Sexual reproduction, hybridization, apomixis, and polyploidization in the genus Boechera (Brassicaceae)

Abstract

Of the 340 genera in the Brassicaceae, apomictic reproduction is found only in the North American genus Boechera. We investigated phylogenetic relationships, ability to hybridize, mating system, and ploidy levels of 92 lines sampled from 85 populations and representing 19 Boechera species. Phylogenetic analyses based on chloroplast DNA sequences identified three lineages in the genus. Reciprocal crosses of each line were made to a common sexual diploid B. stricta tester. The resulting F(1) progeny were analyzed for the inheritance of polymorphic microsatellite loci, genome size, and seed production. Intraspecific B. stricta crosses confirmed that this species is mostly diploid and sexual. Interspecific crosses revealed many other species were diploid and sexual and could be successfully hybridized with the tester. We also found obligate and facultative apomictic diploid and triploid lines. De novo F(1) polyploids (either triploids or tetraploids) were derived from the union of nonreduced (from an apomictic parent) and reduced (from the tester) gametes. However, seed production of these F(1) plants was generally low, suggesting a failure in the transmission of apomixis. The creation of a wide array of segregating genetic populations will facilitate future research on the evolution and inheritance of quantitative variation in Boechera.

Hybrid apomicts trapped in the ecological niches of their sexual ancestors

“…These results demonstrated that (i) some taxa that were previously classified as purely sexual [e.g., B. stricta (48) and B. crandallii (49)] are likely to contain apomicts ( Fig. 1A and Table S1) and (ii) taxa formerly considered as purely apomictic [e.g., B. divaricarpa (37,48)] are likely to be characterized by both sexual and apomictic members (Fig. 1A and Table S1).…”

“…Introgression of apomixis alleles from apomictic diploids into different diploid sexual genotypes (43,48) may have facilitated the enormous genetic diversity characteristic of apomictic Boechera (Table S4) (see also ref. 43).…”

“…S3, and Tables S3 and S4). Widespread hybridization (44,48) reflects similar apomixis frequencies across the major cpDNA-haplotype lineages (lineage I = 54.26%, lineage II = 47.74%, and lineage III = 50.64% apomixis) (Figs. 1B and 2 and Tables S2 and S4), corroborates previous findings of high genotypic diversity in other agamic complexes (e.g., T. officinale) (52), and is furthermore supported by a computational study (53).…”

“…Hybridization can be considered as a potential inducer of apomixis (19). Intra-and interspecific gene flow from apomicts to sexuals via apomictic pollen is possible (48) and likely facilitated the horizontal transfer of apomixis across Boechera (43, 51). Nevertheless, in Boechera, hybridization and apomixis are closely (47) but not exclusively (43) associated.…”

The shrunken genome of Arabidopsis thaliana

“…We do not address polyploidization in this paper but it is a relatively well understood phenomenon and evidence for multiple rounds of polyploidization in the evolutionary past of A. thaliana has already been documented (Simillion et al 2002;Wendel 2000). Furthermore, polyploidy is known to occur at the tips of the Brassicaceae tree, for example in the genera Arabidopsis and Boechera (Clauss and Koch 2006;Schranz et al 2005). Meanwhile, the importance of transposable elements in genome evolution is also well documented (Bartolome et al 2002;Bennetzen 2000;Biemont and Vieira 2005;Kumar and Bennetzen 1999;Lenoir et al 2005).…”