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

*Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, NL-6703 HA, Wageningen, The Netherlands;
^‡Apomixis Research Group, Department of Cytogenetics and Genome Analysis, Leibniz Institute of Plant Genetics and Crop Plant Research, D-06466 Gatersleben, Germany;
^§Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Kruislaan 318, NL-1098 MS, Amsterdam, The Netherlands;
^¶Department of Biology, Duke University, Durham, NC 27708; and
^‖Laboratory of Genetics, Wageningen University, Arboretumlaan 4, NL-6703 BD, Wageningen, The Netherlands

Communicated by Maarten Koornneef, Wageningen University and Research Centre, Wageningen, The Netherlands, July 15, 2007 (received for review May 20, 2007)

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.

Footnotes

**To whom correspondence should be addressed. E-mail: hans.dejong{at}wur.nl
Author contributions: H.d.J. designed research; L.K. and T.M.-O. performed research; L.K., T.F.S., M.E.S., and H.d.J. analyzed data; and T.F.S., M.E.S., S.d.V., and H.d.J. wrote the paper.
↵ ^†Present address: Kasetsart University, Kampaengsaen Campus, Nakorn Pathon 73140, Thailand.
The authors declare no conflict of interest.
Abbreviations:

FCSS,

flow-cytometric seed screen;

GISH,

genomic in situ hybridization.