grandis: A ca. 3 myr-Old Species of Cucurbitaceae with the Largest Y/Autosome Divergence in Flowering Plants

view of plants with heteromorphic sex chromosomes reveal no relationship between species age and degree of sex chro-mosome dimorphism. Its relatively small genome size (0.943 pg/2C in males), large Y chromosome, and phylogenetic proximity to the fully sequenced Cucumis sativus make C. grandis a promising model to study sex chromosome evolu-tion. Copyright © 2012 S. Karger AG, Basel Sex chromosomes in land plants are known from 48 species in 20 families of liverworts, gymnosperms, and flowering plants, where they evolved independently and over widely different time spans [Ming et al., 2011]. In-deed, the sex chromosomes of liverworts differ so funda-mentally from those of vascular plants in functioning during the haploid phase of the life cycle, that they might better be considered a third chromosomal sex-determin-ing system, besides X/Y and W/Z systems [Bachtrog et al., 2011]. These independent origins offer the opportunity to compare incipient sex chromosomes, such as those of Pa-paya and Fragaria, which are just 0.5–2.2 myr old [Liu et al., 2004; Spigler et al., 2008, 2010; Yu et al., 2008], with older ones, such as those of Silene or Rumex, which are thought to be over 10 myr old [Moore et al., 2003; Nava-jas-Pérez et al., 2005; but see Discussion section]. So far