Carica papaya and sexual dimorphism

It was a cool, a bit cloudy day when I decided to take a stroll around the neighbourhood. For the past four months since the commencement of the lockdown around here, I have not done a lot of walking and I felt a bit heavy. A sunless day is a really good day to try and burn those useless fats that might have accumulated in my body system.

As a lover of nature, my eyes will not just stop roving around the flora and fauna of the neighbourhood. I am always on the lookout for anything that will pique my scientific interest. It was not long until I got one in the form of the image below.

^{Picture perfect: dioecious nature of pawpaw}

The image is that of a plant commonly known as pawpaw, scientifically known as Carica papaya. There are two plants of the same species in the image - the first one is carrying fruits while the second one that grows directly beside it is carrying only flowers, no fruits. The normal thing for a plant to do before producing fruits is to flower right? However, the flowering individual in the picture does not produce fruit in nature.

When it comes to flowering, a plant could be monoecious, or dioecious in nature. A monoecious plant is one that produces male and female flowers on the same individual plant. The male and female flowers could be on separate branches in some plant species such as we have it in the corn and banana plant, while male and female organs (androecium and gynoecium) could be embedded in a single flower in some other plant species such the popular hibiscus plant and lilies. Such flowers with gynoecium and androecium embedded as one are said to be perfect.

Dioecious plants, on the other hand, usually have their male flowers on a separate plant and their female flowers on another plant of the same species just as shown in the picture of the pawpaw plant above. Another plant species that comes to the mind in this case is the cannabis plant.

^Cannabis

One question that should come to the mind of my readers is that if male and female flowers are located separately on different plants of the same species, how does pollination now takes place? Mind you, pollination general involves the transfer of pollen grains from the male organs (the anther of a flower) to the female organs (the stigma of a flower). Of course, pollination can be carried out by pollinating agents such as wind, insects, or even mammals. More about pollination can be seen here.

The unique case of C. papaya

Papaya, as it is favouritely called by some people is unique in a way. It can exist as a dioecious plant or gynodioecious species. Gynodieocious plant species usually exist as a hermaphrodite plant with perfect, hermaphroditic flowers and as a female plant with female flowers.

The hermaphrodite plant with hermaphroditic flowers does not really need to sweat much in order for pollination to occur since male and female organs are on the same flower on the same plant. However, self-pollination is generally undesirable in nature as this causes inbreeding and allow some recessive, undesirable traits to surface in the progeny. Consequently, a variety of mechanisms exist that limits self-pollination in nature. These include dichogamy, herkogamy, and self incompatibility.

It thus means that the hermaphrodite C. papaya still have to depend on their male counterparts for the supply of pollen grains for pollination. Up till today, however, the mode through which the pollen from the male/hermaphroditic plant gets to the female/hermaphroditic plant is not well understood. While some argue that it happens by wind, others leaned towards insect pollination while a school of thought argues that its the combination of the two agents.

In a nutshell, the image of C. papaya taken during my stroll consist of either a male and a hermaphrodite plant (gynodioecious) or a male and a female plant (dioecious). Perhaps if I had been able to catch some flowers from the fruiting plant, I would have been able to make a clear distinction.

What do you guys think?

Resources

• Preferential reproduction mode of hermaphrodite papaya plant (Carica papaya L; Caricaceae)
• Papaya
• Sexual dimorphism in flowering plants