in Multimodal Courtship Behavior in Drosophila virilis,D. lummei and D. littoralis

The courtship rituals of Drosophila include an exchange of several signals with different modalities, chemical, visual, acoustic and tactile stimuli, between sexes. Using a video recording method, we studied the role of acoustic communication in courtship behavior in three species of the Drosophila virilis group, D. virilis, D. lummei and two populations of D. littoralis. Five series of experiments were performed: tests with intact flies (control), tests with mute flies (wingless males or females), and tests with deaf flies (aristaless males or females). We distinguished the two situations: either a female did not hear a male or vice versa, males did not hear females. When females did not hear males, the reduction in the copulation number was found in D. virilis and both populations of D. littoralis, but not in D. lummei. When males did not hear females, the reduction in the copulation number was only found in D. littoralis. The ablation of the male aristae in D. virilis and D. lummei even increased the mating success as compared to the control, which may be explained by the ‘sensory overload’ hypothesis. The changes in courtship temporal structure usually included the delayed onset of the main courtship elements (tapping, licking, and singing), and the variation in their duration and the total time of courtship. These effects were, however, more substantial in D. virilis and both populations of D. littoralis than in D. lummei. Thus, the effect of blocking the acoustic channel was different in the three species regardless of their phylogenetic relationship, and the role of acoustic communication in courtship behavior seemed to increase in the order D. lummei – D. virilis – D. littoralis.

     

Mitochondrial DNA Polymorphism in Natural Populations of the Drosophila virilis Species Group

Restriction fragment length polymorphism (RFLP) analysis has been used to evaluate mitochondrial DNA (mtDNA) variation in 12 sibling species forming the Drosophila virilis species group. The variation thresholds corresponding to the interspecific and interstrain levels have been determined. The results indicate that interspecific hybridization has significantly contributed to the evolutionary history of the virilis species group.     

Unusual prophase structures and multiple nucleoli in male meiosis of Drosophila species of the virilis group

With silver nitrate (Ag-NOR) staining, unusual fibrillar structures, apparently coupled to the nucleolus, were found is several species of the D. virilis group. In D. littoralis, beaded strings appear in connection with these structures, whereas the late prophase is characterized by the appearance of multiple nucleoli in the nucleoplasm. In D. virilis, the nucleus has a prominent pointed protrusion in the region of the nucleolus and often a fibril protrudes from this point. Small nucleoli are budding from the nucleolus during prophase. The multiple nucleoli at late prophase are smaller and fewer. A nucleolar body with black spots appears at prometaphase and persists through metaphase and anaphase. In D. lummei, the nucleolus becomes surrounded by fibrils, which are released into the nucleoplasm and on which multiple nucleoli are synthesized.These phenomena are similar to the events described in oocyte meiosis of many animals, where rDNA amplification, coupled to the synthesis of multiple nucleoli in late prophase, has been established.