The phylogeny of nine species of the Drosophila obscura group inferred by the banding homologies of the chromosomal regions. IV. Element C

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.

The phylogenetic position of Drosophila eskoi deduced from P element and Adh sequence data

PCR screening with primers specific for the T-, M-, and O-type P element subfamilies was performed to investigate the interspecific distribution in 18 species and to reconstruct the phylogenetic history of the various types within the obscura species group. T-type elements occur in D. ambigua, D. tristis, D. obscura, D. subsilvestris, and D. eskoi. In the genomes of D. subobscura, D. madeirensis, and D. guanche they are present in the form of terminally truncated T-type derivatives. The wide distribution suggests that the T-type subfamily had a long evolutionary history in the obscura lineage. In contrast, the patchy occurrence of M- and O-type elements can be ascribed to four independent events of horizontal invasion of different lineages. The cladogenesis of the obscura group was investigated using a partial sequence of the Adh gene as a marker. In contrast to earlier findings, the position of D. eskoi had to be revised. D. eskoi appears as the closest relative of the D. ambigua clade, whereas D. tsukubaensis is the sister taxon of the species pair D. bifasciata/D. imaii. This result is in good accordance with the P element data, where high sequence similarity (95%) was found among the T-type elements of D. eskoi and those of D. ambigua and D. tristis.

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).