Characterization and genomic organization of PERI, a repetitive DNA in the Drosophila buzzatii cluster related to DINE-1 transposable elements and highly abundant in the sex chromosomes.

BACKGROUND/AIMS: Transposable elements (TEs) are dynamic components of eukaryotic genomes. We aimed to characterize TEs to help elucidate their impact on the genomic architecture, diversity and evolution of chromosomes in the D. buzzatii cluster of species (repleta group). METHODS: A full TE element of D. buzzatii, named PERI, was identified in a BAC clone available in GenBank. PERI was further analysed using bioinformatics tools, PCR and in-situ hybridization to metaphase chromosomes and DNA fibers. RESULTS: PERI shares several structures in common with DINE-1, an abundant TE found widespread along the Drosophila genus. The central region of PERI is very dynamic but revealed a disrupted pattern of nucleotide variability among its internal tandem repeats. The minimal sequence variation in D. serido suggests recent amplification. PERI accumulates near or at heterochromatic regions of all 6 pairs of chromosomes, especially on the sex chromosomes, with some clustering. CONCLUSIONS: PERI is an abundant type of DINE-1 transposon but with characteristic sequence signatures and probably restricted to the buzzatii complex. The conservation of different central domains and association with genes suggests selective constraints. Although at or near heterochromatin, the distribution of PERI does not overlap with satDNAs, probably a consequence of functional or molecular constraints.

Evolutionary dynamics and sites of illegitimate recombination revealed in the interspersion and sequence junctions of two nonhomologous satellite DNAs in cactophilic Drosophila species.

Satellite DNA (satDNA) is a major component of genomes but relatively little is known about the fine-scale organization of unrelated satDNAs residing at the same chromosome location, and the sequence structure and dynamics of satDNA junctions. We studied the organization and sequence junctions of two nonhomologous satDNAs, pBuM and DBC-150, in three species from the neotropical Drosophila buzzatii cluster (repleta group). In situ hybridization to microchromosomes, interphase nuclei and extended DNA fibers showed frequent interspersion of the two satellites in D. gouveai, D. antonietae and, to a lesser extent, D. seriema. We isolated by PCR six pBuM x DBC-150 junctions: four are exclusive to D. gouveai and two are exclusive to D. antonietae. The six junction breakpoints occur at different positions within monomers, suggesting independent origin. Four junctions showed abrupt transitions between the two satellites, whereas two junctions showed a distinct 10 bp tandem duplication before the junction. Unlike pBuM, DBC-150 junction repeats are more variable than randomly cloned monomers and showed diagnostic features in common to a 3-monomer higher-order repeat seen in the sister species D. serido. The high levels of interspersion between pBuM and DBC-150 repeats suggest extensive rearrangements between the two satellites, maybe favored by specific features of the microchromosomes. Our interpretation is that the junctions evolved by multiples events of illegitimate recombination between nonhomologous satDNA repeats, with subsequent rounds of unequal crossing-over expanding the copy number of some of the junctions.

Low courtship song variation in south and southeastern Brazilian populations of Drosophila meridionalis (Diptera, Drosophilidae).

Drosophila meridionalis is a cactus-breeding species with a wide distribution in South America. Most populations of this species are geographically isolated, what provides a promising scenario for studying evolution. Former studies of this species revealed a remarkable karyotypic variation among its populations. Up to six distinct metaphase chromosomes were described, showing that this species is polymorphic at least at the chromosomal level. In order to elucidate the taxonomic status of populations showing different metaphase chromosomes, we analyzed the courtship song of five populations of D. meridionalis in South and Southeastern Brazil. In addition, we analyzed the metaphase chromosomes of each population. Our results show that, despite the two karyotype observed, most courtship song parameters did not vary among the populations. Altogether, our results suggest that D. meridionalis from South and Southeastern Brazil represents one species with an inter-population chromosomal variability.

The Drosophila serido speciation puzzle: putting new pieces together.

The D. serido superspecies is a complex mosaic of populations distributed over a vast part of South America and showing various degrees of genetical divergence. We have analyzed its chromosomal constitution in 16 new localities of southeastern and southern Brazil. Both the metaphase and salivary gland chromosomes show a sharp split of these populations in two groups. Four populations, fixed for inversion 2e8 and showing the type I karyotype, represent the southwestern limit of D. serido type B, which inhabits the Cerrado in central-western Brazil. The remaining populations are homozygous for 2x7, an inversion also fixed in the Caatinga populations of northeastern Brazil. However, their karyotype, in those populations analyzed, belong to a different type (V) from that of the Caatinga populations. Populations in this second group are polymorphic for five inversions on chromosome 2 plus another on chromosome 5 and show considerable interpopulation differentiation. The breakpoints of chromosome 2 inversions are described and the inversion loops of several heterokaryotypes are presented. Biogeographical information suggests that there are clear ecological differences between the two groups of populations as well as among the populations within the second group. Thepossible role of host plants in promoting the genetic divergence among the D. serido populations is discussed.

Geological record and reconstruction of the late Pliocene impact of the Eltanin asteroid in the Southern Ocean.

In 1995, an expedition on board the research vessel FS Polarstern explored the impact site of the Eltanin asteroid in the Southern Ocean, the only known asteroid impact into a deep ocean basin. Analyses of the geological record of the impact region place the event in the late Pliocene (approximately 2.15 Myr) and constrain the size of the asteroid to be >1 km. The explosive force inferred for this event places it at the threshold of impacts believed to have global consequences, and its study should therefore provide a baseline for the reconstruction and modelling of similar events, which are common on geological timescales.