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Papayas, Carica papaya L., nutritious melon-like fruit enjoyed for breakfast and other meals, are grown in the tropics, subtropics and in greenhouses in some temperate zones and are exported worldwide. The 2016 world production was 13 million t (FAOSTAT, 2016) with India (5.6), Brazil (1.4), Mexico (0.95), Indonesia (0.90), Dominican Republic (0.86) and Nigeria (0.84) being the top producers. Papaya production worldwide is ranked 13th after lemons and limes at 13.05 million t in 2016 (FAOSTAT, 2016; Statista, 2018).

The USA acreage data for 2014 are mostly in Hawaii, i.e. 600 ha (bearing) (NASS, 2017) and yielded 10,669 t. The USA annual farm gate value at US$ 0.48/lb was US$ 1.13 million. The May 2018 volcanic eruption in the major growing area in Hawaii, Puna on Hawaii island, had a devastating effect on papaya production (Wu, 2018). The researchers and agents from the state of Hawaii Department of Agriculture, University of Hawaii College of Tropical Agriculture and Human Resources and the Daniel K. Inouye Pacific Basin Agricultural Research Center are cooperating to establish seed production for the papaya industry.

C. papaya L. is monotypic (Badillo, 2000, 2001) because of its great divergence from c.21 former Carica species, now in the older taxon, Vasconcellea St. Hil. (Badillo 1971, 2000, 2001). Papaya belongs to the family Caricaceae, and its closest relatives are three species from the southern Mexican/Guatemalan clade, Jarilla chocola Standl., Jarilla heterophylla (Llave) Rusby (syn. with Jarilla nana (Benth.) McVaugh) and the thin-stemmed, herbaceous, monotypic Horovitzia cnidoscoloides (Lorence & R. Torres) V.M. Badillo (Carvalho and Renner, 2012). If important disease and abiotic stress resistance, good quality and flavour genes are discovered, these four species probably can be crossed with papaya; the more distantly related South American Vasconcellea species have been crossed with papaya (Manshardt and Wenslaff, 1989a; Chen et al., 1991; Magdalita et al., 1996; Drew et al., 1998; Siar et al., 2011).

Papaya wild germplasm was assessed in herbarium and field samples to determine the level of genetic diversity in wild populations (Pesqueira et al., 2017). Papayas were most diverse in Mexico but were distributed from its tropics to southern Nicaragua. Twenty-four wild populations were compared to the commercial ‘Maradol’. Estimated migration rates were long, and there was high genetic diversity in wild papayas compared to low diversity in domesticated accessions. The influence of humans was considered to be a threat to conservation of diversity.

The most distantly related genus is the African Cylicomorpha Urb. (two species). The Caricaceae is believed to have originated in Africa before the continents separated and the family became fragmented along with the landmasses (Carvalho and Renner, 2012). Papayas probably originated along the Caribbean coast of Central America (Manshardt, 1992) with a centre of diversity in north-western South America (Olson, 2002). The closest family to the Caricaceae is the Moringaceae (Olson, 2002).

Compared to papaya, two other family members are grown on a small scale, the mountain papaya, Vasconcellea pubescens (also known as Vasconcellea cundinamarcensis, Vasconcellea candamarcensis) and ‘babaco’. Cultivated cold-tolerant mountain papayas, V. pubescens, used as canned fruit and for juice, have been studied because of their disease resistance and latex with high proteolytic activity (Carrasco et al., 2017). V. pubescens was crossed with papaya with consideration for cold tolerance (Manshardt and Wenslaff, 1989a; Drew et al., 1998). Babaco is a sterile hybrid from the Ecuadorean highlands of unknown parentage but is believed to be a natural hybrid of Vasconcellea stipulata and V. pubescens (Horovitz and Jimenez, 1967) and closely related to Vasconcellea × heilbornii. Babaco is grown in Ecuador and New Zealand where it is used as a fruit and in drinks (Kemplar and Kabaluk, 1996; Van Droogenbroeck et al., 2004; Dornier et al., 2006). All babaco are female, parthenocarpic and propagated clonally as rooted lateral branches (Kemplar and Kabaluk, 1996; Dornier et al., 2006); pistillate plants are sexually incompatible with related species (Horovitz and Jimenez, 1967), but a few hybrids with papaya have been reported (Manshardt and Hoover, 2017).

Papayas are grown commercially in field plantings, backyards and protected greenhouses/shade houses in the tropics, subtropics and cooler areas worldwide. Early explorers are believed to have dispersed dry seeds. The Spaniard Don Francisco Marin is credited with transporting papaya in the early 1800s from the Marquesas to Hawaii (Storey, 1941). They became an export crop of Hawaii in 1948 (Yee et al., 1970).

The fruits are breakfast staples or accompany other meals, while green fruit are cooked with chicken or pork for stews and soups, consumed green, rehydrated or used as pickled salads in South-east Asia. The ripe fruit are prepared in salsas, drinks, preserves and condiments. The papain has long been extracted from scored green fruit for meat tenderizers, for the brewing industry to clarify beer, in tanning, in face cream and skin treatment additives and in cheese-making as an alternative to rennet (Rana et al., 2017).