Nature and evolution of B chromosomes in plants: A non-coding but information-rich part of plant genomes

Abstract

This work reviews recent advances providing insights on the origin and evolution of B chromosomes (Bs) in representative plant species. Brachyome dichromosomatica has large and micro Bs. Both carry an inactive ribosomal gene cluster. The large Bs contain the B-specific Bd49 family, mainly located at the centromere. Multiple copies are present in the A chromosomes (As) of related species, whereas only a few copies exist in B. dichromosomatica As. The micro Bs share sequences with the As, the large Bs and have the B-specific repeats Bdm29 and Bdm54. It seems that the large and micro Bs are related in origin. It is very unlikely that the Bs originated by simple excision from the As. Rye Bs are composed of sequences predominantly shared with the As. B-specific sequences are located at the heterochromatic end of the long arm. Probably, they originated from the As after many rearrangements, with a tendency for duplication. The E3900 family derives from a Ty3 gypsy retrotransposon, but the D1100 family shows no evidence of genic origin. The overall composition of maize As and Bs is similar suggesting a common origin. Several B-specific sequences have been found, the most studied being pZmBs, which is located at the B centromere. It shows partial homology to the centromere of chromosome 4 and to the knobs. It is not known whether the B centromere derives from centromere 4, or whether both have a more distant common origin. The dynamics of Bs in populations depends on their non-Mendelian mechanisms of transmission, their effects on carrier fitness and on A genes modulating their parasitic properties. Three representative examples are reviewed. The Bs of Allium schoenoprassum are transmitted at a mean lower than Mendelian and adversely affect vigour and fertility. However, there is a differential selection operating in favour of B-containing seedlings. Rye Bs undergo strong drive, which is counteracted by harmful effects on fertility and instabilities at meiosis. Both nondisjunction and meiotic behaviour, and consequently the establishment of B polymorphisms, mainly depend on the Bs themselves. B nondisjunction in maize is controlled by the B, but the As control preferential fertilisation. Considering the non-equilibrium model, the Bs of Allium seem to have been neutralised by the A genome, the As of maize provide defence against B attack, whereas the Bs of rye are only slightly neutralized.

Mitotic karyotype stability and meiotic irregularities in the families Loranthaceae Juss. and Viscaceae Miq

“…We observed a marked reduction in the size of some chromosomes in two of the three species analysed, suggesting the existence of structural changes within these karyotypes. Wiens (1964) and Barlow and Martin (1984) noted the occurrence of B chromosomes in this genus, which is also indicative of karyotypic rearrangements (Puertas 2002). A rare and extreme case of structural changes is observed in several species of Viscum, where heterozygosity for large chromosomal interchanges are associated with a complex sexdetermining chromosome system and dysploid chromosome numbers (Wiens and Barlow 1979;Aparicio 1993).…”

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

“…These chromosomes, which have the capacity to accumulate through generations or during mitosis, have generally been considered to be genetically, although not functionally, inert. There is ample evidence that they have a great variety of exo-and endo-phenotypic effects ( Jones and Rees 1982;Puertas 2002).…”

Centromere Function and Nondisjunction Are Independent Components of the Maize B Chromosome Accumulation Mechanism

“…Such dispensable chromosomes, called B chromosomes, have a neutral or slightly negative impact on the fitness of individuals that carry them (Puertas, 2002;Jones and Houben, 2003) and are typically highly heterochromatic, reflecting their gene poor nature. Because they are not maintained by fitness selection, successful B chromosomes that have spread through populations have acquired accumulation mechanisms that allow them to be transmitted to progeny at frequencies higher than predicted by Mendelian ratios (Puertas, 2002;Jones and Houben, 2003). B chromosomes are undoubtedly derived from A chromosomes at some point (Jones and Houben, 2003).…”