Unlike many plant species, papaya has sex chromosomes. Just like you and me, papayas exist in a population made up of males and females. What makes papaya unique from us, however, is the existence of a third sex type: hermaphrodites. A hermaphrodite papaya tree has both male and female reproductive organs on the same flower. Female papaya plants are XX, males are XY, and hermaphrodites have a slightly different Y chromosome, Yh, and, thus, are XYh.
Top row shows the relative size and morphology of a female papaya flower. The middle row is a typical hermaphrodite flower and the bottom row shows a typical cluster of male papaya flowers.
The male Y chromosome and the hermaphrodite Yh chromosome are almost identical. The differences between those sex chromosomes lie in a region comprising only ten percent of the chromosome. Additionally, within that small 10 percent region, the Y and Yh are 98.8 percent similar to each other. What I study, with Dr. Ray Ming at the University of Illinois at Urbana-Champaign, is that small region, which has been designated as the Hermaphrodite Specific region of the Yh chromosome (HSY), or the Male Specific region of the Y chromosome (MSY). It’s amazing to think that a region so small can determine something as essential as sex type. As a plant geneticist, I am looking for the genes that make male papayas male, those that make females look like females, and those that make hermaphrodites what they are.
Papaya is a multi-million dollar industry. According to the United States Recommended Daily Allowance (USRDA), compared to other fruits (apples, bananas, oranges, etc.) papaya fruit is a leader in vitamin A and C, potassium, folate, niacin, thiamine, riboflavin, and iron content. Its sex chromosomes are comparatively young (only 7 million years old compared to humans, who are older than 167 millions years). The young age of the sex chromosomes allows researchers to see what sex chromosomes go through in early stages of development.
Young male and hermaphrodite papaya plants look identical. It is only after they begin to flower that differences emerge. It is obvious when you come across a flowering male papaya; the tree is covered from top to bottom in long strands of cream white flowers. Each individual flower is small, but the effect of seeing hundreds of flowers in bloom at once is breathtaking. Hermaphrodite trees seem bare in comparison; whereas there is a long chain of flowers on the male plants, hermaphrodites have only a single flower. This ability to produce long strands of flowers is one of the main distinguishing characteristics of male papaya plants.
Male trees are seemingly at a reproductive disadvantage. Hermaphrodite trees are able to not only reproduce with females, they can also self-pollinate and self-reproduce, whereas male trees can only reproduce with females. At first, this may not seem like a huge disadvantage, but you need to remember that plants are sessile organisms (attached at the base). In order for a male to reproduce, it has to hope and pray that one of its pollen grains (a plant equivalent of sperm) will catch the breeze just right, float over to a female, and land in just the right spot for fertilization to take place. To sum up, females can reproduce with both males and hermaphrodites; hermaphrodites can reproduce with both themselves and females. The only way for a male to reproduce, however, is to find a female. It is technically possible for a male to fertilize a hermaphrodite, but this almost never happens because hermaphrodites self-fertilize.
Thus, it seems that males would not stand a fighting chance against hermaphrodites. The hermaphrodites have more opportunities to successfully reproduce and pass their genes onto the next generation. So how is it that we have male papaya plants at all? Surprisingly, rather than comprising a small fraction of the papaya population, male papayas dominate over the hermaphrodites in wild populations. Why does this happen? Are there advantages to being a male papaya plant? The answer lies in those long chains of male papaya flowers.
Plant reproduction requires the pollen from a male (or hermaphrodite) flower to reach the carpel of a female flower. Pollen comes from each individual flower. Those white flower chains on male plants are outputting pollen at a rate that makes hermaphrodites green with envy. So what is causing the male plants to grow such long peduncles (the stalk the flowers grow out of)? Current theory states that selective pressures favor high outputs of pollen in order for males to remain competitive against hermaphrodites. This explains why this characteristic developed in males, but what is currently causing this to happen? As a plant geneticist, I ask the question, “What gene is controlling this?”
In order to answer this question, I study the DNA of papaya, looking for the gene (a stretch of DNA telling the plant to do something), that controls peduncle length in male papaya. There are many pathways a scientist can take to identify a gene that’s controlling a specific aspect of an organism. What I do is study the DNA and genetics of papaya and compare the differences between males and hermaphrodites. This may seem overwhelming at first. The papaya genome has 442 million individual DNA nucleotides (your A, T, G, and Cs) telling the papaya to grow up to be a papaya tree and not an apple tree. Fortunately, long peduncles grow only on male papaya plants, not hermaphrodites or females. This means the gene controlling peduncles is on that MSY region mentioned earlier. The MSY is relatively small, only eight million nucleotides. It is much more manageable, especially when compared to 442. Even better, the MSY is 98.8 percent similar to the HSY. This means that 98.8 percent of that small eight million nucleotide region is exactly the same between males and hermaphrodites. What I am looking for cannot be in regions that are identical because what I’m looking for is the trait that makes males and hermaphrodites different. Suddenly things are starting to look up.
Flowering male papaya trees are amazing to look at. They should be outcompeted by hermaphrodites, but instead they come out ahead in the wild. They are very similar to hermaphrodites — both physically and genetically — yet their DNA makeup is different enough to allow for not only different sexes, but also sex-related characteristics, such as peduncle length. A seemingly insignificant characteristic, peduncle length, might be the cause of keeping and allowing males to dominate in wild papaya populations. What is the identity of this gene? The gene causing long peduncles has to be in that small region of the Y chromosome that is different from the hermaphrodite and females. The MSY. Somewhere in that stretch of eight million nucleotide bases lies the answer to my question.
Written by Jennifer Han, Department of Plant Biology
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