George L. Gabor Miklos

Functional Aspects of Satellite DNA and Heterochromatin

Publisher Summary This chapter discusses the functional aspects of satellite DNA and heterochromatin. Despite all attempts to formulate simple rules governing satellite evolution, it is now clear that each case so far analyzed has brought with it its own claims for generalization, none of which have proven sufficiently all-embracing. One initial hypothesis on satellite evolution was that satellites wax and wane with amazing rapidity in evolutionary terms, so that closely related species differ drastically in amount or type of satellite. However, improved methods of DNA sequencing have led to the suggestion that closely related species appear to modulate their satellites from a common library. Because some kinds of heterochromatin are now known to contain satellite DNA and because a large literature exists on the properties of heterochromatin, it is necessary to consider the relationships between heterochromatin and satellite DNA in some detail. The term “heterochromatin” was initially used to define chromosomes or chromosome segments which did not uncoil at mitotic telophase and so maintained a condensed or heteropycnotic state throughout interphase and into the subsequent prophase of the next division cycle.

Telomeric satellite DNA functions in regulating recombination.

Molecular and cytogenetical analyses of three sibling species of Australian grasshopper, Atractomorpha australis, A. species-1 and A. similis, resolves one of the long standing problems of highly repeated DNA. In this system satellite DNA functions in regulating the level and position of recombination, irrespective of whether the repeated DNA is located in telomeric or centric regions. — Even though the three species do not differ in their euchromatic genome sizes, their relative DNA contents are 1.00/1.10/ 1.41, the difference in genome size being due solely to visible centric or telomeric blocks of heterochromatin. — Antibiotic analytical and preparative ultracentrifugation, in situ hybridization and renaturation kinetic analyses reveal that a large cryptic satellite of A. similis constitutes the heterochromatic telomeric blocks of nearly all autosomes and that the DNA of this satellite is highly repeated. — Comparison of these grasshopper data with published literature of heterochromatic rearrangements in Drosophila and with heterochromatin distribution and recombination patterns in diploid plant species reveals that in every case heterochromatin is implicated in some form of alteration in the meiotic recombination system.

Genetic studies on heterochromatin in Drosophila melanogaster and their implications for the functions of satellite DNA.

In Drosophila melanogaster the centromeric heterochromatin of all chromosomes consists almost entirely of several different satellite DNA sequences. In view of this we have examined by genetic means the meiotic consequences of X chromosomes with partial deletions of their heterochromatin, and have found that the amount and position of recombination on each heterochromatically deleted X is substantially different from that of a normal X. It appears that the amount of heterochromatin is important in modifying the “centromere effect” on recombination. — In all the deleted Xs tested, chromosome segregation is not appreciably altered from that of a nondeleted control chromosome. Thus satellite DNA does not appear to be an important factor in determining the regular segregation of sex chromosomes in Drosophila. Additionally, since X chromosomes with massive satellite DNA deficiencies are able to participate in a chromocenter within salivary gland nuclei, a major role of satellite DNA in chromocenter formation in this tissue is also quite unlikely. — In order to examine the mechanisms by which the amount of satellite DNA is increased or decreased in vivo, we have measured cytologically the frequency of spontaneous sister chromatid exchanges in a ring Y chromosome which is entirely heterochromatic and consists almost exclusively of satellite DNA. In larval neuroblast cells the frequency of spontaneous SCE in this Y is approximately 0.3% per cell division. Since there is no meiotic recombination in D. melanogaster males and since meiotic recombination in the female does not occur in heterochromatin, our results provide a minimum estimate of the in vivo frequency of SCE in C-banded heterochromatin (which is predominantly simple sequence DNA), without the usual complications of substituted base analogs, incorporated radioactive label or substantial genetic content. — We emphasise that: (a) satellite DNA is not implicated in any major way in recognition processes such as meiotic homologue recognition or chromocenter formation in salivaries, (b) there is likely to be continuous variation in the amount of satellite DNA between individuals of a species; and (c) the amount of satellite DNA can have a crucial functional role in the meiotic recombination system.

Genetic dissection of heterochromatin in Drosophila: The role of basal X heterochromatin in meiotic sex chromosome behaviour

We have examined the female meiotic behaviour of three X chromosomes which have large deletions of the basal heterochromatin in Drosophila melanogaster. We find that most of this heterochromatin can be removed without substantially altering pairing and segregation of the two Xs. To compare the role of heterochromatin in male meiosis we have constructed individuals which carry two extra identical heterochromatic mini X chromosomes. These minis behave as univalents even though their heterochromatin is known to contain satellite DNA. We conclude therefore that this satellite DNA is not sufficient to allow effectively normal meiotic behaviour. In all other respects our results in the male extend and confirm Coopers postulate that there exist specific pairing sites in the X heterochromatin. Thus we find no support in either female or male meiosis for the concept that satellite DNA is involved in meiotic chromosome pairing of either a chiasmate or an achiasmate kind.

Meiotic Drive in Drosophila: New Interpretations of the Segregation Distorter and Sex Chromosome Systems

Publisher Summary This chapter reviews that in many plants and animals, heterozygotes for a genetic marker do not yield the expected equality of the two alleles in their progeny. This departure from mendelian expectation is in some cases shown to result from a meiotic event rather than by simple gametic or zygotic lethality; these have been termed cases of meiotic drive, the implication being that particular alleles or chromosomes can be “driven” to a higher population frequency as a consequence of the meiotic event. It discusses that a recent review has considered many of these systems; prominent among the systems they discussed were several, which have been described in male drosophila melanogaster. This chapter is concerned mainly with recent developments in the understanding of these Drosophila systems. It focuses on the segregation distorter (SD) system where the understanding of the genetic and morphological basis of drive has drastically altered in the last two years. In addition, we will examine a number of sex chromosome meiotic drive systems and present a new hypothesis as to their basis.

Nucleotide sequences of highly repeated DNAs; compilation and comments

The nucleotide sequence data from highly repeated DNAs of invertebrates 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 sequence per se is unlikely to be under significant biological constraints and may sensibly be examined by analogy to Kimuras neutral theory for allelic variation. It is not possible with the present evidence to discriminate between the roles of neutral and selective mechanisms 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 the cellular level is unlikely to prove profitable. The presence or in some cases the amount of 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 regions rec~, 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.

Mitochondrial genetics, circular DNA and the mechanism of the petite mutation in yeast.

We propose a general hypothesis involving properties of circular DNA which can explain such phenomena as the petite mutation, suppressiveness, and the polarity observed in mitochondrial recombination in the yeast Saccharomyces cerevisiae . This hypothesis involves excision and insertion events between circular DNA molecules as well as structural rearrangements in the DNA generated by these events. The special properties of circular DNA have been considered in analysing recombination, and a number of results are obtained which are not intuitively apparent. This hypothesis can be applied to any situation involving circular DNA such as bacterial plasmids and cytoplasmic circular DNAs, where the opportunity exists for recombination and rearrangement events.

Restriction endonuclease and molecular analyses of three rat genomes with special reference to chromosome rearrangement and speciation problems

When differences are found between related species of organisms, it is often assumed that the differences themselves are causal factors either in speciation itself or in processes related to speciation. Two recent proposals on the functions of satellite DNA (Hatch et al., 1976 and Fry and Salser, 1977) are that (a) large amounts of satellite DNA are important in facilitating chromosome rearrangements and hence cytogenetic evolution, and (b) satellite DNA differences between homologous chromosomes lead to pairing difficulties and are important in generating infertility barriers and hence speciation. If these proposals were to have some generality, one could expect organisms with very low amounts of highly repeated DNA to exhibit few chromosome rearrangements and to be evolutionarily conservative in a cyto-genetic sense. — We have chosen two very closely related species of rat which are phenotypically almost indistinguishable and which have undergone massive genome reorganization. They differ by 11 major centric rearrangements (2n=32, 2n=50). We have characterised their genomes by restriction endonuclease digestions, thermal denaturations, analytical ultracentrifugations and reassociation techniques, and have found that they have virtually no highly repeated DNA. Thus the 11 major chromosomal rearrangements have been fixed in present day genomes with hardly any highly repeated DNA, centric or otherwise. — It appears therefore that a large amount of highly repeated DNA is not obligatory for the formation and fixation of chromosome rearrangements. In addition, the existing literature reveals that one can find almost any situation at all, from species groups with high amounts of satellite DNA and no gross chromosomal rearrangements, to ones such as those described here, with tiny amounts of highly repeated DNA and massive chromosomal reorganisation. Since direct experimental data indicates that satellite DNA differences per se between homologous chromosomes do not cause infertility, speculations concerning modes of speciation based on satellite DNA differences between otherwise homologous chromosomes would appear to be ill founded.

Localization of the genes shaking-B, small optic lobes, sluggish-A, stoned and stress-sensitive-C to a well-defined region on the X-chromosome of Drosophila melanogaster

Using deletion mapping and complementation tests, we have localized 5 behavioral mutations: shaking-B2, small optic lobesKS58, sluggish-AEE85, stonedts1, and stress-sensitive-C1 to 4 genetic complementation groups at the base of the X-chromosome. Shaking-B2 is an allele of the lethal complementation group R-9-29 near band 19E3; small optic lobesKS58 and sluggish-AEE85 belong to adjacent complementation groups, between lethals W2 and A112 near band 19F4; and stonedts1 and stress-sensitive-C1 are both alleles of the 8P1 lethal complementation group between lethals 114 and 13E3 near bands 20B-C.

The Mutations Previously Designated as Flightless-I3, Flightless-O2 and Standby are Members of the W-2 Lethal Complementation Group at the Base of the X-Chromosome of Drosophila Melanogaster

By using a well defined panel of chromosomal deficiencies, duplications and lethals, we have mapped three mutations causing flightlessness, flightless-I3, flightless-O2 and standby, to a single lethal complementation group (termed W-2) at the base of the X-chromosome of D. melanogaster. We also show that a fourth flightless mutation, termed grounded, previously mapped near to the base of the X-chromosome, is distal to the cytogenetic interval 18F to 20F. Mutants homozygous for the flightless-I3, flightless-O2 and standby mutations exhibit abnormalities of myofibrillar arrangements in the indirect flight muscles. They have distorted Z-bands and the myofibrils are often displaced from their normal parallel arrangement. These viable flightless mutations are all hypomorphs since the homozygous deficiency of the W-2 X-chromosomal region is lethal to the organism.