Drosophila Saltans Research Articles

Dissection of signalling modalities and courtship timing reveals a novel signal in Drosophila saltans courtship

Courtship signalling, necessary for the recognition of potential mates, is often complex, using many modalities with multiple components. Drosophila courtship comprises chemical, tactile, visual and acoustic stimuli. Ablation of single sensory channels, either signal production or reception, can determine the roles of individual modalities in overall reproductive success. Adding measures of courtship timing, particularly courtship latency, the time for the male to initiate courtship, and courtship duration, the time from courtship initiation until the female accepts the male for copulation, allows us to identify the stage of courtship at which a signal acts. This study focuses on Drosophila saltans, a member of the saltans species group. Little is known about sexual behaviour of species in this group, part of the Sophophora subgenus. In the present study, we found that ablation of any one signal in D. saltans did not eliminate mating, thus courtship is multimodal. In addition to examining the signals and signal reception common to most Drosophila species, we also examined the role that the midtarsi play in courtship. The removal of the female's midtarsi significantly reduced mating occurrence. Using timing data, we discerned that the absence of midtarsi in the female did not affect the male's ability to identify the female as a potential mate, but the male may be unable to sufficiently stimulate the female to copulation. Measuring courtship latency and courtship duration, as well as the occurrence of courtship and copulation, allowed us to determine whether a particular signal plays a role in activating the male to initiate courtship or stimulating the female to mate.

Scanning electron microscopy of male terminalia and its application to species recognition and phylogenetic reconstruction in the Drosophila saltans group.

The Drosophila saltans group consists of five subgroups and 21 species, most of which have been identified only by morphological aspects of the male terminalia revealed by drawings using a camera lucida and a bright-field microscope. However, several species in the group, mainly those included in the saltans subgroup, are difficult to differentiate using only these characteristics. In this study, we used scanning electron microscopy (SEM) to analyze 19 structures of the male terminalia in 10 species from the five saltans subgroups. Among these structures, nine could be identified only through SEM analysis. We aimed to find other characteristics useful for morphological recognition of these species and to use these characteristics for phylogenetic reconstruction. These morphological differences enabled us to effectively distinguish among sibling species. These findings confirmed the monophyly of this group as previously determined in evolutionary studies based on other markers. The single most parsimonious tree (CI = 87 and RI = 90) indicated that the cordata subgroup is the most basal lineage and the saltans subgroup is the most apical lineage, as shown in earlier studies based on morphological data. However, our findings differed somewhat from these studies with respect to the phylogenetic relationships of species in the saltans group indicating that this group is still a puzzle that remains to be deciphered.

Phylogenetic Relationships Among Species Subgroups in the <I>Drosophila saltans</I> Group (Diptera: Drosophilidae):Can Morphology Solve a Molecular Conflict?

Proper phylogenetic reconstruction is crucial for understanding many evolutionary phenomena. In spite of the great success of molecular phylogenetics, DNA signal still may be limited by some intrinsic constraints such as codon usage bias. The phylogenetic relationships between the five species subgroups of the Drosophila saltans group are a good example of conflicting molecular phylogenies drawn from different genes due to an ancestral substitutional shift. Here, forty morphological characters were analyzed using the same set of species used in previous molecular studies, with at least a single representative of each subgroup. The cladistic analysis was in disagreement with most of the previous hypotheses in placing the sturtevanti subgroup as an early branch, whereas the four remaining subgroups form a well supported clade that can be further subdivided into two sister clades: one containing the cordata and the elliptica subgroups, whereas the second includes the parasaltans and the saltans subgroups. The molecular evolution (codon usage bias) of the saltans group were revised in light of the present finding. The analysis highlights the important role of morphology in phylogeny reconstruction and in understanding molecular evolutionary phenomena.

A New Nucleolar Body Appears inDrosophila saltansSalivary Gland Cells Before Histolysis, in Programmed Cell Death

The salivary glands of Drosophila saltans (saltans group, saltans subgroup) analyzed in an advanced stage of programmed cell death showed the appearance of a single, round, nucleolar corpuscle inside the highly altered nucleus of every gland cell, at a time during which the integrity of the original nucleolus was already lost and the original nucleolar material apparently disappeared. In the same nuclei, which already had also lost the characteristic chromosome structure, some delicate chromosome threads were maintained. In many cells, the new nucleolar corpuscle and these chromosome threads are associated. These findings are novel. However, the hypothesis put forward concerning their meaning remains dependent on other studies.

Minor shift in background substitutional patterns in the Drosophila saltans and willistoni lineages is insufficient to explain GC content of coding sequences

BackgroundSeveral lines of evidence suggest that codon usage in the Drosophila saltans and D. willistoni lineages has shifted towards a less frequent use of GC-ending codons. Introns in these lineages show a parallel shift toward a lower GC content. These patterns have been alternatively ascribed to either a shift in mutational patterns or changes in the definition of preferred and unpreferred codons in these lineages.Results and discussionTo gain additional insight into this question, we quantified background substitutional patterns in the saltans/willistoni group using inactive copies of a novel, Q-like retrotransposable element. We demonstrate that the pattern of background substitutions in the saltans/willistoni lineage has shifted to a significant degree, primarily due to changes in mutational biases. These differences predict a lower equilibrium GC content in the genomes of the saltans/willistoni species compared with that in the D. melanogaster species group. The magnitude of the difference can readily account for changes in intronic GC content, but it appears insufficient to explain changes in codon usage within the saltans/willistoni lineage.ConclusionWe suggest that the observed changes in codon usage in the saltans/willistoni clade reflects either lineage-specific changes in the definitions of preferred and unpreferred codons, or a weaker selective pressure on codon bias in this lineage.

Further Study on the Esterase Patterns of Sibling Species in the Drosophila saltans Subgroup (saltans Group): Intraspecific and Interspecific Variations in the Development

Twenty of the 32 esterase bands previously detected in the adults of D. prosaltans, D. saltans and D. austrosaltans were found in larvae and pupae studied in this work. The results showed that, in addition to expressing the highest number of esterase bands, the adult stage of the three species exhibited the highest degree of expression (amount of synthesis) for most of the bands. Differences between larval and pupal stages were detected in the degree of expression (amount of synthesis) of the bands and in the frequency of samples expressing them. The frequencies of expression of the bands corresponding to genes in loci 1-3 were greater in pupae than in larvae while the frequencies of expression of the bands corresponding to genes in loci 4-9 were predominantly expressed in larvae or were equal in both developmental stages. Like the adults, larvae, pupae and empty pupal cases (which were also studied in this work) showed specific esterases. Taken together, the observations showed that, in the species studied, every developmental stage is characterized by specific bands and by specific frequency and degree of expression of the bands shared with other stages.

Distribution and insertion numbers of transposable elements in species of the Drosophila saltans group

Abstract Information about the distribution and insertion numbers of many transposable elements is restricted to few speciesof Drosophila , although these elements are widely distributed throughout the genus . The aim of this work was to de-scribe the distribution and insertion numbers of four retrotransposons ( copia , gypsy , micropia , I ) and four trans-posons ( hobo , mariner , Minos and Bari-1 )inthe saltans group of Drosophila. Our data shows that, except for mariner , all the other elements are widespread within the saltans group and show variable insertion numbers of up to24 copies. Key words: Drosophila , genomic insertion, saltans group, retrotransposons, transposons.Received: March 11, 2005; Accepted: September 20, 2005. Introduction Transposable elements are a major component of thegenomes of most species and are widespread throughoutthe genus Drosophila. Most of our knowledge on trans-posable elements in Drosophila comes from studies car-ried out in natural populations of

Tree Rooting with Outgroups When They Differ in Their Nucleotide Composition from the Ingroup: The Drosophila saltans and willistoni Groups, a Case Study

Rooting is frequently the most precarious step in any phylogenetic analysis. Outgroups can become useless for rooting if they are too distantly related to the ingroup. Specifically, little attention has been paid to scenarios where outgroups have evolved different nucleotide frequencies from the ingroup. We investigate one empirical example that arose seeking to determine the phylogenetic relationship between the saltans and the willistoni groups of Drosophila (subgenus Sophophora). We have analyzed 2085 coding nucleotides from the xanthine dehydrogenase (Xdh) gene in 14 species, 6 from the saltans group and 8 from the willistoni group. We adopt a two-step strategy: (1) we investigate the phylogeny without outgroups, rooting the network by the midpoint method; (2) we reinvestigate the rooting of this phylogeny using predefined outgroups in both a parsimony- and a model-based maximum-likelihood framework. A satisfactory description of the substitution process along the Xdh region calls for six substitution types and substitution rate variation among codon positions. When the ingroup sequences are considered alone, the phylogeny obtained using this description corroborates the known relationships derived from anatomical criteria. Inclusion of the outgroups makes the root unstable, apparently because of differences between ingroups and outgroups in the substitution processes; these differences are better accounted for by a simplified model of evolution than by more complex, realistic descriptions of the substitution process.

Fluctuating mutation bias and the evolution of base composition in Drosophila.

The idea that the pattern of point mutation in Drosophila has remained constant during the evolution of the genus has recently been challenged. A study of the nucleotide composition focused on the Drosophila saltans group has evidenced unsuspected nucleotide composition differences among lineages. Compositional differences are associated with an accelerated rate of amino acid replacement in functionally less constrained regions. Here we reassess this issue from a different perspective. Adopting a maximum-likelihood estimation approach, we focus on the different predictions that mutation and selection make about the nonsynonymous-to-synonymous rate ratio. We investigate two gene regions, alcohol dehydrogenase (Adh) and xanthine dehydrogenase (Xdh), using a balanced data set that comprises representatives from the melangaster, obscura, saltans, and willistoni groups. We also consider representatives of the Hawaiian picture-winged group. These Hawaiian species are known to have experienced repeated bottlenecks and are included as a reference for comparison. Our results confirm patterns previously detected. The branch ancestral to the fast-evolving willistoni/saltans lineage, where most of the change in GC content has occurred, exhibits an excess of synonymous substitutions. The shift in mutation bias has affected the extent of the rate variation among sites in Xdh.

Molecular Evolution and Phylogeny of the Drosophila saltans Species Group Inferred from the Xdh Gene

The Drosophila saltans group of the subgenus Sophophora consists of five species subgroups whose phylogenetic relationships are poorly known. We have analyzed 2085 coding nucleotides from the xanthine dehydrogenase (Xdh) gene in six species, at least one from each subgroup. We follow a model-based maximum likelihood framework. We first model the substitution process using a tree topology that is approximately accurate. Then we evaluate several candidate tree topologies using a working model of nucleotide substitution. We found that a minimally realistic description of the substitution process along the Xdh region should allow two transition and four transversion rate parameters and different fixed rates for codon positions, which are distributed statistically according to different gamma distributions. The phylogeny obtained using this description differs in significant respects from a phylogeny based on anatomical criteria. We have also analyzed data from five additional (three nuclear and two mitochondrial) gene regions. In our analysis, these relatively short DNA sequences, either separately or jointly, fail to discriminate statistically among alternative phylogenies. When the data for these five gene regions are combined with the Xdh sequences, the strong phylogenetic signal emerging from Xdh becomes somewhat diluted rather than reinforced. The phylogeny of the species and biogeographical considerations suggest that the D. saltans group originated in the tropics of the New World, similarly as the closely related D. willistoni group.