FIGURE 1.

Meiosis and apomeiosis in 64 Boechera accessions (organized by reproductive mode). (A) Frequencies by taxon of ovules exhibiting sexual tetrads, sexual or Taraxacum-type diplosporous dyads, Hieracium-type aposporous gametophytes, or Antennaria-type diplosporous gametophytes (accession numbers correspond to those in Supplementary Table S1). Tetrad, dyad, and Antennaria-type diplosporous gametophyte frequencies per accession sum to 100%. Aposporous gametophyte frequencies are listed separately (red bars). These develop adventitiously while meiotic tetrads form and degenerate. Median (Med.) numbers of SSR alleles observed among homozygous samples of each sexual accession (Supplementary Table S2) are shown (Med.) as are numbers (No.) of correctly staged ovules analyzed per accession. (B) A hypothesis of evolutionary cycling between hybridization induced apomixis and apomixis facilitated reticulate evolution of new sexual species. Blue box: taxa with mostly 2n pollen with some reduced and shrunken pollen, which suggests meiotic anomalies due to recent interspecific hybridization but with a transition from diplospory to apospory occurring in some taxa possibly due to early genome diploidization events associated with infrequent selfing (15–21); red box: taxa with mostly fertile reduced pollen coupled with apospory and sexual tetrad formation and degeneration, which suggests more extensive genome diploidization with a gradual restoration of sexual fertility; green box: sexually fertile anthers and pistils with no cytological evidence of apomeiosis.