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Papayas have been bred for local taste and size preferences, high yield, good fruit quality, pest and stress resistance and commercial popularity.

BLEMISHED FRUIT. Some popular papaya cultivars have numerous fruit freckles during certain growing periods (Chan, 2004). Consumers select produce by visual appearance and likely select papayas with few blemishes. Hawaii consumers of red-fleshed ‘Sunrise’, ‘Sunset’ and ‘SunUp’ papayas and to a lesser extent other gynodioecious yellow-fleshed papayas are familiar with freckled fruit. Breeding for reduced freckling was initiated in Hawaii using a commercial red-fleshed fruit (T. Matsumoto, unpublished results). Seedlings of the red-fleshed hermaphrodite had few blemishes under Hawaii conditions and were further selected for the trait. Crosses were made with Papaya ringspot virus (PRSV)-resistant but freckle-prone ‘SunUp’, and after 5 years, resistant, reduced-freckling, red-fleshed selections were obtained. Crossing and backcrossing is currently ongoing to select for the highest quality yellow-fleshed papayas with reduced blemishing. Although the condition is less evident on cultivars other than the red ‘Sunrise’, ‘Sunset’ and ‘SunUp’ (all closely related), the yellow hybrids and inbreds will be made available to growers. ‘Eksotika’ in Malaysia has numerous freckles like its parent ‘Sunrise’ (Chan, 2007). ‘Eksotika II’ is a result of a cross between ‘Eksotika’ and sibling Line 19. The new hybrid has reduced freckling, is more attractive than ‘Eksotika’ and has firmer flesh.

UNIFORM STABLE FRUIT SIZE AND REDUCED CARPELLODY IN HERMAPHRODITES. In commercial production, breeders develop female/hermaphrodite types in warmer climates where fruit of the preferred hermaphrodites are stable. Females are only required for breeding because hermaphrodites are self-fertile and uniform in size, i.e. elongated with a relatively small central cavity containing seeds, and are easier to pick and pack for transport. Females are removed from seedling fields. Depending on the method of seed production, either selfing hermaphrodites for inbred lines or crossing hermaphrodites with females to capitalize on heterosis (Chan, 2004), seedlings segregate either one female to two hermaphrodites from selfed seeds or one female to one hermaphrodite from F1 hybrids. Intermediate sex types of hermaphrodites were described by Storey (1953). The labour-intensive task of thinning a field of seedlings is practised in most papaya growing areas.

Hermaphrodite flower types are less stable than females and are affected by temperature changes (Storey, 1953; Arkle and Nakasone, 1984; Nakasone and Paull, 1998). During excessively hot or cold periods, staminate flowers result in gaps in fruit production and in cool periods, carpellodic flowers develop from normally perfect, self-fertilizing hermaphrodite flowers. Carpellodic fruit, if not too severely deformed can be sold as seconds, but they lower the total value of the winter crop. For that reason, cultivars that develop minimal carpellodic fruit are desired.

Dioecious papayas are grown in subtropical regions because of the more stable female flower type in both cool and excessively warm seasons. Fruit from the females are more uniform if pollination is efficient. Therefore, breeding for uniform fruit shape and size simplifies selection.

BACKCROSSING TRANSGENIC PAPAYAS. The first transgenic, commercialized PRSV-resistant plant was a red-fleshed ‘Sunset’ named ‘SunUp’ (Manshardt, 1998). Since the industry preference was yellow-fleshed ‘Kapoho’, an F1 hybrid, ‘SunUp’ × ‘Kapoho’, that was yellow-fleshed was developed (Manshardt, 2007). By adopting F1 hybrids, growers benefitted from increased production as result of heterosis (Chan, 2007), reduced carpellody and staminate flower production in the ‘Laie Gold’ hybrid (Fig. 6.1.1; M. Murakami and C. Fukuyama, papaya growers, Hawaii, 2018, personal communication). ‘Laie Gold’, a complex F1 of the inbred selection ‘Kamiya’ crossed with ‘Rainbow’ F2 seedlings containing the PRSV resistance transgene (Fitch, 2002a,b), was backcrossed to ‘Kamiya’ four times and seedlings named 94Km were distributed; 94Km had more inconsistent flowering, producing staminate flowers during the cold season that resulted in decreased production or crowded fruit columns of variable fruit size that required thinning. Carpellody in ‘Laie Gold’ occurred, but it was minimal when growers managed nitrogen input. The 94Km inbred was crossed with ‘Rainbow’ inbreds to create ‘Laie Gold’-like hybrids, and seedlings are being observed for improved or detrimental traits (M. Murakami, Hawaii, 2018, personal communication). Further breeding of 94Km with ‘Kapoho’ will be used to create transgenic hybrids that produce fruit larger than ‘Rainbow’.

Fig. 6.1.1. A 4.5-month-old ‘Laie Gold’ cutting or micropropagated plant in the field in Kahuku, Hawaii.

Several transgenic ‘Laie Gold’-like papayas sold by several growers and referred to by their label names, e.g. ‘Kamiya’, ‘Kahuku’, ‘Song Special’, ‘Rainbow’ and ‘SunUp’, are grown where PRSV is prevalent. Non-transgenic ‘Sunrise’ papayas are grown on the PRSV-free islands. Large papayas used for cooking and green papaya salad could be transgenic hybrids via outcrossing and grown in virus-infested areas or non-transgenic and grown on virus-free islands.

Transferring the resistance transgene into other cultivars by breeding commenced before deregulation was completed. ‘Laie Gold’ was developed for markets where yellow fruit larger than ‘Rainbow’ with PRSV resistance was preferred. Transformation was reported (Fitch et al., 1998), but the effort and cost of deregulation made breeding a simpler task and resulted in ‘Laie Gold’ (Fitch, 2002b). Seeds are usually produced using a homozygous yellow inbred transgenic ‘Rainbow’ line and the original non-transgenic ‘Kamiya’, a low-bearing selection from ‘Waimanalo’ (X77) that was developed for the weathered, lowland soils of Oahu (Nakasone et al., 1972).