Abstract
Supernumerary 'B' chromosomes are non-essential components of the genome present in a range of plant and animal species-including many grasses. Within diploid and polyploid ryegrass and fescue species, including the forage grass perennial ryegrass (Lolium perenne L.), the presence of B chromosomes has been reported as influencing both chromosome pairing and chiasma frequencies. In this study, the effects of the presence/absence of B chromosomes on genetic recombination has been investigated through generating DArT (Diversity Arrays Technology) marker genetic maps for six perennial ryegrass diploid populations, the pollen parents of which contained either two B or zero B chromosomes. Through genetic and cytological analyses of these progeny and their parents, we have identified that, while overall cytological estimates of chiasma frequencies were significantly lower in pollen mother cells with two B chromosomes as compared with zero B chromosomes, the recombination frequencies within some marker intervals were actually increased, particularly for marker intervals in lower recombination regions of chromosomes, namely pericentromeric regions. Thus, in perennial ryegrass, the presence of two B chromosomes redistributed patterns of meiotic recombination in pollen mother cells in ways which could increase the range of allelic variation available to plant breeders.
Highlights
B chromosomes are small supernumerary chromosomes which are found in a wide variety of both animal and plant species
Through genetic and cytological analyses of these progeny and their parents, we have identified that, while overall cytological estimates of chiasma frequencies were significantly lower in pollen mother cells with two B chromosomes as compared with zero B chromosomes, the recombination frequencies within some marker intervals were increased, for marker intervals in lower recombination regions of chromosomes, namely pericentromeric regions
There would be many DArT markers mapped in common across all six families as this would have allowed for multiple comparisons between 2B and 0B families of recombination frequency (RF) for the same marker intervals
Summary
B chromosomes are small supernumerary chromosomes which are found in a wide variety of both animal and plant species. While they are reported as having a diverse range of potential phenotypic effects on individual fitness and reproductive biology, they are non-essential to the normal functioning of the organism. B chromosomes do not recombine with the ‘normal’ autosomal (A) chromosome complement and can show non-Mendelian inheritance. This can result in lineage-specific increases in the numbers of B chromosomes per cell, within limits imposed by negative effects on fertility.
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