Today we continue our ongoing discussion of gender and how
it relates to biological reproduction and the amazing diversity we see in
nature. To review, we know that individuals we call females make ova or eggs,
and individuals we call males make sperm, and in humans, this comes with a
whole lot of extra baggage (to be fair, many animals exhibit the differences in
appearance and behavior called sexual dimorphism, but I believe we are the only
species to have come up with misogyny and homophobia among other things).
When it comes to reproductive equipment, flowering plants,
technically called angiosperms, have all other groups of organisms beat. As a
group, angiosperms have the highest variety of reproductive structures on
Earth. In this group, the most common sexual form is the hermaphrodite, meaning
both female and male components are found in the same flower. These flowers can
also be referred to as bisexual, and are botanically classified as “perfect”.
It is thought that all angiosperms evolved from a hermaphroditic ancestor, and
single sex, non hermaphroditic strategies evolved later. This is why 75 to 85%
of angiosperms are hermaphrodites.
What are the benefits of being a hermaphroditic plant? The
one that jumps to mind initially is that if there are no other potential mates
around, you can fertilize your self. In theory this works, but most plants
actually go to great lengths to avoid this phenomenon (called “selfing”). Like interbreeding in royal families, selfing
tends to bring out genetic weaknesses over several generations. Think of
selfing as a last chance scenario, especially for annual plants, who live only
one growing season and “winter over” in seed form. If the plant doesn’t get to
mate with anyone else, at least it can sustain itself over the winter as a seed
generated from a self fertilization; ideally in not too many generations in a
row.
Another way that hermaphrodism increases an individual’s
chances of mating is in a low density potential mate situation; a less extreme
version of what we just talked about. If there are very few potential mates
around, what happens if they are all male? Or all female? They aren’t the
isolated individuals of the former scenario but they still can’t mate
successfully in a single sex group. Hermaphrodism in plants ensures both sexes are
represented, even in a low density population.
Yet another benefit of hermaphrodism in plants is that it
extends their mating season. Hermaphrodites can functionally bias their flowers
towards one sex or the other for various reasons. While the flower will have
both male and female sexual structures (the androecium and gynecium
respectively), they may not be functional at the same time. For example the
male may mature first, and produce pollen. Later the female parts will mature,
ready to accept pollen from a separate individual. The plant has then
essentially doubled the amount of time it is spreading and mixing its DNA, and
has eliminated the chance of selfing as well.
The bet hedging, flowering species of the plant kingdom have
clearly benefited from utilizing both genders simultaneously for millions of
years, yielding extraordinary biodiversity and resilience. And with all of our
ideas about gender and value, sexuality and stereotypes, most of us just see
the flower.
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.
University of Oxford’s Science Blog features a conversation
from March 2009, with Dr. John Pannell of the Oxford Dept. of Plant Sciences
Dr. Spencer Barrett’s “The evolution of plant sexual
diversity” is lengthy and quite technical, but dedicated readers may find much
to ponder. April 2002, Vol. 3 www.nature.com/reviews/genetics
