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The MeGI and SiMeGI genes are the driving factors behind sex determination in Persimmons and can be analyzed to gain understanding of the sex determination systems of other plants
Plant sexuality is a fascinating topic that far too many people lack understanding of. It may seem difficult to grasp the reproductive strategies of organisms who are vastly different from ourselves, but the process is actually no more complex than that of humans. Like people, plants contain reproductive structures. The female structures of plants are known as pistils. The male structures as a whole are known as stamen. These are made up of long stalks called filaments that support rounded heads known as anthers. The role of anthers is to produce plant sperm, which we commonly refer to as pollen. When pollen reaches and enters a pistil the ovules are fertilized and eventually develop into seeds to be planted. Many plants are hermaphroditic which means that they possess both of these reproductive structures and are able to self-reproduce in a process known as “selfing”. Other plants have evolved a system called dioecy where individuals only possess one system and are either male or female. This trait is fairly rare yet is possessed by various non-closely related plant species which suggests that multiple evolution events for the trait took place independently from one another. This begs the question, “What molecular and evolutionary changes are behind such unique changes?”.
To gain insight on this subject, scientists sequenced the genetic code of the Caucasian persimmon (Diospyros lotus) a species belonging to the Asterid family that is widely grown in southwest Asia and southeast Europe. The persimmon along with kiwifruits and papayas are among some of the few fruits that scientists have been able to identify specific sex determining genes within. To further characterize their data, the researchers compared the differences between the DNA sequences of female and male persimmons as well as the number of genes found within their persimmon data as a whole and that of other closely related fruits like tomatoes and kiwifruits. These numbers were found to be very similar as one would expect. Researchers then selected 12,058 of the persimmon’s genes that they found to be unique or rare for further study.
Analyzing these genes, researchers compared the persimmon genes with those of grapes, kiwifruits, and tomatoes in regard to the mutation rate of their gene pairs. One section of persimmon genes showed a clear peak in this rate which suggested that a whole genome duplication occurred so that individuals then possessed extra copies of their genome. When compared to the genomes of other fruits, all of which were from different genera, showed that all the fruits within the Diospyros genus, including persimmons, underwent three whole genome duplications that differentiated them from the genus Actinidia, which contains kiwifruits.
Following the last of these three whole genome duplications, MeGI and its duplicate copy SiMeGI were among nine pairs of chromosomes found to have experienced positive selection and were syntenic meaning their distance from each other on the chromosome and order was conserved. These genes most likely arose from the whole genome duplication, but that cannot be fully proven. Further observations of the genes found that 45.5% of gene pairs showed genetic differentiation between male and female individuals. This is to say that the genes express themselves differently in females and males. These differences may have possibly contributed to the persimmons’ trait of dioecy. By observing the effects of both MeGI and SiMeGI independently in plants, researchers discovered that MeGI most likely played the primary role of reducing inhibiting the growth of male reproductive structures in persimmons.
Researchers also discovered the gene OGI which was linked to the Y chromosome. The regions of DNA surrounding the chromosome were specific to males and showed similarities to the sex chromosomes of animal cells. Researchers noticed that as they went further away from OGI coding regions, the rate at which silent mutations occurred between X and Y alleles decreased. Unlike the other two genes, OGI did not display signs that it was produced during the whole genome duplication and instead showed signs that it was derived from an inverted duplication of the MeGI gene.
Putting all of these observations together, researchers have developed a strong grasp on what they believe to be the evolutionary path taken by this persimmon species towards dioecy. A whole genome duplication event unique only to the genus Diospyros occurred, which led to the appearance of the gene MeGI which began to function as the primary suppressor of the flowers’ stamens. This resulted in the flowers with high levels of activity in the gene MeGI developing as females. This event was soon followed by another duplication of the gene MeGI to create the Y-linked OGI, which could then function as a suppressor of MeGI.
The results produced by this research are incredibly interesting because they show vast similarities with studies done on other plants with dioecy. This would suggest some form of convergent evolution as all of these plants took their own routes but somehow ended up in roughly the same place. I believe that the next step from here would be to study how exactly the shift from hermaphroditism benefits these plants and what kind of cons come with them. It would also be interesting to see if through gene-splicing scientists could introduce dioecy to other plant species and what the effects would be.
Khalil Miller is currently enrolled at Davidson College. Contact him at: khmiller@davidson.edu
References
Akagi, T., Kenta S., Hideki N., Hideki H., Ryutaro T., et al. ” The Persimmon Genome Reveals Clues to the Evolution of a Lineage-Specific Sex Determination System in Plants.” PLOS Genetics, vol. 16, no. 5, 2020, doi:10.1371/journal.pgen.1008845.
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