Are B-chromosomes responsible for the extraordinary genome size variation in selected Anthoxanthum annuals?

Abstract

Nuclear genome size is strongly influenced by the number, size and morphology of chromosomes, and there is often a good correlation between genome size and total chromosome length within a karyotype. Because aneuploidy or the presence of accessory chromosomes has repeatedly been reported within the Anthoxanthumaristatum/ovatum complex (Poaceae), both phenomena have to be considered as potential sources of genome size variability within this group. This variability in nuclear genome size reaches 40 %, not only within the complex but also within single populations. Genome size variation, however, does not necessarily correlate positively with the number of chromosomes, as our data also indicate. Although our karyological survey revealed the presence of at least one B-chromosome in 44 % of individuals, we found almost no correlation between the number of B-chromosomes and genome size variability. The presence of B-chromosomes usually increases individual genome size, but does not affect substantially the extent of variability within the complex or population regardless of whether individuals with accessory chromosomes are included. These findings indicate that changes mainly in A-chromosomes are responsible for a huge fraction of genome size variability in the A. aristatum/ovatum complex.