Diploid apomicts of the Boechera holboellii complex display large-scale chromosome substitutions and aberrant chromosomes

Abstract

We conducted a cytogenetic study of sexual lines of Boechera stricta and Boechera holboellii (2n = 14) and seven diploid apomictic accessions of their interspecific hybrid Boechera divaricarpa and B. holboellii (2n = 14 or 15). By studying chromosome morphology, rDNA repeats, genome painting, male meiosis, pollen morphology, and flow-cytometry seed screens, we revealed an unexpected plethora of chromosome forms, pairing behavior, and hybrid composition in all apomictic lines. Genome painting demonstrated that the apomicts are alloploid with variable numbers of B. stricta and B. holboellii-like chromosomes. We assume that large-scale homeologous chromosome substitutions took place in the apomictic hybrids that resulted from recurrent diploid-polyploid transitions through restitutional meiosis and polyploidy-diploid transitions through reductional meiosis. A second peculiarity was the presence of a largely heterochromatic chromosome (Het) in all apomictic accessions (2n = 14 and 15) and an additional smaller chromosome (Del) in the aneuploids (2n = 15). Both chromosomes share repetitive pericentromere repeats with those from the sexual B. stricta, suggesting that they originated from this species. Pairing and behavior at meiosis I of the Het share features with both Y and B chromosomes and suggest that the Del arose from a translocation event or homeologous recombination between a B. holboellii (or related taxon) and a B. stricta chromosome. Based on its presence exclusively in apomictic accessions, we propose that the Het chromosome plays a role in the genetic control of apomixis.

Fig. 1.

Ideograms of the sexual diploid B. holboellii (BH208) and B. stricta (BS2) and the 15-chromosome B. holboellii apomicts (BH1 and BH74) based on DAPI fluorescence and FISH with the 45S and 5S rDNA probes. The chromosome sets of the aneuploid BH1 and BH74 demonstrate excessive chromosomal heteromorphy and the two aberrant chromosomes, Het (star) and Del (diamond).

Fig. 2.

Meiosis in DAPI-stained pollen mother cells of B. holboellii apomicts. The Het chromosome (star) is often clearly heteropycnotic and can form a heteromorphic bivalent. The Del chromosome (diamond) is much shorter than the other chromosomes and can associate with two other chromosomes to form a heteromorphic trivalent. (a–c) BH1 at metaphase I. (d and e) BH74 at asynaptic pachytene (d) and metaphase I (e). (f) BH115 at metaphase I.

Fig. 3.

Genome painting of metaphase complements obtained according to the one- and two-color GISH. (a) One-color painting of a metaphase I complement of BH1. The green Het (stars) and Del (diamonds) indicate that these chromosomes have the B. stricta pericentromere repeat. (Scale bar: 3 μm.) (b) Two-color painting of a BH74 mitotic metaphase. (c–g) Karyograms of the two-color painting of the diploid sexual species B. stricta (c), B. holboellii (d), the B. divaricarpa apomicts BDi175 (e) and ES9 (f), and the B. holboellii apomict BH1 (g).Chromosomes with green signals have the B. stricta repeat. The red signals denote to the B. holboellii origin. The yellow signals in the chromosomes sets of d and g resulted from rDNA hybridizations from genomic DNAs from both B. stricta and B. holboellii genomic DNA.

Fig. 4.

Model for explaining chromosome substitutions in Boechera. Several processes may lead to allopolyploid hybrids, but only two examples are drawn here: an allodiploid resulting from 2n gametes and an allotriploid resulting from a reduced B. holboellii gamete and an unreduced (2n) gamete from B. stricta that may come from B. holboellii as well. Once such a hybrid (either amphitriploid or amphidiploid) adopts the apomictic nature, it can donate various combinations of red and green chromosomes through random interchromosomal recombination in the male meiosis.

Fig. 5.

Schematic representation of the possible origin of aberrant Het′ and Het + Del chromosomes in the 14- and 15-chromosome apomictic Boechera accessions. We assume asynapsis as a possible switch to modify chromatin and epigenetic status of a chromosome region. This region is then involved in a stepwise chromosome degeneration and heterochromatinization, leading ultimately to the Het chromosome. The Del chromosome is considered a translocation or recombinant chromosome between a B. stricta and B. holboellii chromosome.