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Charles
Darwin missed the boat on linking his sexual
and
natural selection in animals to plants as well.
This
is odd because he was quite the botanist and
spent
many years studying grasses and such with his
son.
The parallels between selection in plants and
animals
might have strengthened his initial argument,
but
there would still be dissent about the descent of man.
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Does it seem weird to talk gender differences in plants? Yes, they
do have genders and sex chromosomes, but it's way more complicated in plants than in
animals. More complicated probably means more exceptions, but now let’s focus on the main types of reproductive systems
in plants and the types of sexual dimorphism they create.
We have talked about how plants come in many
varieties-angiosperms, gymnosperms, bryophytes, etc. in previous posts. Many can reproduce both
sexually and asexually, but for today, let’s limit ourselves to angiosperms (flowering plants) and
sexual reproduction.
More than 90% of flowering plants have flowers with male
reproductive organs (stamen) and female reproductive organs (pistil) on the
same individual plant. The term in botany for this is monoecy – the plant is monoecious (mono = one and ecious =
house).
The anther produces pollen (the male microgametophyte, see
this post) and the carpel produces the ovule with the egg cells (female
microgametophyte). How could a plant have differences based on sex if both
sexes are on the same flower? Can a flower be dimorphic with itself? Well…. no,
but it isn’t always that simple.
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Carnovali’s
painting of Hermaphroditus and Salmacis
from
1856. The nymph became so smitten with him on
first
sight that she prayed they would never be parted.
The
wish was granted. Note the soft outlines, this was a
radical
move in Italy in the 1800’s and got some of his
paintings
rejected by the church.
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In science we use the term more strictly; it means one
individual that has both and female reproductive organs. This happens to rarely in animals. But in flowering plants it’s the rule rather than the
exception.
But in some monoecious plants, the flowers aren’t perfect.
Individual flowers will have either male or female reproductive parts. These
are called…. you guessed it, imperfect
flowers.
So, can an imperfect monoecious plant be sexually dimorphic?
On one hand, it is a hermaphroditic individual plant, just like a plant with
perfect flowers; both male and female reproductive organs are found in one
individual. This would argue that it can’t be sexually dimorphic, just like the
perfect plant.
But on the other hand, it has two different types of
flowers, and they look different because one has staminate structures (anther + filament) and the other has pistillate structures (pistil = ovary +
stigma + style). Because of this, the two types of flowers are different morphs
(different shaped versions), and that makes them sexually dimorphic. Does it
matter that they’re on the same individual plant? I leave that argument to you.
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The
top line drawing is a perfect flower, both male and
female.
The bottom drawings are of a male (left) and
female
flower. These can exist on the same plant –
monoecious
imperfect, or on dioecious plants. It’s not
quite
this simple, but wait until next week for that.
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Sexual dimorphism in plants comes in two main flavors, just
like in animals. One is obvious; differences in reproductive organs (primary sex characteristics) will make
the flowers look different. The second type is more interesting. You can have
difference in characteristics not directly related to reproductive organs (secondary sex characteristics). Dimorphisms could include the shape, color, number, or smell of the flowers, or even differences in the vegetation of the plants. Who knew that plant sexual characteristics
could be so complicated?
In general, male flowers are smaller and more numerous than
female flowers. Think about it. Males need to spread as much pollen as
possible, whereas females spend much more energy to make fruits and seeds. Just
like in animals, males make lots of reproductive cells, and females make fewer
– so more, smaller flowers in males makes sense.
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The
male soapwort flowers are on the left. They have
smaller
flowers and more variability in coloring. The
female
flower is on the right. The tall structures are the
pistil.
Compare them to the anthers in the male flowers.
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Scent can be another dimorphism. Most often, the scent of male and female flowers is very
similar; this is so they can attract the same pollinators. But an exception is Phyllanthaceae
plants as shown in a 2013 paper. A parasitic moth is the pollinator. It
gathers pollen from males, offers it to the female flowers where it lays its eggs. The
larvae then eat the seeds - so the females need to be fertilized for this system to work. This means that the
beetle really needs to find male flowers first.
Mated female beetles prefer the smell of the male flower, so female beetles being mated drives them to collect pollen by attracting them to the male flowers more than the female ones. Then, when it visits the female
flower to lay its eggs, it brings along the pollen to ensure a food source
(seeds) for the larvae.
The differences between the sexes can be seen in the plants
as a whole too. Longer living dioecious plants
often have males that are larger. Male seeds will be heavier and
germinate earlier than females. The extra endosperm gives them a chance at
establishing themselves and growing larger, and the early germination also
gives them a head start on the females. So no wonder they are often bigger.
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The
quaking Aspen is an exception. It is long lived, but
the
females are usually bigger and have more clonal
propagation.
And the exception to the exception – Pando,
one
of the oldest and largest organisms on Earth is a
clonal
population of quaking aspens in Utah –
but
they’re male!
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Another example of plant size dimorphism is the Rumex hastatulus from a paper from 2012.
This is a wind pollinated and wind dispersed (for seeds) plant. The male grows
taller than the females early, so it's taller when the pollen is dispersed
(better distance). Then the female grows more and is taller than the males when the wind
disperses the seeds (better distance again). Neat how that works out.
On the other hand, plants that live shorter lives usually
have females that are bigger. In the perennial plant Silene latifolia, growth and survival are same in male
and female until reproduction begins, the females grow bigger and live longer.
There is a live fast and die young strategy for the males – their job is
done first, a lot like female spiders that eat the males after mating. By providing
their mate with a meal (himself), the male spider improves the chance she will lay healthy
eggs.
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On
the left is the male L. xanthoconus
plant with flowers.
These
provide the Pria beetle in the middle with nectar.
The
female plant and flowers on the right are shaped
very
differently and provide the beetle with shelter. It
doesn’t
mean to pollinate the female, it’s just trying to
get
out of the rain.
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The same plant
demonstrates another ecologically driven dimorphism. Males that maximize their number of flowers get visited by
more beetles, but the investment makes them die sooner. Bond and Maze in 1999
showed that the males spend more on non-photosynthetic flowers (because they aren't green) and that these flowers cover
up more of the photosynthetic leaves. The dimorphism is that the female plants live longer.
In many dioecious
plants, the sex ratio is nearly 1:1 male:female. That makes sense, unless there are longevity issues induced
by one sex or the other (like females dying younger because they put more into
making fruits or the example immediately above). However, in some plants the ratio may be way off. Males may be
bigger, and they may make less defense toxins, therefore, they may get eaten by
herbivores more. This example not withstanding, male bias is more common than female bias.
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Biologically,
an interesting picture. Stephen Colbert is
about
six feet tall, sexually dimorphic than most females
based
on height and haircut. The bald eagle has reverse
sexual
size dimorphism, the females are usually larger.
And
the pistachio tree is dioecious and has a sexually
dimorphic
ratio; there is about one male for every
10
females.
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A report in 2010
stated that SSS usually works out so the males end up in areas of less
resources and females in areas of more. Females need the resources more, and
while the less ideal areas mean fewer competitors for the males. That’s the
general rule – males are limited by competition, and females are limited by
resources – think about that.
Next week, the sexual dimorphism of plants
seems strange to us because we don’t really see the difference between males and females (maybe in holly plants). But it does get weirder. Animals have
males, females, and hermaphrodites. But plants take it to a whole other level.
For
more information or classroom activities, see:
Monoecious/dioecious
–
Spatial
segregation of sexes –
Serotiny
–
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