Localization of Drosophila nasutoides satellite DNAs in metaphase chromosomes

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.

Cytophotometric DNA determinations and autoradiographic studies in salivary gland nuclei from larvae with different karyotypes in Drosophila melanogaster

Cytophotometric DNA determinations in Feulgen stained mitotic diploid chromosome sets of neuroblasts from larvae of Drosophila melanogaster stocks, which possess different karyotypes, show significant differences between the 4C values, caused by an additional or deficient X- and Y-chromosome depending on the karyotype. The ranges of polytenic DNA size classes are theoretically expected to be doublings of the corresponding 4C mean value of each karyotype. The extinction integral data of nuclei with completely duplicated 4C quantities exclusively fall into the range of the expected size classes. Not all data falling into the range of a size class necessarily originate from duplicated nuclei, because the limits of the DNA size classes cannot be determined by measurements, but must be estimated from the confidence limits of the corresponding 4C mean value. The validity of the mitotic 4C values of the karyotypes X/X and X/Y is tested using data from non-labeled interphase nuclei, where extinction integral data accumulate in two groups. The larger values (= G2-nuclei) confirm the 4C values of mitotic chromosome sets, and the lower values (= G1-nuclei) are just half of these. Extinction integrals from individual, 3H-thymidine non-incorporating polytene salivary gland nuclei accumulate in distinct, non-overlapping groups which are always complete doublings of the preceding smaller group. In each karyotype, the most frequent data of each group are in accord with the 4C doublings. The data from labeled nuclei alternate with those from unlabeled nuclei. The measured DNA values of individual polytene nuclei that did not incorporate any 3H-thymidine, demonstrate that all chromosomal DNA replicates completely during polytenization of the chromosomes in the larval salivary gland nuclei of Drosophila melanogaster. Specifically, this would mean that the heterochromatic Y-chromosome replicates as well as the partially heterochromatic X-chromosome along with the autosomes. There is no indication of underreplicating heterochromatin.

Nucleotide sequences of highly repeated DNAs; compilation and comments

The nucleotide sequence data from highly repeated DNAs of inverte-brates and mammals are summarized and briefly discussed. Very similar conclusions can be drawn from the two data bases. Sequence complexities can vary from 2 bp to at least 359 bp in invertebrates and from 3 bp to at least 2350 bp in mammals. The larger sequences may or may not exhibit a substructure. Significant sequence variation occurs for any given repeated array within a species, but the sources of this heterogeneity have not been systematically partitioned. The types of alterations in a basic repeating unit can involve base changes as well as deletions or additions which can vary from 1 bp to at least 98 bp in length. These changes indicate that sequenceper seis unlikely to be under significant biological constraints and may sensibly be examined by analogy to Kimura's neutral theory for allelic variation. It is not possible with the present evidence to discriminate between the roles ofneutralandselectivemechanisms in the evolution of highly repeated DNA.

Tandemly repeated arrays are constantly subjected to cycles of amplification and deletion by mechanisms for which the available data stem largely from ribosomal genes. It is a matter of conjecture whether the solutions to the mechanistic puzzles involved in amplification or rapid redeployment of satellite sequences throughout a genome will necessarily give any insight into biological functions.

The lack of significant somatic effects when the satellite DNA content of a genome is significantly perturbed indicates that the hunt for specific functions at thecellularlevel is unlikely to prove profitable.

The presence or in some cases theamountof satellite DNA on a chromosome, however, can have significant effects in the germ line. There the data show that localized condensed chromatin, rich in satellite DNA, can have the effect of rendering adjacent euchromatic regionsrec−, or of altering levels of recombination on different chromosomes. No data stemming from natural populations however are yet available to tell us if these effects are of adaptive or evolutionary significance.