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.
References:
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.
