Mechanisms of chromosome number reduction inArabidopsis thalianaand related Brassicaceae species

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Abstract
Evolution of chromosome complements can be resolved by genome sequencing, comparative genetic mapping, and comparative chromosome painting. Previously, comparison of genetic maps and gene-based phylogenies suggested that the karyotypes ofArabidopsis thaliana(n= 5) and of related species with six or seven chromosome pairs were derived from an ancestral karyotype with eight chromosome pairs. To test this hypothesis, we applied multicolor chromosome painting using contiguous bacterial artificial chromosome pools ofA. thalianaarranged according to the genetic maps ofArabidopsis lyrataandCapsella rubella(bothn= 8) toA. thaliana,A. lyrata,Neslia paniculata,Turritis glabra, andHornungia alpina. This approach allowed us to map theA. lyratacentromeres as a prerequisite to defining a putative ancestral karyotype (n= 8) and to elucidate the evolutionary mechanisms that shaped the karyotype ofA. thalianaand its relatives. We conclude that chromosome “fusions” inA. thalianaresulted from (i) generation of acrocentric chromosomes by pericentric inversions, (ii) reciprocal translocation between two chromosomes (one or both acrocentric), and (iii) elimination of a minichromosome that arose in addition to the “fusion chromosome.” Comparative chromosome painting applied toN. paniculata(n= 7),T. glabra(n= 6), andH. alpina(n= 6), for which genetic maps are not available, revealed chromosomal colinearity between all species tested and allowed us to reconstruct the evolution of their chromosomes from a putative ancestral karyotype (n= 8). Although involving different ancestral chromosomes, chromosome number reduction followed similar routes as found within the genusArabidopsis.