Bernard John

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.

Myths and mechanisms of meiosis

A comparative analysis of the meiotic secquence in a wide variety of organisms indicates there is no convincing evidence that: (1) Premeiotic pairing plays any role in the synapsis of homologues. (2) Heterochromatic association facilitates homologous pairing. (3) Chiasmata ever form within segments which are positively heteropycnotic at zygotenepachytene. (4) Localisation of chiasmata depends on prior localisation of pairing or on the occurrence of euchromatin-heterochromatin boundaries. (5) Prior association of centromeres plays any role in determining co-orientation. (6) Any form of supra-chromosomal organisation exists involving permanent association between the members of a haploid complement, and (7) Unequal progeny ratios recovered from structurally modified Drosophila complements arise as a consequence of distributive pairing. — On the other hand there is good evidence that: (1) Interlocking of bivalents can occur regularly in species with a chiasma frequency sufficiently high to regularly produce ring bivalents and in which the chiasmata are localised to the ends of the bivalent. (2) Some forms of terminal association cannot represent terminalised chiasmata. (3) U-type exchanges present at diplotene result from errors in crossing over. (4) Pairing and chiasma formation are not necessary for coorientation, and (5) at least some types of elastic constrictions present at first metaphase represent extended nucleolar organisers.

Karyotype stability and DNA variability in the Acrididae

The Acrididae are frequently quoted as one of the classic examples of karyotypic stability. Within the family, the Cryptosacci, for instance, are characterised by a majority of species having 23 chromosome arms in the male. The members are then related by Robertsonian sequences in which the basic karyotype is believed to consist of 23 acrocentric elements. Thus the 17-chromosome complement of male truxalines is argued to have been derived from the basic type by three successive centric fusions. Such an origin is at variance with the fact that the rod-shaped chromosomes in eight of the nine species utilised in this study turn out in fact to be telocentric. The scheme is also at variance with the finding that significant differences in DNA content exist both between species within the same chromosome group and between member species of the 17 and 23 groups. The concept of karyotypic stability is thus called to question and the relationship of karyotypes within the family must be considerably more complex than has formerly been supposed.

Causes and consequences of Robertsonian exchange.

At least two types of Robertsonian exchange are now known in the acrocentric chromosomes of man. Both types involve breakage in the arms adjacent to the centromere. Evidence is presented for a third type of exchange, one involving breakage within the centromere itself, in the grasshopper Percassa rugifrons. In this species, which is regularly homozygous for a single fusion metacentric, the eighteen rod autosomes have small but pronounced granules at the centric end of the chromosome. When C-banded these granules show differential Giemsa staining and appear to represent centromeric chromomeres; these chromomeres are lacking in the metacentric fusion product. Equivalent fusions may have occurred in some mammal species too and possible examples of this are discussed in sheep and mice. The Percassa fusion has led to a modification in both the frequency and the distribution of chiasmata as judged by a comparison of these properties in the metacentric relative to the two next smallest rod equivalents. Comparable modifications are known to occur in other naturally occurring fusions but these changes are certainly not automatic consequences of fusion since they are not shown in at least some newly produced fusion mutants.

Equilocality of heterochromatin distribution and heterochromatin heterogeneity in acridid grasshoppers

Comparative fluorescence studies on the chromosome of ten species of acridid grasshoppers, with varying amounts and locations of C-band positive heterochromatin, indicate that the only regions to fluoresce differentially are those that C-band. Within a given species there is a marked tendency for groups of chromosomes to accumulate heterochromatin with similar fluorescence behaviour at similar sites. This applies to all three major categories of heterochromatin — centric, interstitial and telomeric. Different sites within the same complement, however, tend to have different fluorescence properties. In particular, centric C-bands within a given species are regularly distinguishable in their behaviour from telomeric C-bands. Different species, on the other hand, may show distinct forms of differential fluorescence at equilocal sites. These varying patterns of heterochromatin heterogeneity, both within and between species, indicate that whatever determines the differential response to fluorochromes has tended to operate both on an equilocal basis and in a concerted fashion. This is reinforced by the fact that structural rearrangements that lead to the relocation of centric C-bands, either within or between species, may also be accompanied by a change in fluorescence behaviour.

Regularities and restrictions governing C-band variation in acridoid grasshoppers

C-band patterns have been analysed in embryonic neuroblast chromosomes of 23 Australian species of acridoids. All of them showed paracentromeric C-bands but these varied considerably in size both within and between species. Many of them also showed interstitial C-bands in from 1–5 members of the haploid complement and in two cases (Atractomorpha similis and Genus nov. 95 ochracea) larger numbers of interstitial bands were present. Terminal C-bands were the least common though again when present they could be found in 1–6 members of the complement except in the cases of A. similis and Genus nov. 95 ochracea where still larger numbers occur. In 5 of the 23 species the megameric chromosome pair was distinctively C-banded. The B-chromosomes found in 3 of the species were also strikingly different in C-band characteristics compared to the standard A-chromosomes. Differences in the number of very small chromosomes present in different species clearly cannot be explained in terms of differences in their C-band content. Neither are differences in genome size simply related to differences in the total amount of C-band material indicating that changes in the size of the genome in this group have involved alterations in both eu and heterochromatin content. Finally similar amounts of C-band material may be distributed throughout the complement in very different ways in different species.

Patterns and pathways of chromosome evolution within the Orthoptera

A series of case histories are provided which confirm the occurrence of telocentric chromosomes within the Orthoptera. These cases necessitate a fairly radical revision of ideas concerning the principles governing chromosome rearrangement within this group. They also challenge several of the dogmas upon which chromosome evolution has in the past been predicated.

Heterochromatin variation in Cryptobothrus chrysophorus

The endemic grasshopper Cryptobothrus chrysophorus is widely distributed throughout S.E. Australia and its populations display an extensive and spectacular pattern of autosomal variation. While the standard telocentric complement of three long (L1–3), six medium (M4–9) and two short (S10–11) autosome pairs is present throughout most of its range, two quite distinct chromosome “races” can be defined within this species. Populations in the northern part of its distribution (northern N.S.W. and southern Queensland-northern “race”) are differentiated from the remainder (southern “race”) by fixed blocks of distal heterochromatin on autosomes M4, 5, 6, 8 and 9 and by differences in the character of the megameric M7 chromosome. Additionally, while many populations in both races show a polymorphic system of supernumerary segments on the two smallest autosomes (S10–11), that found in the northern “race” is both more variable and more complex. On the other hand all the populations of the southern “race” we have examined are polymorphic for a series of centric shifts which convert telocentrics into acro- or meta-centrics. These occur more commonly in the megameric M7 and the two smallest autosomes (S10–11) although in one population (Forbes Creek, N.S.W.) at least 12 different shifts involving 8 of the autosomes (L3, M4, 5, 6, 7, 8, 9 and S10) are known. By contrast, in the northern “erace” only the small autosomes (S10–11) show centric shifts. These several floating and fixed variants thus involve all chromosomes of the standard set other than the two largest autosomes (L1–2) and the X-chromosome, which appear to be invariate. Finally, morphologically distinct supernumerary (B) chromosomes, intermediate in size between the standard S10 and the M9 elements, are found in both “races” but are especially common in Tasmania, the most southerly point of the species range. These B-chromosomes are partly heterochromatic and partly euchromatic so that they too add to the considerable heterochromatin variation in this species.

A polymorphism for heterochromatic supernumerary segments in Chorthippus parallelus

Supernumerary heterochromatic segments have been found in the S8 bivalent of two populations of Chorthippus parallelus. These segments occur both in the heterozygous and the homozygous states. In both populations the frequencies of basic homozygotes, structural heterozygotes and structural homozygotes conform to a Hardy-Weinberg distribution. In the structurally heterozygous and homozygous states there is a significant increase in the chiasma frequency of the bivalents other than the S8 itself. It is suggested that the adaptive role of these supernumerary segments can be explained in terms of this interchromosomal effect.

Population cytogenetics of Atractomorpha similis

A 537 bp Taq 1 restriction fragment was cloned from the satellite-1 DNA of the Dee Why population of Atractomorpha similis from New South Wales. Tritiumlabelled-cRNA copies of this sat-1 probe, or else of a related Sau3A fragment from the same source, were used as in situ hybridisation probes to characterise the molecular organisation of the distal C-bands, which form a permanent and distinctive feature of the chromosomes of this species. Both probes were shown to be uniformly represented throughout all the distal C-bands not only of the Dee Why population itself but additionally in two other Australian populations where the bands are either less numerous (Fraser Island, Queensland) or smaller in size (Fogg Dam, Northern Territory). The same result was found in a population from Morehead, Papua New Guinea, which has a banding pattern similar to that of Fogg Dam. This holds whether the bands are single or multiple, terminal or subterminal. The probes were, however, consistently absent from all the proximal C-bands, whether centric or paracentromeric, as well as from the short arms which are sometimes present in the otherwise telocentric chromosomes. The results show that all the distal C-bands contain tandem blocks of highly repeated DNA from the same family of sequences. Moreover, the numerous polymorphisms which are present in the distal bands of all ten members of the basic mitotic set can be accounted for by differences in the amount of the sat-1 DNA present in a given pair of homologues. Since there is evidence to indicate that the size of the distal C-bands has increased subsequent to the introduction of the species into Australia there are good grounds for concluding that this increase has involved the amplification of the highly repeated sequence DNA present within the C-bands.