The population cytogenetics of Crepis capillaris. II. The stability and inheritance of B-chromosomes

Abstract

Metacentric B-chromosomes in Crepis capillaris undergo somatic non-disjunction in the developing inflorescences. Nondisjunction occurs at several stages in development and is seen as differences in B-number (i) between pollen mother cells (PMCs) within florets, (ii) between florets within capitula, and (iii) between capitula within inflorescences. About half the plants studied showed variation in B-number within the inflorescence. The number of Bs in inflorescences is very variable but in most plants is greatly in excess of the zygotic number, ranging from 1.00 to 3.56 per PMC in lB plants. Two rounds of non-disjunction occasionally occur in the same cell lineage. The inheritance of Es is greatly enhanced over Mendelian expectation with average transmission of 0.90 through the pollen and 0.83 through the egg. The role of accumulation in the maintenance of the C. capillaris B-chromosome polymorphism is considered. Very strong selection pressures must operate against B-containing plants or gametes in natural populations.

The population cytogenetics of Crepis capillaris. III. B-chromosome effects on meiosis

“…In these crosses the average background genotypic effects on chiasma formation should be similar in the two groups of full-sibs which differ solely in respect of B-chromosomes. The effects of B-chromosomes on chiasma conditions in the A-chromosomes can be investigated directly in individuals which show instability of B-distribution during the development of the inflorescence (Parker et a!., 1989). In these plants, different B-numbers can be assessed against a common A-chromosome background genotype.…”

“…In addition two geographically and ecologically distinct British populations have been studied allowing an assessment to be made of variation in patterns of chiasma response to B-chromosomes. The transmission behaviour of C. capillaris Bs derived from these populations has previously been reported and this indicated only slight interpopulation differences (Parker et a!., 1989 Capitula containing florets at the appropriate meiotic stage were collected from the plants when available and immediately fixed in Carnoy's fixative (6: 3: 1) overnight at room temperature and stored at -10°C.…”

Generating high variability of B chromosomes in Eyprepocnemis plorans (grasshopper)

“…For instance: (i) B derivatives could originate as deletions and centric misdivisions in the plants Centaurea scabiosa (Frost, 1957), Secale cereale (Lima-de-Faria, 1963), Aster ageratoides (Matsuda, 1970), Hypochoeris maculata (Parker, 1976;Parker et al, 1982), Allium schoenoprasum (Bougourd & Parker, 1979) and Crepis capillaris (Parker et at., 1989). Remarkably, the rate of centromere misdivision giving rise to iso Bs reported in C. scabiosa (1.76x l0 to 2.6X103) and H. maculata (1.33 x 10 to 1.86>< l0) was very similar to those we have observed in E. plorans (1.4 x 10 to 2.1 x 10 for B21,0; Table 2).…”

Molecular Evidence for Transcription of Genes on a B Chromosome in Crepis capillaris

“…An accession derived from the original collection contains only one type of B, a small metacentric chromosome (Małuszyń ska and Schweizer 1989), which is morphologically identical to the B chromosome present in natural populations in Britain (Parker et al 1989). Jones et al (1989) studied synaptonemal complexes in C. capillaris plants derived from two British populations and found that single B chromosomes showed predominantly foldback meiotic pairing resulting in a symmetrical univalent hairpin loop, which suggests that the B is an isochromosome.…”