Mechanisms of chromosome number reduction inArabidopsis thalianaand related Brassicaceae species

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 of Arabidopsis 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 of A. thaliana arranged according to the genetic maps of Arabidopsis lyrata and Capsella rubella (both n ‫؍‬ 8) to A. thaliana, A. lyrata, Neslia paniculata, Turritis glabra, and Hornungia alpina. This approach allowed us to map the A. lyrata centromeres as a prerequisite to defining a putative ancestral karyotype (n ‫؍‬ 8) and to elucidate the evolutionary mechanisms that shaped the karyotype of A. thaliana and its relatives. We conclude that chromosome ''fusions'' in A. thaliana resulted 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 to N. paniculata (n ‫؍‬ 7), T. glabra (n ‫؍‬ 6), and H. 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 genus Arabidopsis.chromosome painting ͉ genome homeology ͉ karyotype evolution ͉ genome colinearity ͉ phylogeny

Fast Diploidization in Close Mesopolyploid Relatives ofArabidopsis

“…Phylogenetic reconstructions showed that both ACK-like parental genomes (i.e., paralogous gene copies a1 and a2) of the Australian mesopolyploids are nested within the paraphyletic tribe Camelineae and the closely related Boechereae. It was shown that karyotypes of all so far analyzed taxa from three tribes of Lineage I (Camelineae, Boechereae, and Descurainieae) resemble or were derived from the ACK (Figure 4; Lysak et al, 2006 and references therein; Schranz et al, 2007;Roosens et al, 2008). Here, we showed that species of two other Camelineae genera (Crucihimalaya and Transberingia), considered as potential parental genomes of the Australian species, descended from the ACK.…”

Comparative linkage maps suggest that fission, not polyploidy, underlies near-doubling of chromosome number within monkeyflowers (Mimulus; Phrymaceae)

“…In plants, chromosomal fusion (Schubert and Lysak, 2011;Lysak and Schubert, 2012) has been widely inferred as a cause for chromosome number reductions following ancestral polyploidization in model taxa such as Arabidopsis and maize (Lysak et al, 2006;Wang and Bennetzen, 2012). In contrast, fission has received relatively little attention as a mechanism of chromosome number change in plants.…”

The shrunken genome of Arabidopsis thaliana

“…This has revealed a complicated history of genome and chromosome level duplication and rearrangement in A. thaliana as illustrated by Lysak et al (2006) (for a review see Schranz et al 2007). It is clear that simply adding together the micro-level processes will not necessarily provide us with a full understanding of genome evolution (Charlesworth et al 2001;Gregory 2004Gregory , 2005b).…”