Hot on the Trail of the Sex-Apomixis Switch in Boechera (Brassicaceae)

Boechera encompasses »70 sexual diploid species and many apomictic diploid and polyploid hybrids. Diplospory has been reported in B. gunnisoniana and B. holboellii. Herein, we document diplospory in diploid B. duchesnensis and B. lignifera and in triploid hybrids involving B. formosa, B. lincolnensis and B. stricta. We also document apospory in B. laevigata, B. lignifera, B. microphylla, B. pallidifolia and B. thompsonii. We document sexuality for the first time in a tetraploid and in diploid B. crandallii, B. dispar, B. exilis, B. formosa, B. kelseyana in ed., B. lemmonii, B. oxylobula, B. pendulina, B. retrofracta, B. schistacea, B., sparsiflora and B. stricta. Previous studies report that stress-response genes in Boechera ovules are down regulated during diplosporous but up regulated during sexual megasporogenesis. Other studies suggest that stressing diplosporous Boechera, and various other angiospermous apomicts, tends to switch development from apomeiotic to meiotic. Herein, we document the latter in diploid B. lignifera and triploid B. cf. gunnisoniana. When grown under favorable conditions, diplosporous dyads occurred 5.3 and 100‑fold more frequently than sexual tetrads, respectively. But when plants were drought stressed, these values decreased to 1.6 and 3.8-fold, i.e., meiosis became more frequent. When stressed with drought and heat, sexual tetrads in both taxa were observed 2 to 3‑fold more frequently than diplosporous dyads. Switching from apomeiosis to meiosis was correlated with increased expression of stress response genes. Interestingly, under stress, most ovules failed to produce seeds, and based on single-seed flow cytometry, most seeds formed apomictically.