Sequence and evolution of the Drosophila pseudoobscura glycerol-3-phosphate dehydrogenase locus

The Gpdh genomic region has been cloned and sequenced in Drosophila pseudoobscura. A total of 6.8 kb of sequence was obtained, encompassing all eight exons of the gene. The exons have been aligned with the sequence from D. melanogaster, and the rates of synonymous and nonsynonymous substitution have been compared to those of other genes sequenced in these two species. Gpdh has the lowest rate of nonsynonymous substitution yet seen in genes sequenced in both D. pseudoobscura and D. melanogaster. No insertion/deletion events were observed, and the overall architecture of the gene (i.e., intron sites, etc.) is conserved. An interesting amino acid reversal was noted between the D. melanogaster Fast allele and the D. pseudoobscura gene.     

Evolutionary History and Mode of the <Emphasis Type="Italic">amylase</Emphasis> Multigene Family in <Emphasis Type="Italic">Drosophila</Emphasis>

Previous studies indicate that the tandemly repeated members of the amylase (Amy) gene family evolved in a concerted manner in the melanogaster subgroup and in some other species. In this paper, we analyzed all of the 49 active and complete Amy gene sequences in Drosophila, mostly from subgenus Sophophora. Phylogenetic analysis indicated that the two types of diverged Amy genes in the Drosophila montium subgroup and Drosophila ananassae, which are located in distant chromosomal regions from each other, originated independently in different evolutionary lineages of the melanogaster group after the split of the obscura and melanogaster groups. One of the two clusters was lost after duplication in the melanogaster subgroup. Given the time, 24.9 mya, of divergence between the obscura and the melanogaster groups (Russo et al. 1995), the two duplication events were estimated to occur at about 13.96 ± 1.93 and 12.38 ± 1.76 mya in the montium subgroup and D. ananassae, respectively. An accelerated rate of amino acid changes was not observed in either lineage after these gene duplications. However, the G+C contents at the third codon positions (GC3) decreased significantly along one of the two Amy clusters both in the montium subgroup and in D. ananassae right after gene duplication. Furthermore, one of the two types of the Amy genes with a lower GC3 content has lost a specific regulatory element within the montium subgroup species and D. ananassae. While the tandemly repeated members evolved in a concerted manner, the two types of diverged Amy genes in Drosophila experienced frequent gene duplication, gene loss, and divergent evolution following the model of a birth-and-death process.     

Structural and genetic studies of the proliferation disrupter genes of Drosophila simulans and D. melanogaster

To examine whether structural and functional differences exist in the proliferation disrupter (prod) genes between Drosophila simulans and D. melanogaster, we analyzed and compared both genes. The exon–intron structure of the genes was found to be the same – three exons were interrupted by two introns, although a previous report suggested that only one intron existed in D. melanogaster. The prod genes of D. simulans and D. melanogaster both turn out to encode 346 amino acids, not 301 as previously reported for D. melanogaster. The numbers of nucleotide substitutions in the prod genes was 0.0747 ±  per synonymous site and 0.0116 ± 0.0039 per replacement site, both comparable to those previously known for homologous genes between D. simulans and D. melanogaster. Genetic analysis demonstrated that D. simulans PROD can compensate for a deficiency of D. melanogaster PROD in hybrids. The PRODs of D. simulans and D. melanogaster presumably share the same function and a conserved working mechanism. The prod gene showed no significant interaction with the lethality of the male hybrid between these species. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Drosophila subobscura Adh genomic region contains valuable evolutionary markers.

We have sequenced 4 kb of the genomic region comprising the Adh (Alcohol dehydrogenase) gene of Drosophila subobscura. In agreement with other species which belong to the same subgenus, two structural genes, Adh and Adh-dup, are contained in this region. The main features of these two genes of D. subobscura have been inferred from the sequence data and compared with the homologous region of D. ambigua and D. pseudoobscura. Drosophila subobscura Adh and Adh-dup differ from those of D. ambigua at a corrected estimation of 10.1% and 12.5%, respectively, while from those of D. pseudoobscura they differ by 9.5% and 8.1%, respectively. Our data suggest that Adh and Adh-dup are evolving independently, showing a species-specific pattern. Moreover, particular features of some regions of these genes make them valuable evolutionary hallmarks. For instance, replacement substitutions in the third exon of Adh may indicate the branching of the melanogaster-obscura groups, whereas replacement substitutions in the third exon of the Adh-dup could be used to assess speciation within the obscura group.     

Protein Evolutionary Rates Correlate with Expression Independently of Synonymous Substitutions in Helicobacter pylori

In free-living microorganisms, such as Escherichia coli and Saccharomyces cerevisiae, both synonymous and nonsynonymous substitution frequencies correlate with expression levels. Here, we have tested the hypothesis that the correlation between amino acid substitution rates and expression is a by-product of selection for codon bias and translational efficiency in highly expressed genes. To this end, we have examined the correlation between protein evolutionary rates and expression in the human gastric pathogen Helicobacter pylori, where the absence of selection on synonymous sites enables the two types of substitutions to be uncoupled. The results revealed a statistically significant negative correlation between expression levels and nonsynonymous substitutions in both H. pylori and E. coli. We also found that neighboring genes located on the same, but not on opposite strands, evolve at significantly more similar rates than random gene pairs, as expected by co-expression of genes located in the same operon. However, the two species differ in that synonymous substitutions show a strand-specific pattern in E. coli, whereas the weak similarity in synonymous substitutions for neighbors in H. pylori is independent of gene orientation. These results suggest a direct influence of expression levels on nonsynonymous substitution frequencies independent of codon bias and selective constraints on synonymous sites. Electronic Supplementary Material Electronic Supplementary material is available for this article at and accessible for authorised users. [Reviewing Editor: Dr. Nicolas Galtier]     

Distribution of the Transposable Element Minos in the Genus Drosophila

We analyzed 28 species of the genus Drosophilafor the presence of the Tc1-like transposable element Minosusing Southern blot hybridization under high stringency conditions. The Minostransposon was found in members of both the Drosophilaand the Sophophorasubgenus showing a distribution that is wider if compared to other well-studied Drosophilatransposons such as the Pelement, hoboand mariner. The presence of Minos-hybridizing sequences was discontinuous in the Sophophorasubgenus, especially in the melanogasterspecies group. Using the Polymerase Chain Reaction we amplified a portion corresponding to the putative Minostransposase from different Drosophilaspecies. Cloning and sequence analysis of randomly selected Minoscopies from D. mojavensisis, D. saltansand D. willistonisupports the idea that event(s) of horizontal transfer may have contributed to the spreading of this transposon in the Drosophilagenus. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlated Evolution of Synonymous and Nonsynonymous Sites in <Emphasis Type="Italic">Drosophila</Emphasis>

Recent work has shown that Drosophila melanogaster genes with fast-evolving nonsynonymous sites have lower codon usage bias. This pattern has been attributed to interference between positive selection at nonsynonymous sites and weak selection on codon usage. Here we have looked for this correlation in a much larger and less biased dataset, comprising 630 gene pairs from D. melanogaster and D. yakuba. We confirmed that there is a negative correlation between the rate of nonsynonymous substitutions (d_N) and codon bias in D. melanogaster. We then tested the interference hypothesis and other alternative explanations, including one involving gene expression. We found that d_N indeed correlates with the level of gene expression. Given that gene expression is a strong determinant of codon bias, the relationship between d_N and codon bias might be a by-product of gene expression. However, our tests show that none of the hypotheses we consider seem to explain the data fully.This article contains online supplementary material.Reviewing Editor: Dr. John Huelsenbeck     

Evolutionary differences between alternative and constitutive protein-coding regions of alternatively spliced genes of Drosophila

A total of 790 Drosophila melanogaster genes that are alternatively spliced in a coding region and have orthologs in Drosophila pseudoobscura were studied. It proved that nucleotide substitutions are accumulated in alternative coding regions more rapidly than in constitutive coding regions. Moreover, the evolutionary patterns of alternative regions differing in insertion-deletion mechanisms (use of alternative promoters, splicing sites, or polyadenylation sites) differ significantly. The synonymous substitution rate in coding regions of genes varies more strongly than the nonsynonymous substitution rate. The patterns of substitutions in different classes of alternative regions of Drosophila melanogaster and mammals differ considerably.     

A Genomic Comparison of Faster-Sex, Faster-X, and Faster-Male Evolution Between Drosophila melanogaster and Drosophila pseudoobscura

A genomic comparison of Drosophila melanogaster and Drosophila pseudoobscura provides a unique opportunity to investigate factors involved in sequence divergence. The chromosomal arrangements of these species include an autosomal segment in D. melanogaster which is homologous to part of the X chromosome in D. pseudoobscura. Using orthologues to calculate rates of nonsynonymous (d_N) substitutions, we found genes on the X chromosome to be significantly more diverged than those on the autosomes, but it is not true for segment 3L-XR which is autosomal in D. melanogaster (3L) and X-linked in D. pseudoobscura (XR). We also found that the median d_N values for genes having reproductive functions in either the male, the female, or both sexes are higher than those for sequences without reproductive function and even higher for sequences involved in male-specific function. These estimates of divergence for male sex-related sequences are most likely underestimates, as the very rapidly evolving reproductive genes would tend to lose homology sooner and thus not be included in the comparison of orthologues. We also noticed a high proportion of male reproductive genes among the othologous genes with the highest rates of d_N. Reproductive genes with and without an orthologue in D. pseudoobscura were compared among D. melanogaster, D. simulans, and D. yakuba and it was found that there were in fact higher rates of divergence in the group without a D. pseudoobscura orthologue. These results, from widely separated taxa, bolster the thesis that sexual system genes experience accelerated rates of change in comparison to nonsexual genes in evolution and speciation. [Reviewing Editor: Dr. Willie J. Swanson]     

Positive and Negative Selection in the β-Esterase Gene Cluster of the Drosophila melanogaster Subgroup

We examine the pattern of molecular evolution of the β-esterase gene cluster, including the Est-6 and ψEst-6 genes, in eight species of the Drosophila melanogaster subgroup. Using maximum likelihood estimates of nonsynonymous/synonymous rate ratios, we show that the majority of Est-6 sites evolves under strong (48% of sites) or moderate (50% of sites) negative selection and a minority of sites (1.5%) is under significant positive selection. Est-6 sites likely to be under positive selection are associated with increased intraspecific variability. One positively selected site is responsible for the EST-6 F/S allozyme polymorphism; the same site is responsible for the EST-6 functional divergence between species of the melanogaster subgroup. For ψEst-6 83.7% sites evolve under negative selection, 16% sites evolve neutrally, and 0.3% sites are under positive selection. The positively selected sites of ψEst-6 are located at the beginning and at the end of the gene, where there is reduced divergence between D. melanogaster and D. simulans; these regions of ψEst-6 could be involved in regulation or some other function. Branch-site-specific analysis shows that the evolution of the melanogaster subgroup underwent episodic positive selection. Collating the present data with previous results for the β-esterase genes, we propose that positive and negative selection are involved in a complex relationship that may be typical of the divergence of duplicate genes as one or both duplicates evolve a new function. [Reviewing Editor: Dr. Martin Kreitman]     

Molecular phylogeny of the subgenus Sophophora of Drosophila derived from large subunit of ribosomal RNA sequences

RNA sequencing has been used to assess the relationships among species of the subgenus Sophophora of the genus Drosophila. Two divergent domains, D1 and D2, of the large ribosomal RNA (28S), totalling 550 nucleotides have been sequenced using the rRNA direct sequencing method. A tree has been reconstructed from the neighbor-joining algorithm and the confidence intervals were evaluated by the bootstrap procedure. Results have shown that the branching of the willistoni and saltans groups of the subgenus Sophophora is very ancient and probably predates that of the subgenus Drosophila. The other groups and subgroups of Sophophora are clustered in three main lineages: 1) the melanogaster and oriental subgroups; 2) the montium subgroup; 3) the ananassae subgroup of the melanogaster group clustered with the fima and obscura groups. Thus, in comparison with our results, several taxa of various ranks appear paraphyletic (the genus Drosophila, the subgenus Sophophora and the melanogaster group). Our biochemical phylogeny is only in partial agreement with the pattern of Throckmorton's radiations as well as with classical taxonomy, both based on morphological data.     

ADH and phylogenetic relationships ofDrosophila lebanonensis (Scaptodrosophila)

Summary Increasing data onDrosophila alcohol dehydrogenase (ADH) sequences have made it possible to calculate the rate of amino acid replacement per year, which is 1.7×10^–9. This value makes this protein suitable for reconstructing phylogenetic relationships within the genus for those species for which no molecular data are available such asScaptodrosophila. The amino acid sequence ofDrosophila lebanonensis is compared to all of the already knownDrosophila ADHs, stressing the unique characteristic features of this protein such as the conservation of an initiating methionine at the N-terminus, the unique replacement of a glycine by an alanine at a very conserved position in the NAD domain of all dehydrogenases, the lack of a slowmigrating peptide, and the total conservation of the maximally hydrophilic peptide. The functional significance of these features is discussed.Although the percent amino acid identity of the ADH molecule inDrosophila decreases as the number of sequences compared increases, the conservation of residue type in terms of size and hydrophobocity for the ADH molecule is shown to be very high throughout the genusDrosophila. The distance matrix and parsimony methods used to establish the phylogenetic relationships ofD. lebanonensis show that the three subgenera,Scaptodrosophila, Drosophila, andSophophora separated at approximately the same time.     

Evidence for interspecific transfer of the transposable element mariner betweenDrosophila andZaprionus

Summary The transposable element mariner occurs widely in themelanogaster species group ofDrosophila. However, in drosophilids outside of themelanogaster species group, sequences showing strong DNA hybridization with mariner are found only in the genusZaprionus. the mariner sequence obtained fromZaprionus tuberculatus is 97% identical with that fromDrosophila mauritiana, a member of themelanogaster species subgroup, whereas a mariner sequence isolated fromDrosophila tsacasi is only 92% identical with that fromD. mauritiana. BecauseD. tsacasi is much more closely related toD. mauritiana than isZaprionus, the presence of mariner inZaprionus may result from horizontal transfer. In order to confirm lack of a close phylogenetic relationship between the genusZaprionus and themelanogaster species group, we compared the alcohol dehydrogenase (Adh) sequences among these species. The results show that the coding region of Adh is only 82% identical betweenZ. tuberculatus andD. mauritiana, as compared with 90% identical betweenD. tsacasi andD. mauritiana. Furthermore, the mariner gene phylogeny obtained by maximum likelihood and maximum parsimony analyses is discordant with the species phylogeny estimated by using the Adh genes. The only inconsistency in the mariner gene phylogeny is in the placement of theZaprionus mariner sequence, which clusters with mariner fromDrosophila teissieri andDrosophila yakuba in themelanogaster species subgroup. These results strongly suggest horizontal transfer.     

Chromosomal homologies betweenDrosophila lebanonensis andD. melanogaster determined by in situ hybridization

Twelve biotin-labelled recombinant DNA probes were hybridized to polytene chromosomes ofDrosophila melanogaster andD. lebanonesis. Probes were chosen in order to cover the whole chromosomal complement. Six probes correspond to known genes fromD. melanogaster (RpII215, H3–H4, MHC, hsp28/23, hsp83, hsp70), four probes are clones isolated from aD. subobscura library (Xdh, DsubS3, DsubG3, DsubG4) and the remaining two probes correspond to the Adh gene ofD. lebanonensis and to one sequence (262), not yet characterized, from the same species. The chromosomal homologies obtained from the in situ hybridization results allow us to determine that Muller's C and D chromosomal elements are fused in the karyotype ofD. lebanonensis and constitute the large metacentric chromosome. Single pericentric inversions in theE andB elements have generated the medium and small metacentric chromosomes, respectively. No great changes are detected in Muller'sA element, which remains acrocentric. The changes detected in the karyotypic evolution ofD. lebanonensis are frequently observed inDrosophila evolution, as deduced from chromosomal homologies of severalDrosophila species. The results are also consistent with Muller's proposal that chromosomal elements have been conserved during the evolution ofDrosophila.     

Association Between Levels of Coding Sequence Divergence and Gene Misregulation in <Emphasis Type="Italic">Drosophila</Emphasis> Male Hybrids

Previous studies have shown widespread conservation of gene expression levels between species of the Drosophila melanogaster subgroup as well as a positive correlation between coding sequence divergence and expression level divergence between species. Meanwhile, large-scale misregulation of gene expression level has been described in interspecific sterile hybrids between D. melanogaster, D. simulans, D. mauritiana, and D. sechellia. Using data from gene expression analysis involving D. simulans, D. melanogaster, and their hybrids, we observed a significant positive correlation between protein sequence divergence and gene expression differences between hybrids and their parental species. Furthermore, we demonstrate that underexpressed misregulated genes in hybrids are evolving more rapidly at the protein sequence level than nonmisregulated genes or overexpressed misregulated genes, highlighting the possible effects of sexual and natural selection as male-biased genes and nonessential genes are the principal gene categories affected by interspecific hybrid misregulation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Carlo G. Artieri and Wilfried Haerty contributed equally to this publication.     

Diversification and Independent Evolution of Troponin C Genes in Insects

Troponin C (TpnC), the calcium-binding subunit of the troponin regulatory complex in the muscle thin filament, is encoded by multiple genes in insects. To understand how TpnC genes have evolved, we characterized the gene number and structure in a number of insect species. The TpnC gene complement is five genes in Drosophilidae as previously reported for D. melanogaster. Gene structures are almost identical in D. pseudoobscura, D. suboboscura, and D. virilis. Developmental patterns of expression are also conserved in Drosophila subobscura and D. virilis. Similar, but not completely equivalent, TpnC gene repertoires have been identified in the Anopheles gambiae and Apis mellifera genomes. Insect TpnC sequences can be divided into three groups, allowing a systematic classification of newly identified genes. The pattern of expression of the Apis mellifera genes essentially agrees with the pattern in Drosophilidae, providing further functional support to the classification. A model for the evolution of the TpnC genes is proposed including the most likely pathway of insect TpnC diversification. Our results suggest that the rapid increase in number and sequence specialization of the adult Type III isoforms can be correlated with the evolution of the holometabolous mode of development and the acquisition of asynchronous indirect flight muscle function in insects. This evolutionarily specialization has probably been achieved independently in different insect orders.Reviewing Editor: Dr. Rüdiger Cerff     

A statistical analysis of nucleotide substitutions in the Drosophila Adh region reflects irregularities in molecular clocks

Substitutions rates are expected to be rather constant when a gene is compared between species. To analyze this feature, Ka/Ks ratios have been studied for Alcohol dehydrogenase (Adh) and Alcohol dehydrogenase duplication (Adh-dup) genes in Drosophila species. Adh Ka/Ks values are lower in intrasubgenus comparisons involving species of the Sophophora group than when these species are compared to the D. immigrans and S. lebanonensis, and this difference does not occur in the Adh-dup comparisons.     

Variation in Drosophila acetylcholinesterase

We examined the Canton-S strain of Drosophila melanogaster for electrophoretic variation of the enzyme acetylcholinesterase. The pattern of bands obtained (stained with acetylthiocholine) depended on age, sex, and tissue (i.e., head vs. body part and hemolymph). However, through mixing experiments, it was concluded that most of these apparent differences were due to modification of the enzyme by unknown substances located in the fly's body. The electrophoretic pattern of head acetylcholinesterase was altered so that it became characteristic of the body which was present during extraction. For example, when heads of D. melanogaster were homogenized in an extract from D. lebanonensis bodies, the characteristic AChse bands of melanogaster were absent and instead the bands of lebanonensis were found. it was found that extraction of adult heads in 0.1 M potassium phosphate buffer alone or with a 2-min exposure to 1 mg/ml trypsin at 20 C gave the most reproducible results, independent of age and sex. Using these conditions, 25 strains of D. melanogaster and 30 strains of D. pseudoobscura were examined without finding any reproducible electrophoretic variant of acetylcholinesterase. In addition, 53 strains from 39 other Drosophila species produced a total of only six electrophoretic forms of the head enzyme. Additional electromorphs were found when whole flies were used, but these were not studied in detail because of the possibility that they could be due to postextraction modification.This study was supported by the National Research Council of Canada, Grant No. A0629 and A0235.     

Comparative mapping of cosmids and gene clones from a 1.6 Mb chromosomal region of Drosophila melanogaster in three species of the distantly related subgenus Drosophila

The successful hybridization of cosmid clones from Drosophila melanogaster (Sophophora subgenus) to the salivary gland chromosomes of other species as distantly related as those in the Drosophila subgenus attests their great potential for unravelling genome evolution. We have carried out, using 28 cosmids and 13 gene clones, a study of the organization of the D. melanogaster 95A-96A chromosomal region in three Drosophila subgenus species: D. repleta, D. buzzattii and D. virilis. These clones were first used to built an accurate map of this 1.6 Mb region of D. melanogaster chromosome 3R (Muller’s element E). Then, they were hybridized and mapped to the homologous chromosome 2 of the other three distantly related species. The studied region is disseminated over 13 different sites of chromosome 2 in the Drosophila subgenus species, which implies a minimum of 12 inversion breakpoints fixed between the two subgenera. Extrapolation to the entire chromosome gives 90 fixed inversions. The D. melanogaster Pp1-96A-Acr96Aa segment conserved in D. repleta and D. buzzatii is longer than previously thought and is also conserved in D. virilis. In addition, three other D. melanogaster segments conserved in the three Drosophila subgenus species were found. Finally, our data indicate significant statistical differences in the evolution rate of Muller’s element E among lineages, a result that agrees well with the previous cytogenetic data. Received: 22 July 1998; in revised form: 11 November 1998 / Accepted: 12 November 1998