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Friday, April 18, 2014

Oxalis and the Red Queen

Breeding in the genus Oxalis is complicated by the presence of multiple flower types, referred to as heterosytly because of differences in the arrangement of styles (the female part of a flower). Oxalis generally have three forms, so they are specifically described as being tristylous.

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
DNA from their mother. Any sexual progeny will get only 50% of their DNA from their mother. Basic evolutionary theory suggests there would be a strong bias towards species reproducing by asexual means. So, why…?

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
  • Purple leaves, light mark.
  • Pink flowers.
    • 1' : styles
    • 2' : stamens
    • 3' : stamens
Plant #2
  • Green leaves, smaller.
  • White flowers.
    • 1' : stamens
    • 2' : styles
    • 3' : stamens
If my two plants had happened to have the same flower structure, I wouldn't have looked into the topic of heterostyly. I would have tried crossing the flowers and had the cross fail. I would have assumed the plants were incompatible and not thought to look into it further.

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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  2. King KC, Delph LF, Jokela J, and Lively CM. (2009) The Geographic Mosaic of Sex and the Red Queen. Current Biology 19:1438-1441.
  3. Marco DE and Arroyo MTE. (2014) The Breeding System of Oxalis squamata, a Tristylous South American Species. Botanica Acta 111:497-504.
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  7. http://radix4roots.blogspot.com/2009/09/ocasional-update-1-to-bee-or-not-to-bee.html
  8. http://radix4roots.blogspot.com/2009/08/flurry-of-flowers.html
  9. http://wettingthebeds.cultivariable.com/2013/07/oca-maximizing-seed-yield-of-crosses.html