Papaya- the Sex-change King, Queen and more besides

Papaya

The papaw or papaya not only has two well known names but it is said to have 31 different sexes, and presumably a highly complex social life as a result.

These 31 different sexes can be simplified as males, females and hermaphrodites. Some individual plants always remain predictably male or female or hermaphrodite. However, things are more complicated than that. Some hermaphrodite plants only ever have hermaphrodite flowers, which produce both pollen and eventually fruit from the same flower. Other hermaphrodite plants have separate male and hermaphrodite flowers. While other plants can have separate hermaphrodite and female flowers and some indeed have separate male, female and hermaphrodite flowers simultaneously all on the one plant.

Things start to get really complicated when some male and some hermaphrodite plants go through seasonal sex changes or respond to physical damage in the same manner. Thus, some male plants will produce hermaphrodite flowers and subsequently fruit at certain times of year, or if their stems are damaged by blows from a cutlass. This technique is often used by those unfortunate enough to find that their solitary papaw plant in the back garden is male and therefore unlikely to produce fruit if left to its own devices. Similarly, an hermaphrodite plant which normally only produces fully hermaphrodite flowers may sometimes be induced to produce entirely female flowers as well. Hermaphrodite plants which normally produce male and hermaphrodite flowers, may change to stop producing male flowers and produce female flowers instead etc. Confused; just imagine how complex their social life could be!

Then you could ask, how is all this sexual variety regulated? The answer is more complicated still. In fact there are several competing theories, and it would be true to say we just do not know in detail. However, basically it appears to be regulated by sex chromosomes similar to those in humans with XX female and XY male. But papaws also have an another form of the Y chromosome, such that XY2 individuals are hermaphrodites, and combinations with any two Ys are lethal. With this system if you cross a male plant with an hermaphrodite you get equal numbers of males, females and hermaphrodites in the offspring.  But if you cross a female with an hermaphrodite you only get females and hermaphrodites.

In addition to its incredible love life papaws have another amazing ability. The latex of the papaw plant contains not one but two protease enzymes. These chemicals actually digest proteins. Within the plant world this ability is usually only associated with insectivorous plants such as the Venus flytrap and pitcher plants. The papaw must have evolved this ability to discourage grazing animals. After all you are unlikely to consume a plant that is going to digest you rather than visa versa. What is more, we must ask how did the papaw manage to evolve the ability to synthesise these protease enzymes without digesting itself?

The latex is tapped from the unripe green fruits, by making a series of cuts into the fruit with a piece of broken glass. This operation is performed in early in the morning. Throughout the day the latex drips from the fruit into a coconut shell or pot. The sap is then sun dried. About 1000 fruit are required to produce enough sap to make half a kilo of dried product. Mankind has found many different uses for the protein digesting sap of the papaw plant. In addition to the obvious use as a meat tenderiser, papaw extract has been used to digest protein hazes out of cloudy beer, employed medically to dissolve unwanted growths, and to remove hair from hides before tanning. It is used widely to remove unwanted protein residues in many manufacturing processes and routinely to purify DNA extracts in modern molecular genetics. Perhaps the grossest use of papaw extract is, it’s injection into cattle before slaughtering. Under these circumstances the protein in the animal’s muscles actually start to break down while it is still alive. However, such meat understandably comes with the health warning – ‘do not eat rare!’ otherwise you too could be digested by your own dinner.

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