Ribosomal RNA genes and supernumerary B chromosomes of rye

B chromosomes in plants

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. CONTENTS Summary 411 I. Introduction 411 II. Occurrence 411 III. Structure and organization 415 IV. Inheritance 422 V. Phenotypic effects 426 VI. Populations 429 VII. Applications 430 VIII. Origin 430 Acknowiedgements 430 References 431.

The influence of B chromosomes on rDNA organization in rye interphase nuclei

Patterns of rye rDNA organization in interphase nuclei were studied through the use of in situ hybridization in spreads of root meristem cells from plants with and without B chromosomes (Bs). In cells from plants without Bs each rDNA locus is organized as a single perinucleolar knob of condensed chromatin with decondensed chromatin inside the nucleolus. In plants with Bs there is a marked modification of the pattern, found in more than 23% of nuclei, which involves several regions of condensed chromatin interspersed with decondensed chromatin inside the nucleolus. This B-induced alteration in rDNA interphase organization suggests a change in expression of the rRNA genes located on the A chromosomes probably related to the reduction in nuclear RNA observed previously in plants with Bs. The influence of the Bs on the expression of A chromosome genes, through rearrangement of interphase chromatin, could provide the basis of an explanation for some of the known phenotypic effects of B chromosomes in rye.

Analysis of rye B-chromosome structure using fluorescence in situ hybridization (FISH)

Fluorescence in situ hybridization (FISH) has been used to analyse the structure of the rye B chromosome. Genomic in situ hybridization (GISH) demonstrates the high level of overall similarity between A and B chromosomes of rye, as well as the presence of a number of specific sequences. The B-specific repeat families D1100 and E3900 have been analysed in terms of their physical location and possible contiguity. Rye Bs contain members of the rye-specific dispersed repetitive family R173, as well as centromeric regions similar to those of the As. The B chromosomes analysed in our study lack detectable rDNA sequences. Anomalous results have been obtained with a number of subtelomeric repetitive probes from rye. Bs usually lack these sequences, but evidence is presented that in some cases A-B translocation events may relocate such sequences from the As to the Bs. These data are discussed in the context of current models for the origin of the B chromosome.

Origin of B chromosomes in cultivated rye

Cultivated rye (Secale cereale) and its weedy relative (S. segetale) carry B chromosomes. The B chromosomes are known to be morphologically alike at somatic metaphase and they are of the standard type in natural populations. To clarify the cytogenetic relationship between the standard B chromosomes of S. cereale and those of S. segetale, we made four crosses between Afghan S. segetale with two standard B chromosomes as a pistillate parent and Turkish, Iranian, Korean, and Japanese S. cereale, all with two standard B chromosomes as pollen parents. We observed the pairing of B chromosomes at diakinesis in pollen mother cells in all F1 hybrids with four standard B chromosomes, two from each of the pistillate and the pollen parents. The degree of pairing of B chromosomes in all F1 hybrids with four standard B chromosomes was similar to or somewhat lower than, that in parental strains with four standard B chromosomes. These results showed that the standard B chromosomes in S. segetale from Afghanistan are homologous with those in S. cereale from Turkey, Iran, Korea, and Japan. We therefore propose monophyletic origin of the standard B chromosomes in S. segetale and S. cereale.Key words: Secale, B chromosomes, origin, cultivated rye, weedy rye.

Selfish DNA as a method of pest control

The inheritance of most genes is tightly controlled, governed by the rules of mendelian inheritance if nuclear or uniparental inheritance if cytoplasmic. A few notable genes and cytoplasmic genomes have escaped this regulation. Such genes may spread by increasing their own rate of transmission despite reducing host fitness and may be regarded as 'selfish'. Their population genetics are described and it appears they may impose a significant genetic load on the host population. Modern molecular techniques may enable similar loads to be imposed on pest species either by transferring selfish genes between species, or by linking deleterious genes to a selfish locus. Alternatively, 'modifier' genes that eliminate the virulent, or disease vectorial capacity, of the pest population may be introduced by linkage to a selfish locus. Selfish elements present in multiple copies may be preferable to single-copy elements as the former are capable of a larger reduction in host fitness. The practical application of these agents depends on five factors: (i) the rate of 'reversion' to a non-selfish form; (ii) the evolution of host repressor systems; (iii) their effect on host fitness, which determines their rate of invasion; (iv) the mechanism regulating host population size in the field; and (v) their ease of manipulation in the laboratory. The first two factors are the most uncertain in most systems, but should be amenable to experimental analysis. It is proposed that the development of such techniques may result in powerful new methods of population control which may be applied to both agricultural pests and disease vectors.

Ribosomal RNA genes in B chromosomes of Crepis capillaris detected by non-radioactive in situ hybridization

A non-radioactive in situ hybridization method using biotin-labelled rDNA has made it possible to localize rRNA genes not only at the secondary constriction in both homologous chromosomes No. 3 of Crepis capillaris but also in the B chromosomes occurring in the plants employed. Very clear dot-like rDNA signals at the telomeres of both arms were observed in all B chromosomes. Histochemical silver staining, which is indicative of transcriptional activity of rRNA gene clusters, resulted in both darkly-staining nucleolar constrictions of chromosomes No. 3 and silver deposits at the telomeres of Bs. We conclude that the B chromosomes of C. capillaris are isochromosomes with active rRNA genes located near both telomeres.