Some trends in the evolution of very large chromosomes

Abstract

The general features of the arrangement and cytological distribution of repeated sequences in animal chromosomes are reviewed. These features include internal repetitiveness, conservation of clearly functional sequences, rapid divergence of certain classes of repeated sequence with subsequent fixation of families of diverged sequences, and well defined cytological localization of large blocks of sequences in specific parts of the chromosome set. Moderately or ‘middle’ repetitive (m.r.) sequences constitute a large part of the genomes of higher organisms, they seem to accumulate in a balanced manner within chromosome sets, they are mainly responsible for genome growth, and they are interspersed with sequences of other kinds. Little is known about their cytological distribution. Four experiments are described, each of which aimed to locate middle repetitive sequences in the chromosomes of a salamander and a newt. Tritiated m.r. DNA from Plethodon cinereus binds in a non-random fashion to the meiotic diplotene and mitotic chromosomes of the same species, suggesting a non-random distribution of m.r. sequences on these chromosomes. The same DNA, hybridized in situ to the RNA transcripts on the loops of lampbrush chromosomes, produces light and widespread labelling of many loops, but intense labelling of six pairs of loops, each of which lies near to a centromere. Similar experiments in which newt m.r. DNA was hybridized in situ to newt lampbrush chromosomes showed heavy labelling of about 30 loop pairs on each of the long heteromorphic arms of chromosome I, but very little labelling elsewhere. Autoradiographs of newt mitotic chromosomes hybridized with newt m.r. DNA showed rather even labelling of all chromosomes including chromosome I. The significance of the heavy labelling of lampbrush loops near centromeres and on the heteromorphic arms of the newt chromosome I is discussed in relation to possible cytological and molecular mechanisms for generating and preserving families of tandemly linked and cytologically localized m.r. sequences.

The energy components of stacked chromatin layers explain the morphology, dimensions and mechanical properties of metaphase chromosomes

“…It can be observed that the diameter and length of the chromatids increases with the amount of DNA that they contain. In agreement with this observation, individual species, such as Drosophila melanogaster [43] and chicken [44], having chromosomes that contain very different amounts of DNA, have karyotypes with a great heterogeneity of chromosome sizes ( [19,45,46]; figure 1b,c). The measurements shown in table 1 cover from the small chromatids (approx.…”

“…(a) Schematic illustration of different models for chromatin structure in metaphase chromosomes: (a) chromatin fibres (grey lines) form loops that are bound to a central protein scaffold (brown) [8][9][10], (b) chromatin fibres are irregularly folded [11][12][13], and (c) chromatin has a laminar structure with many staked layers oriented perpendicular to the chromatid axis [2,[14][15][16][17][18]; parallel lines represent the side view of the stacked layers. (b) Examples showing the differences in metaphase chromosome sizes that exist among animal species: crested newt (T. cristatus) (a), human (b), and D. melanogaster (c); the three chromosome sets are represented at the same magnification; figure reproduced from [19] with permission from the Royal Society.…”

Initial characterization of the large genome of the salamander Ambystoma mexicanum using shotgun and laser capture chromosome sequencing

“…Based on previous cytogenetic observations, we anticipate that the smallest salamander chromosomes should be slightly larger than the largest human chromosomes, likely exceeding 250 Mb 37 38 , indicating that our laser capture/amplification approach will be applicable to a diversity of organisms, including those with genomes that are substantially smaller than that of A. mexicanum . Previous studies sequenced material from small numbers of pooled chromosomes that were captured from slides 39 or within microfluidic devices 40 , which also provide invaluable scaffolding/anchoring information.…”

The nuclear content of desoxyribonucleic acid and some problems of Mammalian phylogenesis

“…Even the variation of the amount of constitutive heterochromatin (McGregor 1978) does not entirely explain the changes of GS. On the other hand, almost paradoxycally (see the "DNA-c value paradox" ; Cavalier-Smith 1978), it is becoming clearer that only a small part of DNA in a nucleus transcribes, i.e.…”