ORIGIN AND EVOLUTION OF B CHROMOSOMES IN THE Boechera holboellii COMPLEX: ARE THEY THE CAUSE OR RESULT OF APOMICTIC REPRODUCTION?

¹ Max Planck Institut für Chemische Ökologie, Beutenberg Campus, Winzerlaer Str. 10, D-07745 Jena, Germany.

² Institute of Plant Genetics and Crop Plant Research (IPK), Corrensstrasse 3, D 06468 Gatersleben, Germany

³ Laboratory of Genetics, Wageningen University, Arboretumlaan 4, 6703 BD, Wageningen, The Netherlands

⁴ Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, 6703 HA, Wageningen, The Netherlands

The Boechera holboellii complex comprises B. holboellii and B. drummondii, both of which can reproduce through sex or apomixis. Sexuality is associated with diploid individuals, whereas apomictic individuals are diploid, aneuploid or triploid. Aneuploid individuals were found in geographically and genetically distinct populations. It is unknown whether the extra chromosomes are shared by common descent (single origin) or have originated via genomic processes associated with the repeated transition from diploidy to triploidy. Diploid plants containing the extra chromosome(s) reproduce apomictically, suggesting that the supernumerary chromosome be associated with apomixis. Karyotype analyses revealed that the supernumerary chromosome is easily distinguishable in the metaphase complement by its heterochromatic morphology and differs in length between apomictic plants from different populations. DNA sequence analyses have furthermore demonstrated elevated levels of non-synonymous substitutions in alleles found on the aneuploid chromosome, a result consistent with relaxed selection pressure acting on the asynaptic element. Although the extra chromosome in the apomictic Boechera does not pass normal reductional meiosis, in which it may get eliminated or accumulated by a B-chromosome specific process, its variable and heterochromatic nature does meet the criteria for a genuine B chromosome in other species. Its prevalence and conserved genetic composition nonetheless implies that this chromosome, if truly a B, may be atypical with respect to its influence on its carriers.