Highly differentiated heteromorphic sex chromosomes were found in the karyotypes of Pyxicephalus adspersus (= Rana adspersa) and analysed with the various banding methods. The W chromosome is considerably smaller than the Z chromosome and consists to a great proportion of constitutive heterochromatin. The chromosome lengths and the DNA content of this species have the lowest values determined in the Ranidae to date. The nucleolus organizer regions in the chromosomes reveal a high frequency of inter-individual variations (duplications, triplications) and aberrations. In one female animal, a translocation between the W chromosome and a nucleolus organizer was identified. Preliminary results indicate, that the same chromosome pair which in P. adspersus constitutes the heteromorphic sex chromosomes is still in an initial stage of morphological differentiation in P. delalandii. Although the ZZ/ZW sex chromosomes in Pyxicephalus have developed independently of the ZZ/ZW sex chromosomes in birds and reptiles, they seemed to have passed through identical stages of morphological differentiation. In this process, the heterochromatinization of the W chromosome had a decisive function.

Chromosome banding in Amphibia

summary
This review of supernumerary B chromosome systems in flowering plants deals mainly with work published in the last 15 yr, hut also includes older material which has not hitherto been presented systematically. The term B chromosome (B) is defined, and there is an introductory overview dealing with general characteristics and the significance of Bs as a widespread chromosome polymorphism. Detailed sections are then presented covering the occurrence of Bs in different taxa, their structure and molecular organization, their irregular modes of inheritance, their phenotypic effects, population dynamics and origin. Particular attention is paid to the research growth points in molecular analysis of the structure and genome organization of Bs, and to transmission genotypes in the context of their adaptive versus their selfish properties. Where appropriate, reference is also made to likely future lines of research, and also to the usefulness of B chromosomes in genetic analysis and as model systems to study general phenomena of genome organization and evolution, nuclear physiology and architecture, chromosome polymorphism and selfish DNA.

B chromosomes in plants

In the parasitic wasp, Nasonia vitripennis, males are haploid and usually develop from unfertilized eggs, whereas females are diploid and develop from fertilized eggs. Some individuals in this species carry a genetic element, termed psr (paternal sex ratio), which is transmitted through sperm and causes condensation and subsequent loss of paternal chromosomes in fertilized eggs, thus converting diploid females into haploid males. In this report the psr trait was shown to be caused by a supernumerary chromosome. This B chromosome contains at least three repetitive DNA sequences that do not cross-hybridize to each other or to the host genome. The psr chromosome apparently produces a trans-acting product responsible for condensation of the paternal chromosomes, but is itself insensitive to the effect. Because the psr chromosome enhances its transmission by eliminating the rest of the genome, it can be considered the most "selfish" genetic element yet described.

http://science.sciencemag.org/content/sci/240/4851/512.full.pdf

A "selfish" B chromosome that enhances its transmission by eliminating the paternal genome.

https://www.cell.com/cell/pdf/0092-8674(84)90504-X.pdf

Mapping of DNAase I sensitive regions on mitotic chromosomes

Supernumerary chromosomes arise from portions of the normal chromosome complement through nondisjunction, fragmentation, or other mechanisms. Once present in the genome, they are subject to virtually the same genetic conditions that affect the evolutionary degeneration of heteromorphic sex chromosomes. Y or W chromosomes occur only in the presence of X or Z chromosomes, respectively, just as supernumeraries never occur except in the presence of the complete regular karyotype containing their progenitor sequences. Thus, mechanisms that can account for the evolution of sex-chromosome heteromorphism can also be invoked to explain the degeneration process of supernumerary chromosomes after their origination. Incipient supernumeraries initially have genes identical with those on progenitor chromosomes. This frees them from the evolutionary constraint of carrying nonduplicated genetic information, just as in Y and W chromosomes during early stages of sex-chromosome differentiation. The degeneration of supernume...

Muller's Ratchet and the evolution of supernumerary chromosomes

The restriction enzymes EcoR1, Hpa II and Hae III have been used to cleave nuclear DNA of Muntiacus muntjak Each enzyme gives rise to a different pattern Each pattern is characterized by the presence or absence of bands and by the type of distribution of most of the sequences cleaved Hpa II and Hae III cleave DNA segments which are CCGG and GGCC respectively but they produce quite different patterns with the same DNA Hpa II builds a gradient of large molecules concentrated mainly between 110times103 and 600 base pairs whereas Hae III forms a smear of small molecules concentrated mainly between 34times103 and 200 base pairs This difference in pattern is characteristic of the DNA of over 30 eukaryotic species so far investigated



The action of Hae III on fixed metaphase chromosomes of M muntjak leads to the appearance of a “hairy” structure and a chromomere pattern which is not present in the controls and which does not show up after EcoR1 treatment



The effect of the enzyme on the chromosomes is not understood at present

Action of restriction endonucleases on the DNA and chromosomes of Muntiacus muntjak

The Cervidae show one of the largest variations in chromosome number found within a mammalian family. The five species of the deer family which are the subject of this study vary in chromosome number from 2n = 70 to 2n = 6. Digestion with the restriction enzymes EcoRI, HpaII, HaeIII and MspI reveals that there is a series of highly repetitive sequences forming similar band patterns in the different species. To obtain information on the degree of homology among these conserved sequences we isolated a HpaII restriction fragment of approximately 990 base pairs from reindeer DNA. This DNA sequence was 32P-labelled and hybridized by the Southern blot technique to DNAs cleaved with HpaII and HaeIII from the reindeer and four other Cervidae species. Hybridization to specific restriction fragments was recorded in all species. The patterns of hybridization showed a higher degree of similarity between reindeer, elk and roe deer than between reindeer and the Asiatic species (fallow deer and muntjac). Homologies are still present between the highly repetitive sequences of the five species despite the drastic reorganization that led to a change in chromosome number from 6 to 70.

Conservation of repetitive DNA sequences in deer species studied by Southern blot transfer

Nuclear DNAs extracted from plants of Secale cereale (rye) without and with B chromosomes were cleaved with the restriction endonucleases Hpa II, EcoR1, Bam H-1 and Hae III. Repeated sequences from distinct bands which consist of segments having about 3538 N.P. after EcoR1 digestion; 3692, 3385, 2923, 2615, 2462, 2000 and 1108 N.P. after Bam H-1 cleavage and 3077, 2000, 892, 554, 338 and 262 N.P. after Hae III digestion. In the four enzyme treatments no difference could be detected between the DNAs derived from plants with and without B chromosomes. Moreover, nuclear DNAs were centrifuged to equilibrium in CsCl in the model E ultracentrifuge. No DNA satellites could be detected in these DNAs although the number of B chromosomes varied from 0 to 6. These results support the idea, presented earlier by several authors, that B chromosomes are derived from A chromosomes by a simple and rapid process. Unique, not repeated sequences may be responsible for their formation.

E. Olsson

Papers

Action of restriction endonucleases on the DNA and chromosomes of Muntiacus muntjak

Conservation of repetitive DNA sequences in deer species studied by Southern blot transfer

Comparison of DNA cleavage in rye with and without B chromosomes.