B chromosomes and genome size in flowering plants.

B chromosomes are extra chromosomes found in some, but not all, individuals within a species, often maintained by giving themselves an advantage in transmission, i.e. they drive. Here we show that the presence of B chromosomes correlates to and varies strongly and positively with total genome size (excluding the Bs and corrected for ploidy) both at a global level and via a comparison of independent taxonomic contrasts. B chromosomes are largely absent from species with small genomes; however, species with large genomes are studied more frequently than species with small genomes and Bs are more likely to be reported in well-studied species. We controlled for intensity of study using logistic regression. This regression analysis also included effects of degree of outbreeding, which is positively associated with Bs and genome size, and chromosome number, which is negatively associated with Bs and genome size, as well as variable ploidy (more than one ploidy level in a species). Genome size, breeding system and chromosome number all contribute independently to the distribution of B chromosomes, while variable ploidy does not have a significant effect. The genome size correlates are consistent with reduced selection against extra DNA in species with large genomes and with increased generation of B sequences from large A genomes.