The physical differences between the flower types match hidden differences in chemistry which limit the effectiveness of pollination between like types. These differences, both physical and chemical, discourage inbreeding.
With three forms, the odds that a vegetatively related plant will have the same form is 1/1 while the odds that any random plant will have the same form is 1/3. Failure of pollination from the same form to succeed results in a strong preference against inbreeding.
Inbreeding reduces genetic diversity and increases the odds of revealing deleterious recessive alleles. Some plants, like Oxalis, have systems to discourage inbreeding, others don't. It isn't clear what drives some species to evolve these systems while others don't, but there is related theory that might be informative…
Why do most plants/animals reproduce by sexual means? Any asexual progeny will get 100% of their
The current theory, backed by years of experimental evidence in different systems (fish, snails, nematodes) is that sexual reproduction allows a species to mix its genome up and keep ahead of parasites, which have much shorter generation times and are rapidly evolving to overwhelm the host immune system.
This is referred to as the "Red Queen Hypothesis", in reference to Lewis Carol's character in "Through the Looking-Glass".
"Now, here, you see, it takes all the running you can do, to keep in the same place."
From a theoretical stance, the same pressures which lead to the prevalence of sexual reproduction are likely to be involved in why some plant species have evolved heterostyly. That is to say, a high level of parasitism over evolutionary time periods likely provided the selection pressure leading to the evolution of heterostyly and other mechanisms to minimize inbreeding. Proving this experimentally is a bit harder to do, however, and researchers are actively working on the subject.
What got me interested in this system is that I have two Oxalis triangularis plants I got from a friend. One plant has dark purple leaves with a central lighter mark and pink flowers. The second plant has green leaves and white flowers. I opened up a flower from each plant and discovered they were structured differently.
|Plant #1||Plant #2|
I lucked out and my plants have different flower forms, so my attempts to cross them will likely result in the production of seed. I still have to ponder on the subject of what I might want to breed this species for, but at least I will probably get some interesting combinations of traits in the offspring.
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Marco DE and Arroyo MTE. (2014) The Breeding System of Oxalis squamata, a Tristylous South American Species. Botanica Acta 111:497-504.
Moran LT, Schmidt OG, Gelarden IA, Parrish RCII, and Lively CM. (2011) Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex. Science 333:216-218.
Quental TB and Marshall CR. (2013) How the Red Queen Drives Terrestrial Mammals to Extinction. Science 341:290-292.
Weller SG, Dominguez CA, Molina-Freaner FE, Fornoni J, and Lebuhn G. (2007) The evolution of distyly from tristyly in populations of Oxalis alpina (Oxalidaceae) in the Sky Islands of the Sonoran Desert. American Journal of Botany 94:972-985.
- Dercole F, Ferriére R, and Rinaldi S. (2010) Chaotic Red Queen coevolution in three-species food chains. Procedings of the Royal Society of Biologists 277:2321-2330.