Low rates of homogenization of the DBC-150 satellite DNA family restricted to a single pair of microchromosomes in species from the Drosophila buzzatii cluster

A satellite DNA family, termed DBC-150, comprises slightly GC-rich repeat units of approximately 150 bp that were isolated (by DNA digestions or PCR) from the genome of all seven Drosophila species from the buzzatii cluster (repleta group). The presence of subrepeats suggests that part of the extant DBC-150 monomer originated by the duplication of small sequence motifs. The DBC-150 family is compared to the previously described pBuM satDNA family, an abundant component of the genome of five species of the cluster. The two families are different in several aspects, including primary structure, A + T content, intraspecific and interspecific variability and rates of homogenization (or nucleotide spread). The data indicate a lower rate of homogenization (and absence of complete concerted evolution) of the DBC-150 compared to the pBuM family. FISH on metaphase chromosomes revealed that the DBC-150 family is located exclusively in the microchromosomes. To our knowledge this is the first record of a complex Drosophila satDNA restricted to a single pair of microchromosomes. The observed low rates of homogenization of the DBC-150 family might be related to a presumed reduction or suppression of meiotic recombination in the microchromosomes.     

Chromosomal location and evolution of a satellite DNA family in seven sturgeon species

The Hind III satellite DNA family, isolated from the Acipenser naccarii genome, was used as a probe for fluorescent in-situ hybridization (FISH) on the karyotype of seven sturgeon species, six belonging to the genus Acipenser and one to Huso. All species except one (A. sturio) exhibit from 8 to 80 chromosome hybridization signals, mainly localized at the pericentromeric regions. Eight chromosomes with weak hybridization signals are present in H. huso and A. ruthenus, which are characterized by a karyotype with about 120 chromosomes. The species with 240–260 chromosomes, A. transmontanus, A naccarii, A. gueldenstaedtii, and A. baerii, show from 50 to 80 signals, prevalently localized around centromeres. Moreover, A. transmontanus and A. gueldenstaedtii show from 4 to 8 chromosomes with a double signal. The phylogenetic and evolutionary relationships among sturgeon species are discussed on the basis of number and morphology of signal-bearing chromosomes and on the localization of signals.     

Molecular organization of the Drosophila melanogaster Adh chromosomal region in D. replete and D. buzzatii, two distantly related species of the Drosophila subgenus

The molecular organization of a 1.944-Mb chromosomal region of Drosophila melanogaster around the Adh locus has been analyzed in two repleta group species: D. repleta and D. buzzatii. The extensive genetic and molecular information about this region in D. melanogaster makes it a prime choice for comparative studies of genomic organization among distantly related species. A set of 26 P1 phages from D. melanogaster were successfully hybridized using fluorescence in-situ hybridization (FISH) to the salivary gland chromosomes of both repleta group species. The results show that the Adh region is distributed in D. repleta and D. buzzatii over six distant sites of chromosome 3, homologous to chromosomal arm 2L of D. melanogaster (Muller's element B). This observation implies a density of 2.57 fixed breakpoints per Mb in the Adh region and suggests a considerable reorganization of this chromosomal element via the fixation of paracentric inversions. Nevertheless, breakpoint density in the Adh region is three times lower than that estimated for D. repleta chromosome 2, homologous to D. melanogaster 3R (Muller's element E). Differences in the rate of evolution among chromosomal elements are seemingly persistent in the Drosophila genus over long phylogenetic distances. This revised version was published online in July 2006 with corrections to the Cover Date.     

Organization and molecular cytogenetics of a satellite DNA family fromHoplias malabaricus (Pisces,Erythrinidae)

The chromosomes of the primitive South American teleost fishHoplias malabaricus have been analyzed by classical cytogenetic (C-, AgNOR-, Hoechst 33258-, and Q-banding) techniques. A highly repetitive DNA family has been cloned and sequenced. It is a tandemly repeated sequence of about 355 bp, yielding an overall base pair composition of 67% AT with long runs of >50% As and 70% Ts. Analysis of sequence variation has allowed the further categorization ofHoplias satellite DNA into two evolutionarily related subfamilies A and B, distinguishable by characteristic insertions and deletions within this 355-bp monomer. Subfamily A satellite is found (in diverged form) at the centromeres of mostH. malabaricus chromosomes. Sequence variants are clustered on specific chromosomal subsets. Subfamily B satellite is highly specific for the paracentromeric heterochromatin on one particular chromosome pair by fluorescencein situ hybridization. These results indicate that theHoplias satellite DNA family has evolved in a concerted manner predominantly via recombination events involving homologous, rather than non-homologous chromosome regions. The clones isolated here may be useful for the molecular, genetic, and cytological analysis of the genusHoplias.     

Comparative analysis of the MSAT-160 repeats in four species of common vole (Microtus, Arvicolidae)

The highly repeated tandemly arranged satellite DNA from the MSAT-160 family has been studied in 4 species of common vole (the Microtus arvalis group). All the monomer units analysed were classified into 4 subfamilies on the basis of similar nucleotide substitutions. The first 3 subfamilies do not show any species specificity since they combine monomers from several of the vole species examined; the fourth subfamily contains monomer units with substitutions specific to M. arvalis. Certain monomers of different species within the first 3 subfamilies display additional identical substitutions, making them more similar. Despite considerable similarity in monomer sequence within the subfamilies, specific features were found for each of the 4 species. A specific ratio of each type of monomer belonging to the corresponding subfamilies is typical of each species. In addition, the genomes of common vole species differ in the abundance of the MSAT-160 DNA, its pericentromeric location and organization. The mechanisms possibly involved in the evolution of the common vole MSAT-160 sequences are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Population variation in the A chromosome distribution of satellite DNA and ribosomal DNA in the grasshopper Eyprepocnemis plorans

The double FISH analysis of two repetitive DNAs (a satellite DNA and ribosomal DNA) in 12 natural populations of the grasshopper Eyprepocnemis plorans collected at the south (Granada and Málaga provinces) and south-east (Albacete and Murcia provinces) of the Iberian Peninsula has shown their widespread presence throughout the whole genome as well as extensive variation among populations. Both DNAs are found in most A chromosomes. Regularly, both DNAs occurred in the S₁₁ and X chromosomes, rDNA in the S₁₀ and satDNA in the L₂ and M₃. No correlation was found between the number of satDNA and rDNA clusters in the A genomes of the 12 populations analysed, and both figures were independent of the presence of B chromosomes. The genomic distribution of both DNAs showed no association with the geographical localization of the populations analysed. Finally, we provide evidence that the supernumerary chromosome segment proximally located on the S₁₁ chromosome is, in most cases, the result of satDNA amplification but, in some cases, it might also derive from amplification of both satDNA and rDNA.     

Cytogenetic and molecular analysis of the satellite DNA of the genusCtenomys (Rodentia Octodontidae) from Uruguay

The genus of subterranean rodentsCtenomys presents the widest range of variability in diploid number among mammals (from2n=10 to2n=70). In Uruguay, this variability is observed in karyotypes with2n=44, 50 or 58 and two geographically isolated populations with 70 chromosomes but different karyotypic structure. The last three populations were analyzed in the present study. They present a satellite DNA, which was isolated from genomic DNA afterAlul digestion.In situ hybridization showed that this satellite DNA is located in the centromeric region of a few chromosomes, coincident with Hoechst 33258 staining and C-banding patterns. A similar satellite DNA was detected in Argentinian species of this genus. We established that, in spite of differences in number of positive heterochromatic blocks per karyotype, the C value is the same in the three populations studied. The nature and possible evolutionary path of this repeated DNA is discussed.Accepted for publication by M. Schmid