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Tuesday, December 1, 2015

Evolutionary Battle of the Sexes

Most of the familiar animals are found in male and female versions. Some others switch between male and female and then there some are both at the same time. Plants mix it up a bit. Most of the familiar ones are both male and female at the same time, with gender-specific sex organs packed together into flowers. Some others have separate male and female flowers. Still others follow our pattern, with separate individuals being male and female.

When the two genders are split into different organisms, an interesting evolutionary dynamic comes into play:
  1. Suppose male births are less common than female.
  2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
  3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
  4. Therefore the genes for male-producing tendencies spread, and male births become more common.
  5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
  6. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
This simple chain of logic is often attributed to R. Fisher (as "Fisher's Principle"), but originates in an older paper by W. D. Hamilton. The consequence of this is that we would expect species to have the 1:1 equilibrium ratio. Any species that doesn't fit this pattern... become interesting. They don't follow the pattern because something else is going on.

A Dandelion on an Alaskan peak.
Dandelions (Taraxacum officinalis) are interesting. Bees collect the male pollen and distribute it to the female pistils, as with most flowers, but the Dandelion diverges from the common pattern at this point. The pollen induces seed development in a process called apomixis, where the resulting seeds show no genetic contribution from the pollen. Each Dandelion is a clone of its female parent, so the population is at an extreme 0:1 male to female ratio. Any genetic elements that would increase the chance of male genetics contributing to the next generation would have a severe evolutionary advantage, but the males have completely lost the fight.

The Dandelion female 'parent' invests far more energy in producing seeds than the male 'parent', so writing the male entirely out of the genetic equation may not be so strange in the end.

Saharan Cypress (Cupressus dupreziana) represents another extreme case... but in the exact opposite direction. The seeds of the Saharan Cypress show no genetic contribution from the mother. The plant will even act as a surrogate for pollen from the related Mediterranean Cypress (C. sempervirens), producing baby C. sempervirens plants. That C. sempervirens will also play the surrogate role for C. dupreziana pollen, producing baby C. dupreziana plants, says that the behavior is probably seen more generally in this group.

Writing the female entirely out of the genetic equation is a very strange concept. Given that it is the female parent that invests so much energy into seed production, the female genetics should remain important.

If you've followed along so far, you might not have noticed I pulled a fast one on you. Hamilton's logic works in mammals like us because we have distinct genetic elements that are associated with the male and female genders. Genes on the Y-chromosome or on the X-chromosomes can selectively respectively favor the male or female gender. Most plants produce both male and female gametes and so there are no genetic elements that are associated with either one in particular. This disconnect is probably what has allowed plants with such extreme reproductive patterns to have evolved. Because no genetic elements would be stuck with only one gender parent, there is no selective force to maintain parity of genetic transmission through both genders. Genetic influence passing through female gametes or male gametes alone then becomes as viable as any other mechanism of vegetative reproduction a plant may use.

Though Dandelions are the traditional example of apomixis, not all Dandelions use this strategy. There are versions that have a more typical sexual pattern (and the difference seems to come down to chromosome copy number). The Cypress also sometimes mixes up the genetics from pollen with those from eggs. Thus both types of plants persist in generating some of the genetic diversity needed to survive in the long-term race against the Red Queen (the-biologist-is-in.blogspot.com/2014/04/oxalis-and-red-queen.html), even if they go about doing in a peculiar way.


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