GENETIC DISTANCE AND EVOLUTIONARY RELATIONSHIPS IN THE DROSOPHILA OBSCURA GROUP

EVOLUTIONARY IMPLICATIONS OF DNA DIVERGENCE IN THE DROSOPHILA OBSCURA GROUP

Using DNA-DNA hybridization, we have determined the degree of single-copy DNA (scDNA) divergence among eight species of the Drosophila obscura group. These include Old World and New World species as well as members of two subgroups. Contrary to classical systematics, members of the affinis subgroup are more closely related to American members of the obscura subgroup than are Old World species. The Old World species are not a monophyletic group. The degree of scDNA divergence among species is not necessarily correlated with morphology, chromosomal divergence, or ability to form hybrids. A unique pattern of hybrid formation was found: species separated by a ΔT_m of 6.5°C can form hybrids whereas species separated by a ΔT_m of 2.5°C cannot. As with other groups of Drosophila, the obscura group has discrete parts of the genome evolving at very different rates. The slow evolving fraction of the nuclear genome is evolving at about the same rate as mitochondrial DNA. The additional scDNA divergence accompanying the step from partial reproductive isolation (between North American pseudoobscura and the isolated Bogotà population) to full isolation is very small. The resolution of the technique was challenged by five closely related taxa with a maximum ΔT_m of 2.5°C separating them; the taxa were unambiguously resolved and the "correct" phylogeny recovered. Finally, there is some indication that scDNA in the obscura group may be evolving considerably slower than in the melanogaster subgroup.

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.

Phylogeny of the Drosophila obscura species group deduced from mitochondrial DNA sequences

Approximately 2 kb corresponding to different regions of the mtDNA of 14 different species of the obscura group of Drosophila have been sequenced. In spite of the uncertainties arising in the phylogenetic reconstruction due to a restrictive selection toward a high mtDNA A+T content, all the phylogenetic analysis carried out clearly indicate that the obscura group is formed by, at least, four well-defined lineages that would have appeared as the consequence of a rapid phyletic radiation. Two of the lineages correspond to monophyletic subgroups (i.e., affinis and pseudoobscura), whereas the obscura subgroup remains heterogeneous assemblage that could be reasonably subdivided into at least two complexes (i.e., subobscura and obscura).

Nuclear and mitochondrial ribosomal RNA variability in the obscura group of Drosophila

Parts of 28S (nuclear) and 12S (mitochondrial) ribosomal RNA of Palearctic, Nearctic and African species of the obscura group have been sequenced by the direct method of sequencing. Rates of nucleotide substitutions in both molecules were compared. The nucleotide divergence is higher in the mitochondrial rRNA. Average distances of species taken in pairwise were compared to results obtained with the melanogaster subgroup: the divergence of nuclear rRNA appears lower, that of the mtDNA higher whereas genetic distances (allozymes) and sncDNA distances are similar. Noticeable variability of evolutionary rates can be observed even in low taxonomical levels. Phylogenetic trees for the obscura group are in general agreement with those obtained with other characters.

Mitochondrial DNA evolution in the obscura species subgroup of Drosophila

Mitochondrial DNA (mtDNA) restriction site maps for nine species of the Drosophila obscura subgroup and for Drosophila melanogaster were established. Taking into account all restriction enzymes (12) and strains (45) analyzed, a total of 105 different sites were detected, which corresponds to a sample of 3.49% of the mtDNA genome. Based on nucleotide divergences, two phylogenetic trees were constructed assuming either constant or variable rates of evolution. Both methods led to the same relationships. Five differentiated clusters were found for the obscura subgroup species, one Nearctic, represented by Drosophila pseudoobscura, and four Palearctic, two grouping the related triads of species Drosophila subobscura, Drosophila madeirensis, Drosophila obscura, Drosophila subsilvestris, and two more represented by one species each, Drosophila bifasciata, and Drosophila tristis. The different Palearctic clusters are as distant between themselves as with the Nearctic one. For the related species D. subobscura, D. madeirensis, and D. guanche, the pair D. subobscura-D. madeirensis is the closest one. The relationships found by nucleotide divergence were confirmed by differences in mitochondrial genome size, with related species sharing similar genome lengths and differing from the distant ones. The total mtDNA size range for the obscura subgroup species was from 15.5 kb for D. pseudoobscura to 17.1 for D. tristis.

Functional and biochemical features of alcohol dehydrogenase in four species of the obscura group of Drosophila

The biochemical features of ADH of four Drosophila species of the obscura group have been studied. The relationship between ethanol tolerance and ADH activity has been investigated. Propan-2-ol and acetone concentrations have been determined in propan-2-ol treated flies and ADH activity has been followed during 96 h of propan-2-ol treatment. Data on the ADH system confirm constructed phylogenies based on electrophoretic variation and chromosome homologies.

Puff activity after heat shock in two species of the Drosophila obscura group

When individuals of Drosophila guanche are submitted to heat shock, five new puffs are induced. These puffs usually do not appear during normal development. Comparing these results with those obtained in Drosophila subobscura, also belonging to the obscura group, differences between the induced puffing pattern of both species have been found.

The banding pattern of polytene chromosomes of Drosophila guanche compared with that of D. subobscura

A detailed map of the salivary gland chromosomes of Drosophila guanche is presented and compared to the standard gene arrangements of D. subobscura. Generally, the polytene chromosome banding patterns of the two species show a high degree of homology. Only Segment I of the sex chromosome (Chromosome A) shows marked differences. The banding pattern proposed for this segment in D. guanche could have originated from a cluster of overlapping inversions including A1 arrangement.

Gene dosage compensation and the evolution of sex chromosomes

Dosage compensation is a mechanism by means of which the activity of X-linked or Z-linked genes is made equal in the two sexes of organisms with an XX compared to XY or ZZ compared to ZW basis of sex determination. In mammals, compensation is achieved by the inactivation of one X chromosome in somatic cells of females. In Drosophila, compensation does not involve inactivation. The two X chromosomes in females as well as the single X in males are regulated, and individual genes are thought to respond independently to the regulatory mechanism. It is proposed that in both groups of organisms the evolution of heteromorphic sex chromosomes was gradual and occurred as the direct result of the evolution of dosage compensation rather than the reverse.

Construction of phylogenetic trees for proteins and nucleic acids: empirical evaluation of alternative matrix methods

The methods of Fitch and Margoliash and of Farris for the construction of phylogenetic trees were compared. A phenetic clustering technique--the UPGMA method--was also considered. The three methods were applied to difference matrices obtained from comparison of macromolecules by immunological, DNA hybridization, electrophoretic, and amino acid sequencing techniques. To evaluate the results, we used the goodness-of-fit criterion. In some instances, the F-M and Farris methods gave a comparably good fit of the output to the input data, though in most cases the F-M procedure gave a much better fit. By the fit criterion, the UPGMA procedure was on the average better than the Farris method but not as good as the F-M procedure. On the basis of the results given in this report and the goodness-of-fit criterion, it is suggested that where input data are likely to include overestimates as well as true estimates and underestimates of the actual distances between taxonomic units, the F-M method is the most reasonable to use for constructing phylogenies from distance matrices. Immunological, DNA hybridization, and electrophoretic data fall into this category. By contrast, where it is known that each input datum is indeed either a true estimate or an underestimate of the actual distance between 2 taxonomic units, the Farris procedure appears, on theoretical grounds, to be the matrix method of choice. Amino acid and nucleotide sequence data are in this category.

Isozymes and allozymes: alternate forms of protein adaptation?

The hypothesis is put forward that constant or predictable environments will select for multiple locus enzymes (isozymic variation) while variable or nonpredictable environments will select for multiple alleles at single loci (allozymic variation). If this hypothesis is valid, isozymes and allozymes should represent alternative forms of enzyme adaptation and the levels of the two types of variation should be negatively correlated. This prediction was tested from data reported in the literature for five groups of organisms: mammals, other vertebrates, insects, other invertebrates and plants. Single substrate enzymes showed the predicted negative relationship between allozymic and isozymic variation. Multiple substrate enzymes did not show a significant relationship. Isozymic variation was greater for insects, other invertebrates and plants, in multiple substrate enzymes compared to single substrate enzymes. For mammals and vertebrates, allozymic variation for multiple substrate enzymes was approximately double that of single substrate enzymes, while isozymic variation remained constant.

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

Drosophila melanogaster 5S ribosomal RNA labeled with 125I was used as an in situ hybridization probe to localize complementary sequences in chromosomes of species in the Drosophila virilis group. Whereas virilis, the ancestral species, has two different 5S gene loci, the derived species show only one of these loci; in the two lines that have evolved from virilis it is the opposite locus that is conserved. The possible events leading to such an arrangement are discussed.