Sunday, July 15, 2012
Gender Part Four: Plants Gone Wild
Note: This show originally aired on July 14, 2012.
We have been spending the past few weeks looking at the biological basis for and variation in gender. This week we continue our look at sexuality in plants.
The vast majority of flowering plants or angiosperms are hermaphroditic, having both male and female reproductive structures on the same plant and usually in the same flower, we discussed those in detail a couple of weeks ago. A small minority of plant species have bucked the trend and evolved to have fully separate sexes, with males and females occurring in entirely different individuals. These polymorphic plants are called dioceious. Gender in dioecious plants is determined genetically and there is, to use the word of one researcher, a “bewildering” level of variability in the sexual structures of dioecious angiosperms.
Plant sexual polymorphism (poly meaning many, morph meaning shape) is a relatively new invention in angiosperms, and this newness is the reason for the variability. Amazingly this form of sex determination is estimated to have evolved over 100 separate times. This means that many many lines of angiosperm plants have, in evolutionary terms, solved the same problem in the same or similar ways. These plants have evolved an XX/XY sex chromosome system, much like ours, but it is relatively new in evolutionary terms, so it is much more complicated and less well resolved than the relatively simple mammalian system. Dioecy (or the state of being dioecious) is not at all the straight forward boy meets girl scenario we humans are used to; in addition to the male meets female system, there are male meets female biased hermaphrodite and female meets male biased hermaphrodite, and mixes of all three as well. These plants are experimenting wildly with sexual reproduction, beyond the dreams of even the most creative humans. This chaos (and it is chaos, if you don’t believe me, dive into the literature) is a result of the relative youth of the dioecious reproductive system. We’ve already said that angiosperms have the most diverse reproductive structures of any group of organisms. Think of these dioecious outliers as those on the forefront of a new evolutionary wave. Sometimes they succeed, other times they fail. While there is some evidence that this system yields a lower overall reproductive rate, I think it is too soon to judge the results of this evolutionary experiment.
As if all this sexual experimentation weren’t enough, it gets really interesting with the fact that many unisexual plants are diphasic, meaning they change sex, starting as one, ending as another. One reason for this life strategy is environmental stress. A close to home example is the unusual wood and wetland species Jack in the Pulpit, aka Arisaema triphyllum. This is a multi year perennial plant, and when it first emerges, it is non flowering. Then it develops into a male plant. Traditional thinking has it that pollen is a less energy intensive gamete to produce than the larger ovule, hence smaller plants with less resources to devote to gamete production will be male. If the environment is stressful enough, in terms of nutrient levels or space competition, the plant remains male. If it is able to gather enough resources to grow larger, it will switch sexes and present as female in later years. There is evidence that the sex ratio of the plants around it also effects its sex determination. These plants are able to do this because the have a “plastic genotype” that allows them to develop one of a few options depending on the environmental conditions they are presented with in any give growing season. This plasticity, I believe, is a result of the relative youth of this sexual system. This is the wild west of biology, the rules aren’t fully written yet, and these plants are testing the limits of the system, which, as noted above, can lead as easily to failure as to reward.
The other thing we should note, is that if these individuals can change sex, that means that they must have the genetic code for both sexes in their genome, even if they are using a proto version of an X/Y sexual determination system similar to ours. Many vertebrates have no sexual determination genetics, and rely totally on environmental factors for determining gender. As mammals evolved on a branch of the tree of life away from other vertebrates, is it possible that our gender development was as plastic as these plants we now observe? We will consider that in the coming weeks as we continue our exploration of the origins of gender.
From the Indian Academy of Sciences, a pair of articles in their science education journal Resonance. From 1998, Volume 3 No. 4 R.M. Borge’s Gender in Plants: Why do plants change sex? and Vol. 3 No. 11 R. M. Borges Gender in Plants: More about Why and How Plants Change Sex. Dated, but useful.
Barret, Spencer “The Evolution of Plant Sexual Diversity” Nature Reviews: Genetics April 2002, Vol. 3
Charlesworth, D. “Plant Sex Determination and Sex Chromosomes” Heredity 2002 (88) 94-101 Very technical, good luck with this one.
http://www.sciencedaily.com/releases/2008/08/080807144242.htm “Gene for sexual switching in melons provides clues to evolution of sex”
http://www.pitt.edu/~kalisz/Research.html Website for the University of Pittsburgh Kalisz Lab, a plant research lab.