How Single Molecule Real-Time Sequencing and haplotype phasing have enabled reference-grade diploid genome assembly of wine grapes.
Domesticated grapevines (Vitis vinifera) have relatively small genomes of about 500 Mb (Lodhi and Reisch, 1995; Jaillon et al., 2007; Velasco et al., 2007), which is similar to other small-genomes species like rice (430 Mb; Goff et al., 2002), medicago (500 Mb; Tang et al., 2014), and poplar (465 Mb; Tuskan et al., 2006). Despite their small genome size, the sequencing and assembling of grapevine genomes is difficult because of high levels of heterozygosity. The high heterozygosity in domesticated grapes may be due, in part, to their domestication from an obligately outcrossing, dioecious wild progenitor. Domesticated grapes can be selfed, in theory, because their mating system transitioned to hermaphroditic, self-fertile flowers during domestication. In practice, however, selfed progeny tend to be non-viable, presumably due to a high deleterious recessive load and resulting inbreeding depression. As a consequence of these fitness effects, most grape cultivars are crosses between distantly related parents (Strefeler et al., 1992; Ohmi et al., 1993; Bowers and Meredith, 1997; Sefc et al., 1998; Lopes et al., 1999; Di Gaspero et al., 2005; Tapia et al., 2007; Ibáñez et al., 2009; Cipriani et al., 2010; Myles et al., 2011; Lacombe et al., 2013).