Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 192

A draft genome sequence has been known for 10 years (Ming et al., 2008), and although it should have represented a boost to further improvement of papaya, unfortunately the trend against transgenics and the high cost of getting transgenics deregulated and commercialized has halted progress.

Sequence data from the draft should assist functional genomics studies to see if traits that breeders and growers have observed over time actually have sequence confirmation. For example, in the Xanthorrhoeaceae, taxonomists dissected three species, Haworthia, Haworthiopsis and Tulista, from one species, Haworthia, that were placed into subgenera based on phenotypic characteristics like flower morphology (Gildenhuys and Klopper, 2017). A phylogram (Manning et al., 2014) showing maximum parsimony bootstrap support below the branches and Bayesian posterior probabilities above the branches provided visual evidence of the data uncovered by sequencing and clarify relatedness of species. Papaya and its relatives will eventually possess a similar bootstrap analysis where breeders can decide on which species can be crossed, possibly by bridge-crossing (Drew and O’Brien, 2001), to incorporate useful genes to improve papaya. Papaya, as was shown by VanBuren et al. (2015), was farmed by humans >6200 years ago for qualities important for farming and consumption near the centre of diversity without regard for conserving wild papayas that could possess useful genes for future breeding, e.g. the apparent absence of virus resistance genes in the wild C. papaya populations (Manshardt, 1992). The importance of habitat and wild germplasm conservation now evident to researchers was likely not important for early hunters/gatherers. Gene transfer has helped papaya growers in Hawaii, but in southern China, of the many PRSV strains extant in this large area, gene transfer of only one resistance gene has proved fruitless (Wu et al., 2018).

Using data in the hermaphrodite genome sequence, VanBuren et al. (2015) ascertained that humans domesticated papaya in Mesoamerica >6200 years ago followed by selection for the hermaphrodite type c.4000 years ago, coinciding with the rise of the Maya. Hermaphrodites have lower nucleotide diversity than males.

Sequence information was used to verify the identity of the STK gene for PRSV resistance in V. pubescens (Dillon et al., 2005a) and information that distinguished papaya relatives for useful genes could be useful for crop improvement (Carrasco et al., 2017).