Plant 203

Carica papaya L. (Caricaceae)



Papaya, sole member of the genus Carica, is naturally distributed in Mesoamerica, where it was probably first domesticated. The ease with which papaya propagates and disperses means it is now found naturalised throughout the tropics.

Traditionally, Carica contained many superficially similar species and had a wide native distribution in the Americas extending from Mexico into the Andes. Detailed morphological investigations and breeding experiments, confirmed by DNA analyses, show that the Andean papayas belong to a distinct genus (Vasconcellea), distantly-related genus to papaya. Phylogenetic analysis shows that papaya is most closely related to species from two small Mesoamerican genera (Horovitzia and Jarilla).

Papaya is best known as a ubiquitous tropical fruit, and is eaten either ripe or green. However, it also produces an enzyme called papain, which breaks down proteins. Consequently, papaya extracts are used as meat tenderisers, fabric softeners and in beer production. Crushed, pungent papaya seeds have also been used as pepper substitutes. The flavour is produced by glucosinolates, a biochemical character shared by the Caricaceae and about a dozen other families related to the cabbages.

Papaya is a squat, latex-producing, night-flowering tree with all its large, palmately-lobed leaves crowded at the top of a single stem that can reach ten metres in height. Wild populations of papaya are dioecious, having separate male and female plants. The vast majority of cultivated papayas have a third gender, so-called hermaphrodite plants where otherwise male plants have a few female flowers. Hermaphrodite plants are capable of self-pollination, making them reliable fruit producers. Genetic analyses have shown papaya has an XY system of sex chromosomes (similar to humans); males are XY and females XX. The hermaphrodite plants have a slightly modified Y chromosome.

The fruit is an orange-fleshed berry containing large numbers of deeply-furrowed seeds that have a translucent, whitish sarcotesta (fleshy seed coat). The fruit is one of the best ways to separate wild and cultivated papayas. Wild papayas have small (up to 7 cm wide), spherical, thin-fleshed fruits. In contrast cultivated papayas have large (up to 30 cm wide), cylindrical, thick-fleshed fruits.

More than a dozen papaya cultivars are grown commercially but papaya is prone to pests and pathogens that have significant effects on productivity. An outbreak of one of these pathogens, papaya ringspot virus, led to the approval and release of a genetically-modified papaya resistant to the virus. Today the majority of papayas commercially grown on Hawaii are genetically modified.

Further reading

Liu Z et al. 2004. A primitive Y chromosome in papaya marks incipient sex chromosome evolution. Nature 427: 348-352.

Carvalho FA and Renner SS 2012. A dated phylogeny of the papaya family (Caricaceae) reveals the crop's closest relatives and the family's biogeographic history. Molecular Phylogenetics and Evolution 65: 46-53.

Ming R et al. 2008. The draft genome of the transgenic tropical fruit tree papaya (Carica papaya Linnaeus). Nature 452: 991-996.

Stephen Harris