Molecular and chromosomal phylogeny in the obscura group of Drosophila inferred from sequences of the rp49 gene region

Long-read based assembly and synteny analysis of a reference Drosophila subobscura genome reveals signatures of structural evolution driven by inversions recombination-suppression effects

BACKGROUND

Drosophila subobscura has long been a central model in evolutionary genetics. Presently, its use is hindered by the lack of a reference genome. To bridge this gap, here we used PacBio long-read technology, together with the available wealth of genetic marker information, to assemble and annotate a high-quality nuclear and complete mitochondrial genome for the species. With the obtained assembly, we performed the first synteny analysis of genome structure evolution in the subobscura subgroup.

RESULTS

We generated a highly-contiguous ~ 129 Mb-long nuclear genome, consisting of six pseudochromosomes corresponding to the six chromosomes of a female haploid set, and a complete 15,764 bp-long mitogenome, and provide an account of their numbers and distributions of codifying and repetitive content. All 12 identified paracentric inversion differences in the subobscura subgroup would have originated by chromosomal breakage and repair, with some associated duplications, but no evidence of direct gene disruptions by the breakpoints. Between lineages, inversion fixation rates were 10 times higher in continental D. subobscura than in the two small oceanic-island endemics D. guanche and D. madeirensis. Within D. subobscura, we found contrasting ratios of chromosomal divergence to polymorphism between the A sex chromosome and the autosomes.

CONCLUSIONS

We present the first high-quality, long-read sequencing of a D. subobscura genome. Our findings generally support genome structure evolution in this species being driven indirectly, through the inversions' recombination-suppression effects in maintaining sets of adaptive alleles together in the face of gene flow. The resources developed will serve to further establish the subobscura subgroup as model for comparative genomics and evolutionary indicator of global change.

The molecular characterization of fixed inversions breakpoints unveils the ancestral character of the Drosophila guanche chromosomal arrangements

Cytological studies revealed that the number of chromosomes and their organization varies across species. The increasing availability of whole genome sequences of multiple species across specific phylogenies has confirmed and greatly extended these cytological observations. In the Drosophila genus, the ancestral karyotype consists of five rod-like acrocentric chromosomes (Muller elements A to E) and one dot-like chromosome (element F), each exhibiting a generally conserved gene content. Chromosomal fusions and paracentric inversions are thus the major contributors, respectively, to chromosome number variation among species and to gene order variation within chromosomal element. The subobscura cluster of Drosophila consists in three species that retain the genus ancestral karyotype and differ by a reduced number of fixed inversions. Here, we have used cytological information and the D. guanche genome sequence to identify and molecularly characterize the breakpoints of inversions that became fixed since the D. guanche-D. subobscura split. Our results have led us to propose a modified version of the D. guanche cytological map of its X chromosome, and to establish that (i) most inversions became fixed in the D. subobscura lineage and (ii) the order in which the four X chromosome overlapping inversions occurred and became fixed.

Chromosomal inversions promote genomic islands of concerted evolution of Hsp70 genes in the Drosophila subobscura species subgroup

Heat-shock (HS) assays to understand the connection between standing inversion variation and evolutionary response to climate change in Drosophila subobscura found that "warm-climate" inversion O₃₊₄ exhibits non-HS levels of Hsp70 protein like those of "cold-climate" O_ST after HS induction. This was unexpected, as overexpression of Hsp70 can incur multiple fitness costs. To understand the genetic basis of this finding, we have determined the genomic sequence organization of the Hsp70 family in four different inversions, including O_ST , O₃₊₄ , O₃₊₄₊₈ and O₃₊₄₊₁₆ , using as outgroups the remainder of the subobscura species subgroup, namely Drosophila madeirensis and Drosophila guanche. We found (i) in all the assayed lines, the Hsp70 family resides in cytological locus 94A and consists of only two genes, each with four HS elements (HSEs) and three GAGA sites on its promoter. Yet, in O_{ST ,} the family is comparatively more compact; (ii) the two Hsp70 copies evolve in concert through gene conversion, except in D. guanche; (iii) within D. subobscura, the rate of concerted evolution is strongly structured by inversion, being higher in O_ST than in O₃₊₄ ; and (iv) in D. guanche, the two copies accumulated multiple differences, including a newly evolved "gap-type" HSE2. The absence of concerted evolution in this species may be related to a long-gone-unnoticed observation that it lacks Hsp70 HS response, perhaps because it has evolved within a narrow thermal range in an oceanic island. Our results point to a previously unrealized link between inversions and concerted evolution, with potentially major implications for understanding genome evolution.

Dense gene physical maps of the non-model species Drosophila subobscura

The comparative analysis of genetic and physical maps as well as of whole genome sequences had revealed that in the Drosophila genus, most structural rearrangements occurred within chromosomal elements as a result of paracentric inversions. Genome sequence comparison would seem the best method to estimate rates of chromosomal evolution, but the high-quality reference genomes required for this endeavor are still scanty. Here, we have obtained dense physical maps for Muller elements A, C, and E of Drosophila subobscura, a species with an extensively studied rich and adaptive chromosomal polymorphism. These maps are based on 462 markers: 115, 236, and 111 markers for elements A, C, and E, respectively. The availability of these dense maps will facilitate genome assembly and will thus greatly contribute to obtaining a good reference genome, which is a required step for D. subobscura to attain the model species status. The comparative analysis of these physical maps and those obtained from the D. pseudoobscura and D. melanogaster genomes allowed us to infer the number of fixed inversions and chromosomal evolutionary rates for each pairwise comparison. For all three elements, rates inferred from the more closely related species were higher than those inferred from the more distantly related species, which together with results of relative-rate tests point to an acceleration in the D. subobscura lineage at least for elements A and E.

An Unusual Role for doublesex in Sex Determination in the Dipteran Sciara

The gene doublesex, which is placed at the bottom of the sex-determination gene cascade, plays the ultimate discriminatory role for sex determination in insects. In all insects where this gene has been characterized, the dsx premessenger RNA (pre-mRNA) follows a sex-specific splicing pattern, producing male- and female-specific mRNAs encoding the male-DSXM and female-DSXF proteins, which determine male and female development, respectively. This article reports the isolation and characterization of the gene doublesex of dipteran Sciara insects. The Sciara doublesex gene is constitutively transcribed during development and adult life of males and females. Sciara had no sex-specific doublesex mRNAs but the same transcripts, produced by alternative splicing of its primary transcript, were present in both sexes, although their relative abundance is sex specific. However, only the female DSXF protein, but not the male DSXM protein, was produced at similar amounts in both sexes. An analysis of the expression of female and male Sciara DSX proteins in Drosophila showed that these proteins conserved female and male function, respectively, on the control of Drosophila yolk-protein genes. The molecular evolution of gene doublesex of all insects where this gene has been characterized revealed that Sciara doublesex displays a considerable degree of divergence in its molecular organization and its splicing pattern with respect to the rest of dipterans as suggested by its basal position within the doublesex phylogeny. It is suggested that the doublesex gene is involved in Sciara sex determination although it appears not to play the discriminatory role performed in other insects.

Inferring the demographic history of Drosophila subobscura from nucleotide variation at regions not affected by chromosomal inversions

Drosophila subobscura presents a rich and complex chromosomal inversion polymorphism. It can thus be considered a model system (i) to study the mechanisms originating inversions and how inversions affect the levels and patterns of variation in the inverted regions and (ii) to study adaptation at both the single-gene and chromosomal inversion levels. It is therefore important to infer its demographic history as previous information indicated that its nucleotide variation is not at mutation-drift equilibrium. For that purpose, we sequenced 16 noncoding regions distributed across those parts of the J chromosome not affected by inversions in the studied population and possibly either by other selective events. The pattern of variation detected in these 16 regions is similar to that previously reported within different chromosomal arrangements, suggesting that the latter results would, thus, mainly reflect recent demographic events rather than the partial selective sweep imposed by the origin and frequency increase of inversions. Among the simple demographic models considered in our Approximate Bayesian Computation analysis of variation at the 16 regions, the model best supported by the data implies a population size expansion soon after the penultimate glacial period. This model constitutes a better null model, and it is therefore an important resource for subsequent studies aiming among others to uncover selective events across the species genome. Our results also highlight the importance of introducing the possibility of multiple hits in the coalescent simulations with an outgroup.

Introgression in the Drosophila subobscura--D. Madeirensis sister species: evidence of gene flow in nuclear genes despite mitochondrial differentiation

Species hybridization, and thus the potential for gene flow, was once viewed as reproductive mistake. However, recent analysis based on large datasets and newly developed models suggest that gene exchange is not as rare as originally suspected. To investigate the history and speciation of the closely related species Drosophila subobscura, D. madeirensis, and D. guanche, we obtained polymorphism and divergence data for 26 regions throughout the genome, including the Y chromosome and mitochondrial DNA. We found that the D. subobscura X/autosome ratio of silent nucleotide diversity is significantly smaller than the 0.75 expected under neutrality. This pattern, if held genomewide, may reflect a faster accumulation of beneficial mutations on the X chromosome than on autosomes. We also detected evidence of gene flow in autosomal regions, while sex chromosomes remain distinct. This is consistent with the large X effect on hybrid male sterility seen in this system and the presence of two X chromosome inversions fixed between species. Overall, our data conform to chromosomal speciation models in which rearrangements are proposed to serve as gene flow barriers. Contrary to other observations in Drosophila, the mitochondrial genome appears resilient to gene flow in the presence of nuclear exchange.

Gene flow and gene flux shape evolutionary patterns of variation in Drosophila subobscura

Gene flow (defined as allele exchange between populations) and gene flux (defined as allele exchange during meiosis in heterokaryotypic females) are important factors decreasing genetic differentiation between populations and inversions. Many chromosomal inversions are under strong selection and their role in recombination reduction enhances the maintenance of their genetic distinctness. Here we analyze levels and patterns of nucleotide diversity, selection and demographic history, using 37 individuals of Drosophila subobscura from Mount Parnes (Greece) and Barcelona (Spain). Our sampling focused on two frequent O-chromosome arrangements that differ by two overlapping inversions (OST and O(3+4)), which are differentially adapted to the environment as observed by their opposing latitudinal clines in inversion frequencies. The six analyzed genes (Pif1A, Abi, Sqd, Yrt, Atpα and Fmr1) were selected for their location across the O-chromosome and their implication in thermal adaptation. Despite the extensive gene flux detected outside the inverted region, significant genetic differentiation between both arrangements was found inside it. However, high levels of gene flow were detected for all six genes when comparing the same arrangement among populations. These results suggest that the adaptive value of inversions is maintained, regardless of the lack of genetic differentiation within arrangements from different populations, and thus favors the Local Adaptation hypothesis over the Coadapted Genome hypothesis as the basis of the selection acting on inversions in these populations.

Structure and population genetics of the breakpoints of a polymorphic inversion in Drosophila subobscura

Drosophila subobscura is a paleartic species of the obscura group with a rich chromosomal polymorphism. To further our understanding on the origin of inversions and on how they regain variation, we have identified and sequenced the two breakpoints of a polymorphic inversion of D. subobscura--inversion 3 of the O chromosome--in a population sample. The breakpoints could be identified as two rather short fragments (∼300 bp and 60 bp long) with no similarity to any known transposable element family or repetitive sequence. The presence of the ∼300-bp fragment at the two breakpoints of inverted chromosomes implies its duplication, an indication of the inversion origin via staggered double-strand breaks. Present results and previous findings support that the mode of origin of inversions is neither related to the inversion age nor species-group specific. The breakpoint regions do not consistently exhibit the lower level of variation within and stronger genetic differentiation between arrangements than more internal regions that would be expected, even in moderately small inversions, if gene conversion were greatly restricted at inversion breakpoints. Comparison of the proximal breakpoint region in species of the obscura group shows that this breakpoint lies in a small high-turnover fragment within a long collinear region (∼300 kb).

Studies of the species barrier between Drosophila subobscura and D. madeirensis V: the importance of sex-linked inversion in preserving species identity

The X chromosome is known to exert a disproportionately large effect on characters related to post-zygotic reproductive isolation. There is also growing evidence about the important role of the chromosomal regions with reduced recombination (such as inversions) in maintaining the identity of closely related species. Using molecular markers, we examine the effect of different regions of the X chromosome on determination of hybrid traits (viability, testes size, sperm motility and morphological anomalies) in hybrid males between Drosophila madeirensis and Drosophila subobscura. The preponderant effect of a region localized inside the A2 inversion in the X chromosome in all hybrid traits is identified. Other marked regions exert a weaker influence or only influence some of the hybrid trait. Our results confirm the crucial role of sex-linked chromosomal inversion in preserving the identity of species with incomplete reproductive isolation. The specific genomic make-up of parental lines used to perform crosses has a great effect on hybrid fitness.

The gene transformer-2 of Sciara (Diptera, Nematocera) and its effect on Drosophila sexual development

BACKGROUND

The gene transformer-2, which is involved in sex determination, has been studied in Drosophila, Musca, Ceratitis, Anastrepha and Lucilia. All these members of Diptera belong to the suborder Brachycera. In this work, it is reported the isolation and characterisation of genes transformer-2 of the dipterans Sciara ocellaris and Bradysia coprophila (formerly Sciara coprophila), which belong to the much less extensively analysed Sciaridae Family of the Suborder Nematocera, which is paraphyletic with respect to Suborder Brachycera.

RESULTS

The transformer-2 genes of the studied Sciara species were found to be transcribed in both sexes during development and adult life, in both the soma and germ lines. They produced a single primary transcript, which follows the same alternative splicing in both sexes, giving rise to different mRNAs isoforms. In S. ocellaris the most abundant mRNA isoform encoded a full-length protein of 251 amino acids, while that of B. coprophila encoded a protein of 246 amino acids. Both showed the features of the SR protein family. The less significant mRNA isoforms of both species encoded truncated, presumably non-functional Transformer-2 proteins. The comparison of the functional Sciara Transformer-2 proteins among themselves and those of other insects revealed the greatest degree of conservation in the RRM domain and linker region. In contrast, the RS1 and RS2 domains showed extensive variation with respect to their number of amino acids and their arginine-serine (RS) dipeptide content. The expression of S. ocellaris Transformer-2 protein in Drosophila XX pseudomales lacking the endogenous transformer-2 function caused their partial feminisation.

CONCLUSIONS

The transformer-2 genes of both Sciaridae species encode a single protein in both sexes that shares the characteristics of the Transformer-2 proteins of other insects. These proteins showed conserved sex-determination function in Drosophila; i.e., they were able to form a complex with the endogenous Drosophila Transformer protein that controls the female-specific splicing of the Drosophila doublesex pre-mRNA. However, it appears that the complex formed between the Drosophila Transformer protein and the Sciara Transformer-2 protein is less effective at inducing the female-specific splicing of the endogenous Drosophila doublesex pre-mRNA than the DrosophilaTransformer-Transformer2 complex. This suggests the existence of species-specific co-evolution of the Transformer and Transformer-2 proteins.

FISH mapping of microsatellite loci from Drosophila subobscura and its comparison to related species

Microsatellites are highly polymorphic markers that are distributed through all the genome being more abundant in non-coding regions. Whether they are neutral or under selection, these markers if localized can be used as co-dominant molecular markers to explore the dynamics of the evolutionary processes. Their cytological localization can allow identifying genes under selection, inferring recombination from a genomic point of view, or screening for the genomic reorganizations occurring during the evolution of a lineage, among others. In this paper, we report for the first time the localization of microsatellite loci by fluorescent in situ hybridization on Drosophila polytene chromosomes. In Drosophila subobscura, 72 dinucleotide microsatellite loci were localized by fluorescent in situ hybridization yielding unique hybridization signals. In the sex chromosome, microsatellite distribution was not uniform and its density was higher than in autosomes. We identified homologous segments to the sequence flanking the microsatellite loci by browsing the genome sequence of Drosophila pseudoobscura and Drosophila melanogaster. Their localization supports the conservation of Muller's chromosomal elements among Drosophila species and the existence of multiple intrachromosomal rearrangements within each evolutionary lineage. Finally, the lack of microsatellite repeats in the homologous D. melanogaster sequences suggests convergent evolution for high microsatellite density in the distal part of the X chromosome.

Effect of the gene doublesex of anastrepha on the somatic sexual development of Drosophila

BACKGROUND

The gene doublesex (dsx) is at the bottom of the sex determination genetic cascade and is transcribed in both sexes, but gives rise to two different proteins, DsxF and DsxM, which impose female and male sexual development respectively via the sex-specific regulation of the so-called sexual cyto-differentiation genes. The present manuscript addressed the question about the functional conservation of the tephritid Anastrepha DsxF and DsxM proteins to direct the sexual development in Drosophila (Drosophilidae).

METHODOLOGY

To express these proteins in Drosophila, the GAL4-UAS system was used. The effect of these proteins was monitored in the sexually dimorphic regions of the fly: the foreleg basitarsus, the 5th, 6th and 7th tergites, and the external terminalia. In addition, we analysed the effect of Anastrepha DsxF and DsxM proteins on the regulation of Drosophila yolk protein genes, which are expressed in the fat body of adult females under the control of dsx.

CONCLUSIONS

The Anastrepha DsxF and DsxM proteins transformed doublesex intersexual Drosophila flies into females and males respectively, though this transformation was incomplete and the extent of their influence varied in the different sexually dimorphic regions of the adult fly. The Anastrepha DsxF and DsxM proteins also behaved as activators and repressors, respectively, of the Drosophila yolk protein genes, as do the DsxF and DsxM proteins of Drosophila itself. Finally, the Anastrepha DsxF and DsxM proteins were found to counteract the functions of Drosophila DsxM and DsxF respectively, reflecting the normal behaviour of the latter proteins towards one another. Collectively, these results indicate that the Anastrepha DsxF and DsxM proteins show conserved female and male sex-determination function respectively in Drosophila, though it appears that they cannot fully substitute the latter's own Dsx proteins. This incomplete function might be partly due to a reduced capacity of the Anastrepha Dsx proteins to completely control the Drosophila sexual cyto-differentiation genes, a consequence of the accumulation of divergence between these species resulting in the formation of different co-adapted complexes between the Dsx proteins and their target genes.

Divergent evolution and molecular adaptation in the Drosophila odorant-binding protein family: inferences from sequence variation at the OS-E and OS-F genes

BACKGROUND

The Drosophila Odorant-Binding Protein (Obp) genes constitute a multigene family with moderate gene number variation across species. The OS-E and OS-F genes are the two phylogenetically closest members of this family in the D. melanogaster genome. In this species, these genes are arranged in the same genomic cluster and likely arose by tandem gene duplication, the major mechanism proposed for the origin of new members in this olfactory-system family.

RESULTS

We have analyzed the genomic cluster encompassing OS-E and OS-F genes (Obp83 genomic region) to determine the role of the functional divergence and molecular adaptation on the Obp family size evolution. We compared nucleotide and amino acid variation across 18 Drosophila and 4 mosquito species applying a phylogenetic-based maximum likelihood approach complemented with information of the OBP three-dimensional structure and function. We show that, in spite the OS-E and OS-F genes are currently subject to similar and strong selective constraints, they likely underwent divergent evolution. Positive selection was likely involved in the functional diversification of new copies in the early stages after the gene duplication event; moreover, it might have shaped nucleotide variation of the OS-E gene concomitantly with the loss of functionally related members. Besides, molecular adaptation likely affecting the functional OBP conformational changes was supported by the analysis of the evolution of physicochemical properties of the OS-E protein and the location of the putative positive selected amino acids on the OBP three-dimensional structure.

CONCLUSION

Our results support that positive selection was likely involved in the functional differentiation of new copies of the OBP multigene family in the early stages after their birth by gene duplication; likewise, it might shape variation of some members of the family concomitantly with the loss of functionally related genes. Thus, the stochastic gene gain/loss process coupled with the impact of natural selection would influence the observed OBP family size.

Genetic exchange versus genetic differentiation in a medium-sized inversion of Drosophila: the A2/Ast arrangements of Drosophila subobscura

Chromosomal inversion polymorphism affects nucleotide variation at loci associated with inversions. In Drosophila subobscura, a species with a rich chromosomal inversion polymorphism and the largest recombinational map so far reported in the Drosophila genus, extensive genetic structure of nucleotide variation was detected in the segment affected by the O(3) inversion, a moderately sized inversion at Muller's element E. Indeed, a strong genetic differentiation all over O(3) and no evidence of a higher genetic exchange in the center of the inversion than at breakpoints were detected. In order to ascertain, whether other polymorphic and differently sized inversions of D. subobscura also exhibited a strong genetic structure, nucleotide variation in 5 gene regions (P236, P275, P150, Sxl, and P125) located along the A(2) inversion was analyzed in A(st) and A(2) chromosomes of D. subobscura. A(2) is a medium-sized inversion at Muller's element A and forms a single inversion loop in heterokaryotypes. The lower level of variation in A(2) relative to A(st) and the significant excess of low-frequency variants at polymorphic sites indicate that nucleotide variation at A(2) is not at mutation-drift equilibrium. The closest region to an inversion breakpoint, P236, exhibits the highest level of genetic differentiation (F(ST)) and of linkage disequilibrium (LD) between arrangements and variants at nucleotide polymorphic sites. The remaining 4 regions show a higher level of genetic exchange between A(2) and A(st) chromosomes than P236, as revealed by F(ST) and LD estimates. However, significant genetic differentiation between the A(st) and A(2) arrangements was detected not only at P236 but also in the other 4 regions separated from the nearest breakpoint by 1.2-2.9 Mb. Therefore, the extent of genetic exchange between arrangements has not been high enough to homogenize nucleotide variation in the center of the A(2) inversion. A(2) can be considered a typical successful inversion of D. subobscura according to its relative length. Chromosomal inversion polymorphism of D. subobscura might thus cause the genome of this species to be highly structured and to harbor different gene pools that might contribute to maintain adaptations to particular environments.

Principles of genome evolution in the Drosophila melanogaster species group

That closely related species often differ by chromosomal inversions was discovered by Sturtevant and Plunkett in 1926. Our knowledge of how these inversions originate is still very limited, although a prevailing view is that they are facilitated by ectopic recombination events between inverted repetitive sequences. The availability of genome sequences of related species now allows us to study in detail the mechanisms that generate interspecific inversions. We have analyzed the breakpoint regions of the 29 inversions that differentiate the chromosomes of Drosophila melanogaster and two closely related species, D. simulans and D. yakuba, and reconstructed the molecular events that underlie their origin. Experimental and computational analysis revealed that the breakpoint regions of 59% of the inversions (17/29) are associated with inverted duplications of genes or other nonrepetitive sequences. In only two cases do we find evidence for inverted repetitive sequences in inversion breakpoints. We propose that the presence of inverted duplications associated with inversion breakpoint regions is the result of staggered breaks, either isochromatid or chromatid, and that this, rather than ectopic exchange between inverted repetitive sequences, is the prevalent mechanism for the generation of inversions in the melanogaster species group. Outgroup analysis also revealed evidence for widespread breakpoint recycling. Lastly, we have found that expression domains in D. melanogaster may be disrupted in D. yakuba, bringing into question their potential adaptive significance.

The organization of genes on chromosomes changes over evolutionary time. In some organisms, such as fruit flies and mosquitoes, inversions of chromosome regions are widespread. This has been associated with adaptation to environmental pressures and speciation. However, the mechanisms by which inversions are generated at the molecular level are poorly understood. The prevailing view involves the interactions of sequences that are moderately repeated in the genome. Here, we use molecular and computational methods to study 29 inversions that differentiate the chromosomes of three closely related fruit fly species. We find little support for a causal role of repetitive sequences in the origin of inversions and, instead, detect the presence of inverted duplications of ancestrally unique sequences (generally protein-coding genes) in the breakpoint regions of many inversions. This leads us to propose an alternative model in which the generation of inversions is coupled with the generation of duplications of flanking sequences. Additionally, we find evidence for genomic regions that are prone to breakage, being associated with inversions generated independently during the evolution of the ancestors of existing species.

Chromosomal inversion breakpoints were compared between three closely related Drosophila species. Many are associated with inverted gene duplications, suggesting that the prevalent mechanism for their generation involves staggered breakpoints.

Global patterns of sequence evolution in Drosophila

Background

Sequencing of the genomes of several Drosophila allows for the first precise analyses of how global sequence patterns change among multiple, closely related animal species. A basic question is whether there are characteristic features that differentiate chromosomes within a species or between different species.

Results

We explored the euchromatin of the chromosomes of seven Drosophila species to establish their global patterns of DNA sequence diversity. Between species, differences in the types and amounts of simple sequence repeats were found. Within each species, the autosomes have almost identical oligonucleotide profiles. However, X chromosomes and autosomes have, in all species, a qualitatively different composition. The X chromosomes are less complex than the autosomes, containing both a higher amount of simple DNA sequences and, in several cases, chromosome-specific repetitive sequences. Moreover, we show that the right arm of the X chromosome of Drosophila pseudoobscura, which evolved from an autosome 10 – 18 millions of years ago, has a composition which is identical to that of the original, left arm of the X chromosome.

Conclusion

The consistent differences among species, differences among X chromosomes and autosomes and the convergent evolution of X and neo-X chromosomes demonstrate that strong forces are acting on drosophilid genomes to generate peculiar chromosomal landscapes. We discuss the relationships of the patterns observed with differential recombination and mutation rates and with the process of dosage compensation.

A study of wing morphology and fluctuating asymmetry in interspecific hybrids between Drosophila buzzatii and D. koepferae

In this work we investigate the effect of interspecific hybridization on wing morphology using geometric morphometrics in the cactophilic sibling species D. buzzatii and D. koepferae. Wing morphology in F1 hybrids exhibited an important degree of phenotypic plasticity and differs significantly from both parental species. However, the pattern of morphological variation between hybrids and the parental strains varied between wing size and wing shape, across rearing media, sexes, and crosses, suggesting a complex genetic architecture underlying divergence in wing morphology. Even though there was significant fluctuating asymmetry for both, wing size and shape in F1 hybrids and both parental species, there was no evidence of an increased degree of fluctuating asymmetry in hybrids as compared to parental species. These results are interpreted in terms of developmental stability as a function of a balance between levels of heterozygosity and the disruption of coadaptation as an indirect consequence of genomic divergence.

Do Species Converge during Adaptation? A Case Study inDrosophila

Adaptation to novel environments is a crucial theme in evolutionary biology, particularly because ex situ conservation forces populations to adapt to captivity. Here we analyze the evolution of life-history traits in two closely related species, Drosophila subobscura Collin and Drosophila madeirensis Monclus, during adaptation to the laboratory. Drosophila madeirensis, an endemic species from Madeira, is here shown to have less ability to adapt to the laboratory. Early fecundity was the only trait where this species showed a significant improvement with time. By comparison, D. subobscura improved in most traits, and its early fecundity increased faster than that of D. madeirensis. Our findings suggest that different species, even closely related ones, may adapt at different rates to the same environment.

Quantitative genetics of speciation: additive and non-additive genetic differentiation between Drosophila madeirensis and Drosophila subobscura

The role of dominance and epistasis in population divergence has been an issue of much debate ever since the neoDarwinian synthesis. One of the best ways to dissect the several genetic components affecting the genetic architecture of populations is line cross analysis. Here we present a study comparing generation means of several life history-traits in two closely related Drosophila species: Drosophila subobscura, D. madeirensis as well as their F1 and F2 hybrids. This study aims to determine the relative contributions of additive and non-additive genetic parameters to the differentiation of life-history traits between these two species. The results indicate that both negative dominance and epistatic effects are very important in the differentiation of most traits. We end with considerations about the relevance of these findings for the understanding of the role of non-additive effects in speciation.

Parallel evolution of chimeric fusion genes

To understand how novel functions arise, we must identify common patterns and mechanisms shaping the evolution of new genes. Here, we take advantage of data from three Drosophila genes, jingwei, Adh-Finnegan, and Adh-Twain, to find evolutionary patterns and mechanisms governing the evolution of new genes. All three of these genes are independently derived from Adh, which enabled us to use the extensive literature on Adh in Drosophila to guide our analyses. We discovered a fundamental similarity in the temporal, spatial, and types of amino acid changes that occurred. All three genes underwent rapid adaptive amino acid evolution shortly after they were formed, followed by later quiescence and functional constraint. These genes also show striking parallels in which amino acids change in the Adh region. We showed that these early changes tend to occur at amino acid residues that seldom, if ever, evolve in Drosophila Adh. Changes at these slowly evolving sites are usually associated with loss of function or hypomorphic mutations in Drosophila melanogaster. Our data indicate that shifting away from ancestral functions may be a critical step early in the evolution of chimeric fusion genes. We suggest that the patterns we observed are both general and predictive.

Origin and evolution of a chimeric fusion gene in Drosophila subobscura, D. madeirensis and D. guanche

An understanding of the mutational and evolutionary mechanisms underlying the emergence of novel genes is critical to studies of phenotypic and genomic evolution. Here we describe a new example of a recently formed chimeric fusion gene that occurs in Drosophila guanche, D. madeirensis, and D. subobscura. This new gene, which we name Adh-Twain, resulted from an Adh mRNA that retrotransposed into the Gapdh-like gene, CG9010. Adh-Twain is transcribed; its 5' promoters and transcription patterns appear similar to those of CG9010. Population genetic and phylogenetic analyses suggest that the amino acid sequence of Adh-Twain evolved rapidly via directional selection shortly after it arose. Its more recent history, however, is characterized by slower evolution consistent with increasing functional constraints. We present a model for the origin of this new gene and discuss genetic and evolutionary factors affecting the evolution of new genes and functions.

Chromosomal inversion polymorphism leads to extensive genetic structure: a multilocus survey in Drosophila subobscura

The adaptive character of inversion polymorphism in Drosophila subobscura is well established. The O(ST) and O(3+4) chromosomal arrangements of this species differ by two overlapping inversions that arose independently on O(3) chromosomes. Nucleotide variation in eight gene regions distributed along inversion O(3) was analyzed in 14 O(ST) and 14 O(3+4) lines. Levels of variation within arrangements were quite similar along the inversion. In addition, we detected (i) extensive genetic differentiation between arrangements in all regions, regardless of their distance to the inversion breakpoints; (ii) strong association between nucleotide variants and chromosomal arrangements; and (iii) high levels of linkage disequilibrium in intralocus and also in interlocus comparisons, extending over distances as great as approximately 4 Mb. These results are not consistent with the higher genetic exchange between chromosomal arrangements expected in the central part of an inversion from double-crossover events. Hence, double crossovers were not produced or, alternatively, recombinant chromosomes were eliminated by natural selection to maintain coadapted gene complexes. If the strong genetic differentiation detected along O(3) extends to other inversions, nucleotide variation would be highly structured not only in D. subobscura, but also in the genome of other species with a rich chromosomal polymorphism.

Seasonal cycles of allozyme-by-chromosomal-inversion gametic disequilibrium in Drosophila subobscura

Allozyme loci are frequently found non randomly associated to the chromosomal inversions in which they are included in Drosophila. Two opposite views compete to explain strong allozyme-by-inversion gametic disequilibria: they result from natural selection or, conversely, merely represent remnants of associations accidentally established at the origin of inversions. Empirical efforts aimed at deciding between adaptive and historical scenarios have focused on the spatial distribution of disequilibria. Yet, the evolutionary significance of these associations remains uncertain. I report here the results of a time-series analysis of the seasonal variation of alleles at six allozyme loci (Acph, Lap, Pept-1, Ao, Mpi, and Xdh) in connection with the O chromosomal polymorphisms of D. subobscura. The findings were: (1) in the segment I of the O chromosome, Lap and Pept-1 allozymes changed seasonally in a cyclical fashion within the ST gene arrangement, but they changed erratically within the 3 + 4 gene configuration; (2) the frequencies of Lap1.11 and Pept-1(0.40) within ST dropped to their lowest values in early and late summer, respectively, when the seasonal level of the ST arrangement is lowest. Furthermore, Lap1.11 and Pept-1(0.40) covary with ST only within these seasons, yet in a fashion inconsistent with these alleles having a major influence on the dynamics of the inversion; (3) seasonal cycling of alleles within inversions were not detected at Acph, Ao, Mpi, and Xdh, yet these loci are nearly monomorphic at the study population, and/or their sampled series were shorter than those for Lap and Pept-1; and (4) simply monitoring allozyme frequencies separately for each inversion proved to be superior, for evidencing the seasonal cycles of the disequilibria, to the use of the D' coefficient of association. Observed seasonal cycles of allozymes within inversions likely reflect natural selection.

Drosophila OVO regulates ovarian tumor transcription by binding unusually near the transcription start site

Evolutionarily conserved ovo loci encode developmentally regulated, sequence-specific, DNA-binding, C(2)H(2)-zinc-finger proteins required in the germline and epidermal cells of flies and mice. The direct targets of OVO activity are not known. Genetic experiments suggest that ovo acts in the same regulatory network as ovarian tumor (otu), but the relative position of these genes in the pathway is controversial. Three OVO-binding sites exist in a compact regulatory region that controls germline expression of the otu gene. Interestingly, the strongest OVO-binding site is very near the otu transcription start, where basal transcriptional complexes must function. Loss-of-function, gain-of-function and promoter swapping constructs demonstrate that OVO binding near the transcription start site is required for OVO-dependent otu transcription in vivo. These data unambiguously identify otu as a direct OVO target gene and raise the tantalizing possibility that an OVO site, at the location normally occupied by basal components, functions as part of a specialized core promoter.

Characterization of a MEN1 ortholog from Drosophila melanogaster

Multiple endocrine neoplasia type 1 (MEN1) is a familial cancer syndrome characterized by tumors of the parathyroid, entero-pancreatic neuroendocrine and pituitary tissues and caused by inactivating mutations in the MEN1 gene. Menin, the 610-amino acid nuclear protein encoded by MEN1, binds to the transcription factor JunD and can repress JunD-induced transcription. We report here the identification of a MEN1 ortholog in Drosophila melanogaster, Menin1, that encodes a 763 amino acid protein sharing 46% identity with human menin. Additionally, 69% of the missense mutations and in-frame deletions reported in MEN1 patients appear in amino acid residues that are identical in the Drosophila and human protein, suggesting the importance of the conserved regions. Drosophila Menin1 gene transcripts use alternative polyadenylation sites resulting in 4.3 and 5-kb messages. The 4.3-kb transcript appears to be largely maternal, while the 5-kb transcript appears mainly zygotic. The binding of Drosophila menin to human JunD or Drosophila Jun could not be demonstrated by the yeast two-hybrid analysis. The identification of the MEN1 ortholog from Drosophila melanogaster will provide an opportunity to utilize Drosophila genetics to enhance our understanding of the function of human menin.

Nucleotide variation at the yellow gene region is not reduced in Drosophila subobscura: a study in relation to chromosomal polymorphism

In contrast to Drosophila melanogaster and Drosophila simulans, the yellow (y) gene region of Drosophila subobscura is not located in a region with a strong reduction in recombination. In addition, this gene maps very close to the breakpoints of different inversions that segregate as polymorphic in natural populations of D. subobscura. Therefore, levels of variation at the y gene region in this species relative to those found in D. melanogaster and D. simulans may be affected not only by the change in the recombinational environment, but also by the presence of inversion polymorphism. To further investigate these aspects, an approximately 5.4-kb region of the A (=X) chromosome including the y gene was sequenced in 25 lines of D. subobscura and in the closely related species Drosophila madeirensis and Drosophila guanche. The D. subobscura lines studied differed in their A-chromosomal arrangements, A(st), A(2), and A(1). Unlike in D. melanogaster and D. simulans, levels of variation at the y gene region of D. subobscura are not reduced relative to those found at other genomic regions in the same species (rp49, Acp70A, and Acph-1). This result supports the effect of the change in the recombinational environment of a particular gene on the level of neutral variation. In addition, nucleotide variation is affected by chromosomal polymorphism. A strong genetic differentiation is detected between the A(1) arrangement and either A(st) or A(2), but not between A(st) and A(2). This result is consistent with the location of the y gene relative to the breakpoints of inversions A(1) and A(2). In addition, the pattern of nucleotide polymorphism in A(st)+A(2) and A(1) seems to point out that variation at the y gene region within these chromosomal classes is in the phase transient to equilibrium. The estimated ages of these arrangements assuming a star genealogy indicate that their origin cannot predate the D. madeirensis split. Therefore, the present results are consistent with a chromosomal phylogeny where Am(1), which is an arrangement present in D. madeirensis but absent in current populations of D. subobscura, would be the ancestral arrangement.

The current state of insect molecular systematics: a thriving Tower of Babel

Insect molecular systematics has undergone remarkable recent growth. Advances in methods of data generation and analysis have led to the accumulation of large amounts of DNA sequence data from most major insect groups. In addition to reviewing theoretical and methodological advances, we have compiled information on the taxa and regions sequenced from all available phylogenetic studies of insects. It is evident that investigators have not usually coordinated their efforts. The genes and regions that have been sequenced differ substantially among studies and the whole of our efforts is thus little greater than the sum of its parts. The cytochrome oxidase I, 16S, 18S, and elongation factor-1 alpha genes have been widely used and are informative across a broad range of divergences in insects. We advocate their use as standards for insect phylogenetics. Insect molecular systematics has complemented and enhanced the value of morphological and ecological data, making substantial contributions to evolutionary biology in the process. A more coordinated approach focused on gathering homologous sequence data will greatly facilitate such efforts.

The relationship between allozyme and chromosomal polymorphism inferred from nucleotide variation at the Acph-1 gene region of Drosophila subobscura

The Acph-1 gene region was sequenced in 51 lines of Drosophila subobscura. Lines differ in their chromosomal arrangement for segment I of the O chromosome (O(st) and O(3+4)) and in the Acph-1 electrophoretic allele (Acph-1(100), Acph-1(054), and Acph-1(>100)). The ACPH-1 protein exhibits much more variation than previously detected by electrophoresis. The amino acid replacements responsible for the Acph-1(054) and Acph-1(>100) electrophoretic variants are different within O(st) and within O(3+4), which invalidates all previous studies on linkage disequilibrium between chromosomal and allozyme polymorphisms at this locus. The Acph-1(>100) allele within O(3+4) has a recent origin, while both Acph-1(054) alleles are rather old. Levels of nucleotide variation are higher within the O(3+4) than within the O(st) arrangement except for nonsynonymous sites. The McDonald and Kreitman test shows a significant excess of nonsynonymous polymorphisms within O(st) when D. guanche is used as the outgroup. According to the nearly neutral model of molecular evolution, this excess is consistent with a smaller effective size of O(st) relative to O(3+4) arrangements. A smaller population size, a lower recombination, and a more recent bottleneck might be contributing to the smaller effective size of O(st).

Targeted expression of the DNA binding domain of DRE-binding factor, a Drosophila transcription factor, attenuates DNA replication of the salivary gland and eye imaginal disc

The promoters of Drosophila genes encoding DNA replication-related proteins contain transcription regulatory elements consisting of an 8-bp palindromic DNA replication-related element (DRE) sequence (5'-TATCGATA). The specific DRE-binding factor (DREF), a homodimer of the polypeptide with 709 amino acid residues, is a positive trans-acting factor for transcription of DRE-containing genes. Both DRE binding and dimer formation are associated with residues 16 to 115 of the N-terminal region. We have established transgenic flies expressing the full-length DREF polypeptide or its N-terminal fragment (amino acid residues 1 to 125) under the control of the heat shock promoter, the salivary gland-specific promoter, or the eye imaginal disc-specific promoter. Heat shock induction of the N-terminal fragment during embryonic, larval, or pupal stages caused greater than 50% lethality. This lethality was overcome by coexpression of the full-length DREF. In salivary glands of the transgenic larvae expressing the N-terminal fragment, this fragment formed a homodimer and a heterodimer with the endogenous DREF. Ectopic expression of the N-terminal fragment in salivary gland cells reduced the contents of mRNAs for the 180-kDa subunit of DNA polymerase alpha and for dE2F and the extent of DNA endoreplication. Ectopic expression of the N-terminal fragment in the eye imaginal discs significantly reduced DNA replication in cells at the second mitotic wave. The lines of evidence suggest that the N-terminal fragment can impede the endogenous DREF function in a dominant negative manner and that DREF is required for normal DNA replication in both mitotic cell cycle and endo cycle.

The acid phosphatase-1 gene region in the Drosophila species of the subobscura cluster

The acid phosphatase-1 (Acph-1) gene region was sequenced in three species of Drosophila: D. subobscura, D. madeirensis and D. guanche. These three closely related species, which are included in the obscura group, form the subobscura cluster. The different functional regions of the gene were identified by similarity with the sequence of D. melanogaster. The structure of Acph-1 is conserved in the four species. Average divergence at synonymous and nonsynonymous sites between D. melanogaster and the species of the subobscura cluster is Ks = 1.1354 and Ka = 0.1743, respectively. The rather high Ka value confirms that ACPH-1 is a rapidly evolving enzyme in Drosophila, as previously suggested by immunological studies. Amino acid replacements are not randomly distributed along the gene. In fact, an excess of replacements is detected in exon I, indicating that the signal peptide encoded by this exon evolves even faster than the rest of the protein. Divergence at the Acph-1 gene region is further evidence that D. madeirensis and D. subobscura are more closely related than D. guanche is to any of them. In addition, both silent divergence in noncoding regions and synonymous divergence in the coding region indicate that the split of the D. guanche lineage is about twice as old as the split of the lineages leading to D. madeirensis and D. subobscura. These phylogenetic relationships are, however, not supported by divergence at nonsynonymous sites since the lowest Ka estimate is between D. guanche and D. subobscura.

Synonymous rates at the RpII215 gene of Drosophila: variation among species and across the coding region

The region encompassing the RpII215 gene that encodes the largest component of the RNA polymerase II complex (1889 amino acids) has been sequenced in Drosophila subobscura, D. madeirensis, D. guanche, and D. pseudoobscura. Nonsynonymous divergence estimates (Ka) indicate that this gene has a very low rate of amino acid replacements. Given its low Ka and constitutive expression, synonymous substitution rates are, however, unexpectedly high. Sequence comparisons have allowed the molecular clock hypothesis to be tested. D. guanche is an insular species and it is therefore expected to have a reduced effective size relative to D. subobscura. The significantly higher rate of synonymous substitutions detected in the D. guanche lineage could be explained if synonymous mutations behave as nearly neutral. Significant departure from the molecular clock hypothesis for synonymous and nonsynonymous substitutions was detected when comparing the D. subobscura, D. pseudoobscura, and D. melanogaster lineages. Codon bias and synonymous divergence between D. subobscura and D. melanogaster were negatively correlated across the RpII215 coding region, which indicates that selection coefficients for synonymous mutations vary across the gene. The C-terminal domain (CTD) of the RpII215 protein is structurally and functionally differentiated from the rest of the protein. Synonymous substitution rates were significantly different in both regions, which strongly indicates that synonymous mutations in the CTD and in the non-CTD regions are under detectably different selection coefficients.

Molecular population genetics of the rp49 gene region in different chromosomal inversions of Drosophila subobscura

Nucleotide variation at the ribosomal protein 49 (rp49) gene region has been studied in 75 lines of Drosophila subobscura belonging to four chromosomal arrangements (Ost, O3+4, O3+4+8, and O3+4+23). The location of the rp49 gene region within the inversion loop differs among heterokaryotypes: it is very close to one of the breakpoints in heterozygotes involving Ost chromosomes, while it is in a more central position in all other heterokaryotypes. The distribution of nucleotide polymorphism in the different arrangements is consistent with a monophyletic origin of the inversions. The data also provide evidence that gene conversion and possibly double crossover are involved in shuffling nucleotide variation among gene arrangements. The analyses reveal that the level of genetic exchange is higher when the region is located in a more central position of the inverted fragment than when it is close to the breakpoints. The pairwise difference distributions as well as the negative values of Tajima's and Fu and Li's statistics further support the hypothesis that nucleotide variation within chromosomal arrangements still reflects expansion after the origin of the inversions. Under the expansion model, we have estimated the time of origin of the studied inversions.