Chromosomal phylogeny and karyotype evolution in x=7 crucifer species (Brassicaceae)

Abstract

Karyotype evolution in species with identical chromosome number but belonging to distinct phylogenetic clades is a long-standing question of plant biology, intractable by conventional cytogenetic techniques. Here, we apply comparative chromosome painting (CCP) to reconstruct karyotype evolution in eight species with x=7 (2n=14, 28) chromosomes from six Brassicaceae tribes. CCP data allowed us to reconstruct an ancestral Proto-Calepineae Karyotype (PCK; n=7) shared by all x=7 species analyzed. The PCK has been preserved in the tribes Calepineae, Conringieae, and Noccaeeae, whereas karyotypes of Eutremeae, Isatideae, and Sisymbrieae are characterized by an additional translocation. The inferred chromosomal phylogeny provided compelling evidence for a monophyletic origin of the x=7 tribes. Moreover, chromosomal data along with previously published gene phylogenies strongly suggest the PCK to represent an ancestral karyotype of the tribe Brassiceae prior to its tribe-specific whole-genome triplication. As the PCK shares five chromosomes and conserved associations of genomic blocks with the putative Ancestral Crucifer Karyotype (n=8) of crucifer Lineage I, we propose that both karyotypes descended from a common ancestor. A tentative origin of the PCK via chromosome number reduction from n=8 to n=7 is outlined. Comparative chromosome maps of two important model species, Noccaea caerulescens and Thellungiella halophila, and complete karyotypes of two purported autotetraploid Calepineae species (2n=4x=28) were reconstructed by CCP.

Figure 1.

ACK (n=8) and Phylogenetic Relationship of Selected Tribes and Species within the Family Brassicaceae. (A) Scheme of the ACK of crucifer Lineage I comprising eight chromosomes (AK1 to AK8) and 24 GBs (A to X). Modified from Schranz et al. (2006). (B) Three major phylogenetic lineages (Lineages I to III) were recognized within Brassicaceae (Beilstein et al., 2006). The six analyzed x=7 tribes (in boldface) are embedded within an unresolved assemblage of tribes including Lineage II (in blue). A tentative phylogenetic position of the ACK, and species analyzed and/or discussed in this study, are given. Base chromosome number (x) is indicated for each tribe (mult., multiple base numbers). The tree is modified from Koch and Al-Shehbaz (2008).

Figure 2.

Reconstructed Karyotypes of Conringieae, Calepineae, and Noccaeeae Species. (A) Karyotype of C. orientalis (2n=14; Conringieae) and chromosomes of this species revealed by CCP. (B) Karyotype of C. irregularis (2n=28; Calepineae). (C) Karyotype of G. laevigata (2n=28; Calepineae). (D) Karyotype of N. caerulescens (2n=14; Noccaeeae) and Noccaea chromosomes after CCP. (E) Tentative scenario of the origin of translocation chromosomes AK6/8 and AK5/6/8 from ancestral chromosomes AK5, AK6, and AK8. The 24 GBs are indicated by uppercase letters (A to X) and colored according to their positions on chromosomes AK1 to AK8 of the ACK (Figure 1A). Downward-pointing arrows indicate the opposite orientation of GBs compared with the position in the ACK. Karyotypes in (A) to (D) are drawn to scale (bar = 5 Mb). In CCP images, arrowheads point to pericentromeric heterochromatin and the arrow indicates the heterochromatic knob. Bars = 10 μm.

Figure 3.

Reconstructed Karyotypes of Eutremeae, Isatideae, and Sisymbrieae Species. (A) Karyotype of M. perfoliatum (2n=14; Isatideae) and Myagrum chromosomes after CCP. (B) A tentative scenario of the origin of translocation chromosomes AK2/5 and AK2/5/6/8 from chromosomes AK2 and AK5/6/8. (C) Karyotype of G. glastifolia (2n=14; Isatideae). (D) Karyotype of O. aegyptiacum (2n=14; Sisymbrieae). (E) Karyotype of T. halophila (2n=14; Eutremeae). The 24 GBs are indicated by uppercase letters (A to X) and colored according to their positions on chromosomes AK1 to AK8 of the ACK (Figure 1A). Downward-pointing arrows indicate the opposite orientation of GBs compared with the position in the ACK. In O. aegyptiacum, it was not possible to distinguish whether the 5S rDNA loci are located within a pericentromeric region of short or long arms of the analyzed mitotic chromosomes; thus, the 5S rDNA positions are only approximate. Karyotypes in (A) and (C) to (E) are drawn to scale (bar = 5 Mb). In CCP images, arrowheads refer to pericentromeric heterochromatin and arrows indicate heterochromatic knobs. Bar = 10 μm.

Figure 4.

Reconstruction of Karyotype Evolution in Six x=7 Tribes and Brassiceae from the PCK (n=7) and an Ancestral Karyotype (n=8) Based on CCP Data. The 24 GBs are indicated by uppercase letters (A to X) and colored according to their position on chromosomes AK1 to AK8 of the ACK (Figure 1A). Downward-pointing arrows indicate the opposite orientation of GBs compared with the position in the ACK. A tentative number of translocations (transl.) and inversions (inv.) is given at the nodes of the chromosomal phylogeny. 45S rDNA loci are shown as cross-hatched boxes. For the tribe Brassiceae (exemplified by the B. rapa karyotype, n=10; Parkin et al., 2005), only associations of GBs shared with the PCK are displayed; other chromosomes/chromosome regions are shown as black bars.