Genome size variation in the Aloaceae, an angiosperm family displaying karyotypic orthoselection

Chromosomal variation is widespread in angiosperms, with numbers ranging from 2n = 4 in some species to over 600 in others, and with genome sizes from the minute in Arabidopsis thaliana to the very large in Fritillaria spp. Many families show extensive karyotype variation, sometimes within a species, often with a genus. Others display karyotypic orthoselection, with identical karyotypes throughout a genus, or in groups of genera, e.g. Agave and its relatives. The highest orthoselection level occurs in the Aloaceae, which has the same bimodal karyotype in all species (x = 7; four long and three short acrocentrics). Major structural changes to Aloaceae chromosomes are common in the wild, but are not known as homozygotes and take no part in speciation. The ratio of combined volumes of the long and short chromosomes is constant in Aloe species. Despite the karyotypic uniformity, 4C nuclear DNA amounts vary two-fold in diploid species, with lowest values in primitive species. This range is demonstrable in the extreme heteromorphy of homeologues that is visible in hybrids. Supported by the observation that chiasma position in an intergeneric hybrid is random, even where homeologue length differs greatly, these findings suggest that evolutionary increase in DNA amount involves the amplification of many small DNA segments at sites distributed evenly throughout the chromosome complement in numbers proportional to chromosome length, thus maintaining karyotypic uniformity in the genus. (C) 1998 Annals of Botany Company.