Genome Size Evolution In Arabidopsis And The Brassicaceae

MA Lysak, FR Blattner, A Meister, HB Ali and I Schubert

Institute of Plant Genetics and Crop Plant Research (IPK), Gatersleben, D-06466, Germany
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The model plant Arabidopsis thaliana has one of the smallest genomes among flowering plants (1C~0.15 pg). Comparative genetic mapping and chromosome painting (1) indicate that the Arabidopsis karyotype evolved from ancestral ones by chromosome translocations and inversions, accompanied by reduction in chromosome number and genome size. To investigate whether evolutionary changes in karyotype structure and chromosome number are also reflected by genome size variation in other Brassicaceae (Cruciferae) species and whether this variation is correlated with molecular phylogenetic relationships within the family, we performed flow-cytometric genome size estimation in more than 80 taxa. A mean 1C-value of 0.75 pg was obtained; 87% of the species had C-values smaller than 1.0 pg, but extremely large C-values were found in Matthiola and Physaria. Three Matthiola species (2n = 14) had an average 1C-value of 2.7 pg and similar genome sizes were found in Physaria bellii (2.8 pg; 2n = 8) and P. didymocarpa (2.7 pg; 2n = 56), despite the large differences in chromosome number. Although Physaria species are nested within the same phylogenetic clade as Arabidopsis (2), P. bellii has only 2n = 8 chromosomes but a genome size about 18-fold larger than that of Arabidopsis (2n = 10). However, the genome size of Aethionema species (2n = 48), which occupy the basal phylogenetic position within the Brassicaceae, was 0.9 pg.
After comparing genome sizes with current molecular phylogenies, we conclude that both genome size increases and decreases occurred as the Brassicaceae evolved. Despite a tendency for the genomes to be small, we have demonstrated that there have been recent increases in genome size, even in close relatives of Arabidopsis.

1. Lysak MA, Pecinka A, Schubert I. (2003). Chromosome Research. 11: 195-204.
2. Heenan PB, Mitchell AD, Koch M. (2002). New Zealand Journal of Botany 40: 543-562.