Nuclear DNA variation due to B chromosomes

Evolution of B Chromosomes: From Dispensable Parasitic Chromosomes to Essential Genomic Players

B chromosomes represent additional chromosomes found in many eukaryotic organisms. Their origin is not completely understood but recent genomic studies suggest that they mostly arise through rearrangements and duplications from standard chromosomes. They can occur in single or multiple copies in a cell and are usually present only in a subset of individuals in the population. Because B chromosomes frequently show unstable inheritance, their maintenance in a population is often associated with meiotic drive or other mechanisms that increase the probability of their transmission to the next generation. For all these reasons, B chromosomes have been commonly considered to be nonessential, selfish, parasitic elements. Although it was originally believed that B chromosomes had little or no effect on an organism's biology and fitness, a growing number of studies have shown that B chromosomes can play a significant role in processes such as sex determination, pathogenicity and resistance to pathogens. In some cases, B chromosomes became an essential part of the genome, turning into new sex chromosomes or germline-restricted chromosomes with important roles in the organism's fertility. Here, we review such cases of "cellular domestication" of B chromosomes and show that B chromosomes can be important genomic players with significant evolutionary impact.

Rye (Secale cereale) supernumerary (B) chromosomes associated with heat tolerance during early stages of male sporogenesis

BACKGROUND AND AIMS

Rye supernumerary (B) chromosomes have an accumulation mechanism involving the B subtelomeric domain highly enriched in D1100- and E3900-related sequences. In this work, the effects of heat stress during the early stages of male meiosis in 0B and +B plants were studied.

METHODS

In-depth cytological analyses of chromatin structure and behaviour were performed on staged rye meiocytes utilizing DAPI, fluorescence in situ hybridization and 5-methylcytosine immune labelling. Quantitative real-time PCR was used to measure heat effects on the expression of the Hsp101 gene as well as the 3·9- and 2·7-kb E3900 forms in various tissues and meiotic stages.

KEY RESULTS AND CONCLUSIONS

Quantitative real-time PCR established that heat induced equal up-regulation of the Hsp101 gene in 0B and 2B plants, with a marked peak in anthers with meiocytes staged at pachytene. Heat also resulted in significant up-regulation of E3900-related transcripts, especially at pachytene and for the truncated 2·7-kb form of E3900. Cytological heat-induced anomalies in prophase I, measured as the frequency of anomalous meiocytes, were significantly greater in 0B plants. Whereas telomeric sequences were widely distributed in a manner close to normal in the majority of 2B pachytene cells, most 0B meiocytes displayed abnormally clustered telomeres after chromosome pairing had occurred. Relevantly, bioinformatic analysis revealed a significantly high-density heat responsive cis regulatory sequence on E3900, clearly supporting stress-induced response of transcription for the truncated variant. Taken together, these results are the first indication that rye B chromosomes have implications on heat tolerance and may protect meiocytes against heat stress-induced damage.

Variable karyotype with B-chromosomes in Bellevalia saviczii (Liliaceae)

Bellevalia saviczii (Liliaceae), a hexaploid (2n = 6x = 24), was collected from mountain slopes in the Shiraz valley, southern Iran. The basic karyotype consists of one long metacentric chromosome, a slightly shorter acrocentric, and two shorter submetacentrics. In 32% of the plants, one and sometimes two of the acrocentrics (IIa) appeared with a pericentric inversion that changed it into a metacentric (IIm). This metacentric IIm was present in a Hardy-Weinberg ratio throughout the collection range of more than 300 km. A number of aneuploids were found, pentasomics [2n - 1 = 23 (4.9%)], heptasomics [2n + 1 = 25 (2.5%)], and one octasomic [2n + 2 = 26 (0.3%)]. Re-assorted karyotypes with 2n = 24, but with odd numbers of two of the chromosomes, were also present (6.6%). B-chromosomes were found in root meristems and in pedicels in a number (19.1%) of the plants that grew around a 30 km marsh. Their numbers ranged from 1B to 8Bs per plant peaking in the 2B mode. The B-chromosomes (two polymorphs with terminal and near-terminal sticking points) were telocentric with a few metacentric iso-Bs. No B-carrying plants were found in the drier regions away from the marsh. The B-frequency distribution for aneuploids and pericentric inversions was much different than for the standard euploid.

Analysis of the phenotypic effects of B chromosomes in a natural population of Metagagrella tenuipes (Arachnida: Opiliones)

Phenotypic effects of B chromosomes in a natural population of Metagagrella tenuipes (Arachnida: Opiliones) were studied. Mean number of Bs per individual in the population studied was 6.0, and remained stable during two successive summers of 1997 and 1998. In contrast to the number of B chromosomes, ratios between individuals possessing odd and those possessing even numbers of Bs changed during both collection seasons: the proportion of harvestmen with an even number of Bs decreased from June-July to October-November. A possible reason for this may be a difference in susceptibility to parasites between B-odd and B-even harvestmen. In the group of B-even individuals the percentage of infected harvestmen in the June-July samples was much higher compared to the B-odd group. In addition, the infection rate in the B-even group decreased more sharply than among B-odd harvestmen. In the group of B-even harvestmen infection was associated with reduced body size, whereas no such association was found among B-odd harvestmen. In the group of B-even individuals there was a U-shaped relationship between number of Bs and the probability of being infected by parasites, and an inverted-U-shaped relationship between body size and number of Bs. No such associations were found in the group of B-odd harvestmen. Seasonal selection is suggested to be a main factor contributing to the B-chromosome polymorphism in M. tenuipes.

Estimated frequency of B-chromosomes and population density of Astyanax scabripinnis paranae in a small stream

A study was made of the frequency of B-chromosomes and the population density of Astyanax scabripinnis paranae (Pisces, Characidae, Tetragonopterinae) from three stretches of the Cascatinha stream (Botucatu, SP). In the first stretch the population was estimated to be about 212 individuals and among 35 karyotyped specimens, 23 carried one macro B-chromosome; in the second stretch the population was estimated to be about 650 individuals and among 20 specimens karyotyped, two possessed one macro B-chromosome; in the third stretch the population was estimated to be about 107 individuals and among 10 specimens karyotyped, one carried one macro B-chromosome. A significant difference was observed in the frequency of macro B-chromosomes in females (57%) and males (8.7%) (P = 0.0001). These data suggest that the B-chromosome frequency and the populational density are not directly related. The hypothesis of the existence of some adaptive effect conferred by the B-chromosomes to the specimens from the first stretch of the Cascatinha stream is presented and discussed

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 role of B chromosomes in the population dynamics of yellow-necked wood mice Apodemus flavicollis (Rodentia, Mammalia)

Seasonal changes in the frequency of animals with B chromosomes were studied during 1989 in the population of yellow-necked wood mice, Apodemus flavicollis, on the mountain Jastrebac in Yugoslavia. During the study the estimated density of animals reached a very large value (378.8 individuals/ha for the April sample), which produced stress conditions leading to population decline. Six samples with a total of 247 specimens were karyotypically analyzed. Animals with Bs were present in all samples in frequencies that differed significantly at the 5% level. This difference was produced by the reduction in frequencies of specimens with Bs between the April and May samples (0.53 versus 0.29; chi 2(1) = 9.44; p < 0.002) and appeared mostly as a result of increased elimination of B-carrying animals from the prereproductive group. Young animals with Bs were inferior in stress conditions produced by overabundance of animals. From the first to the last sample, the frequency of animals with one B among B-containing animals constantly decreased, while the frequency of animals with more than one B increased. The possible role of Bs in regulating population dynamics is discussed.

B-chromosomes: germ-line parasites which induced changes in host recombination

The object of this paper is to suggest that there may be an unexpected connexion between parasites and the evolution of sex, using for illustration an unfamiliar type of parasite, the selfish chromosome. The major intellectual challenge of sexuality is to an environment which is continually getting worse. The elegant solution given by the Red Queen theory (Levin, 1975; Hamilton, 1980; Bell, 1982; Bell & Maynard Smith, 1988) is that the relevant aspect of the environment is provided by antagonists—pathogens, predators and competitors—which, because they can respond adaptively so as to negate any improvement that has been made, provide a constant stimulus for continued evolution. Sexuality and recombination are favoured because some of the new combinations of genes which they create are resistant to the current population of antagonists. In other respects, sex and recombination are probably highly disadvantageous: outcrossed sex is expensive because it halves the rate of transmission of genes, while recombination breaks up successful combinations of genes. It is only in certain circumstances that the necessity for continual counter-adaptation will overcome these disadvantages: in particular, the damage (reduction in fitness) caused by an antagonist must be substantial, and the amount of damage must depend on a genetic interaction between the antagonistic species. These requirements are often satisfied by host—parasite systems, where both the ecological and genetic interactions between the antagonists may be very severe and highly specific (see reviews by Day, 1974 and Burdon, 1987). It is possible, therefore, that sex and recombination are maintained in natural populations largely through the dynamics of the coevolution of hosts and their parasites. This is certainly compatible with the major ecological patterns shown by sexual systems, with outcrossed sex being more common in the sea than in freshwater, more common at low than at high latitudes, and generally more common in stable, complex, climax environments where interactions between species are expected to be more frequent and intense (Bell, 1982). However, there is as yet no evidence which conclusively supports a direct causal link between the incidence of parasitism and the rate of recombination. In particular, it has never been demonstrated that a particular parasite has the effect of eliciting, directly or indirectly, a greater rate of genetic recombination in its host. We suggest that such a parasite exists; both the parasite and its effects are well known, but have never been interpreted in the context of the evolution of recombination through host—parasite coevolution. It is in many respects a rather unusual parasite. We shall argue thatB-chromosomes represent highly evolved parasitic DNA, transmitted through the germ line and often eliciting greater rates of recombination in the host genome.

The DNA content of the individual chromosomes of rye

The relative DNA content of individual chromosomes of Secale cereale L. was determined in 25 cells by microdensitometry of Feulgen stained preparations. The correlation value between relative DNA content and relative chromosome length was r=0.61 for all 328 chromosomes measured. However, the correlation coefficients calculated for individual cells as well as for mean values always approached 1. Taking into account the structure of rye chromosomes, this indicates that microdensitometric results may not be accurate when large quantities of heterochromatic DNA sequences are present in analyzed material.

Annual cycles in the incidence of b chromosomes in the grasshopper Myrmeleotettix maculatus (acrididae: orthoptera)

The frequency of B chromosomes has been determined for several stages in the grasshopper life-cycle, over three generations, in three natural populations spanning a steep cline in East Anglia. This has revealed a rise in B frequency in prediapause eggs, a fall during the winter in the embryos and a rise in the early nymphs in certain situations. The causes of these effects are discussed and also their importance, along with meiotic drive, in maintaining an environmentally correlated cline of stable adult B frequencies.