Recent developments in genetic technology have brought the once commercially impractical pawpaw to possibly becoming a viable candidate for mass consumption. The common pawpaw, which quickly goes rotten and has many seeds which can deter consumers, is native to North America, and is currently only sold at small farmers markets due to distribution difficulties. (Shore, 2012) A project, led by Kermit Ritland, and partnered with plant propagation firm Bevo Agro, will attempt to commercialise the fruit. The pawpaw produces large amounts of ethylene gas, acting as a hormone to trigger ripening in many fruits, and so causes the pawpaw to move from ripe to rotten approximately 72 hours after being picked. (Kenrick, 2009)
The first aim of the project is to produce a much slower ripening fruit. A gene has already been identified in the apple that controls the production of ethylene gas, and this candidate gene will be targeted in the pawpaw. Researchers plan to produce a slower ripening fruit through a selective breeding program rather than direct genetic manipulation. (Shore, 2012) The project will use natural variation in the gene that controls ethylene release, using several trees over multiple generations. The use of modern techniques, such as the ability to identify certain genetic markers before full maturity of the plant, gives researchers the ability to shorten the breeding process from possibly decades to several years. Studying these markers gives researchers vital information on the presence and prominence of these ethylene genes to cultivate and cross breed the plants with desirable characteristics. The ultimate aim is to produce plants with the effects of these genes reduced or even muted. (Shore, 2012)
The second aim of the project is to produce a seedless version of the fruit by inducing polyploidy, the same state which produces seedless watermelon. This requires a change in the number of chromosomes in the plant. The pawpaw is naturally a diploid, meaning it has two homologous sets of chromosomes. Polyploidy is the state of the cells having more than two homologous chromosomes. The plant is sterile as its gametes have a different chromosomal number to the gametes of fertile plants, preventing successful fertilization and therefore resulting in a plant that cannot reproduce and preventing the production of seeds. (Lowe & Pomper, 2005) Polyploidy can be induced by introducing chemicals such as colchicine into the cellular environment, which interrupts anaphase of mitosis, stopping proper separation of chromosomes. (Painter, 2011) As the plants are sterile, different methods must be used for propagation, and the project will most likely use techniques such as grafting to reproduce plants that naturally occur as seedless polyploids.
The project promises very exciting new horizons for its sponsors, although the actual results are far from being realized. Due to the relatively slow progress of a selective breeding program, and the reliance on natural variation in cultivated plants, results may be many years away, even with modern genetic techniques.