The B-chromosome system of Allium schoenoprasum

Chromosomal and genomic analysis suggests single origin and high molecular differentiation of the B chromosome of Abracris flavolineata

Supernumerary chromosomes (B chromosomes) have been an intriguing subject of study. Our understanding of the molecular differentiation of B chromosomes from an interpopulation perspective remains limited, with most analyses involving chromosome banding and mapping of a few sequences. To gain insights into the molecular composition, origin, and evolution of B chromosomes, we conducted cytogenetic and next-generation sequencing analysis of the repeatome in the grasshopper Abracris flavolineata across various populations. Our results unveiled the presence of B chromosomes in two newly investigated populations and described new satellite DNA sequences. While we observed some degree of genetic connection among A. flavolineata populations, our comparative analysis of genomes with and without B chromosomes provided evidence of two new B chromosome variants. These variants exhibited distinct compositions of various repeat classes, including transposable elements and satellite DNAs. Based on shared repeats, their chromosomal location, and the C-positive heterochromatin content on the B chromosome, these variants likely share a common origin but have undergone distinct molecular differentiation processes, resulting in varying degrees of heterochromatinization. Our data serve as a detailed example of the dynamic and differentiated nature of B chromosome molecular content at the interpopulation level, even when they share a common origin.

Distribution of B chromosomes in age categories of the yellow-necked mouse Apodemus flavicollis (Mammalia, Rodentia)

The presence of B chromosomes (Bs) is frequently found in populations of yellow-necked mouse, Apodemus flavicollis. Differences in frequencies of Bs in age categories were analyzed in 717 animals in order to clarify the mechanisms of their maintenance in populations of A. flavicollis. The absence of statistically significant differences in the frequency of Bs between six age categories indicates that the maintenance of Bs in populations can be explained by in terms of their contribution to overall genetic diversity of the species rather than by parasitic behavior.

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.

Possible heterotic effects of B chromosomes on body mass in a population of Apodemus flavicollis

The adaptive significance of supernumerary (B) chromosomes was studied in yellow-necked mice, Apodemus flavicollis. No significant change in the average frequencies of B chromosomes could be confirmed in a population from northern Bohemia during a period spanning 17 years. However, we found a significant regression between the frequency of B chromosomes and body mass. The best fit to a logistic function was obtained by a model that included the effects of both sex and body mass and their interaction. A significant relationship between the mean number of B chromosomes and body mass was found in males but not in females. It is suggested that a selective advantage for males possessing B chromosomes, possibly in relation to survival during winter, explains the data obtained.

The inheritance of B chromosomes in Allium schoenoprasum L

The inheritance of B chromosomes has been investigated. In Allium schoenoprasum from the River Wye, Powys; controlled crosses between plants of known B chromosome constitution were carried out, and the numbers of Bs present in the progenies scored. There was considerable heterogeneity in B chromosome transmission rates across the range of B chromosome numbers, through both the male and female lines, and in crosses involving odd and even numbers of Bs. The mean B number of the progeny was lower than the Mendelian expectation in 41 of the 57 crosses, significantly so in about a third of cases. No progeny had a mean B number significantly higher than expected. The mean transmission rate per B was not significantly different between parents with different numbers of Bs, in either the male or female line, or between plants carrying odd or even numbers of Bs. Transmission through the male line (0.43) was higher than through the female line (0.35), but not significantly so. The overall mean transmission rate per B (0.4), estimated from all crosses, was significantly lower than the expected rate of 0.5. This study provides conclusive evidence that B chromosomes in A. schoenoprasum from the River Wye lack an effective accumulation mechanism and, furthermore, show significant losses during transmission to the progeny of controlled crosses.

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.

CYTOGENETIC VARIATION IN THE BRITISH FLORA: ORIGINS AND SIGNIFICANCE

Cytogenetic variation in the British flora is discussed in relation to the taxonomy and evolution of species and to their distribution and past history. In polyploids there may be two or more cytotypes in Britain with different distributions and ecology, e.g. Empetrum nigrum and Hippocrepis comosa; alternatively only one cytotype is present in Britain, but it forms part of a wider polyploid pattern, the study of which may clarify past patterns of migration, e.g. Potentilla fruticosa and Sisyrinchium bermudiana. The study of polyploids may also have taxonomic significance, e.g. Cochlearia, Symphytum and Mentha. In some situations gene flow between polyploid levels takes place and may be in both directions. In other polyploid groups sexually sterile cytotypes may be maintained by vegetative reproduction, e.g. Ranunculus ficaria and Holcus mollis or by gametophytic apomixis. In some cases a polyploid and aneuploid series may be maintained in which all the components are fertile; this situation is discussed in relation to the genus Erophila. There has been considerable emphasis in the past in attempting to identify the genomic ancestors of allopolyploids; two British species, Spartina anglica and Senecio cambrensis are among the few examples of recent allopolyploids of known origin; in both cases formation has depended on the establishment of an introduced species as one parent. The significance of B chromosomes is discussed; although a range of British species contain B chromosomes it has only been possible to investigate their adaptive significance in one case. Interchanges exist in British species either in the heterozygous form, e.g. in Alopecurus species, where they can be identified by their meiotic consequences, or in the homozygous form, e.g. Epilobium sect. Epilobium species, where they can be identified by meiotic analysis of hybrids. The use of modern cytogenetic techniques including chromosome banding and in-situ hybridization and the investigation of characteristics such as infra-species variation in DNA amounts are discussed in relation to future cytogenetic investigations of the British flora.