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This
clip shows the mating of hermaphroditic
leopard
slugs. Each may provide male gametes
for
the other, or it may just go one way. They hang
from
a branch to do this, and the male reproductive
organs
spiral around one another. The trait has gone
mad
– in some species, the male organ has reached
92
cm long!
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Some gastropod (snails and such) breeding is strictly sexual; they have male
and female snails (marine), while most terrestrial snails are hermaphrodites.
But even then, most need another gastropod to mate with. Each hermaphrodite
fertilizes the eggs of the other. There are the rare cases where a
hermaphroditic slug will self-fertilize and produce clones, but this is the
exception, not the rule.
These examples show that weird reproduction does take place
in animals, but the plants have us beat by a long shot. Even the simple types of
flower breeding systems turn out to be not so simple. Sometimes being both sexes is the easy
part.
If a single flowering plant (angiosperm) can produce both male and female reproductive cells (pollen and egg),
the plant is called monoecious (one
house). This represents a nice tight bundle, reproduction wise. One plant can make the male gamete
cells (pollen) and the female gamete
(egg) – it’s a hermaphrodite. Let’s look at the types of monoecious flowering
plants, maybe it's not so simple:
Monoecious perfect - These plants have flowers that contain both the male reproductive
structures and the females reproductive structures, so they are said to be perfect flowers. Because they occur on
the same flower, any time the plant flowers, the blooms have both female and
male structures and qualities. These are true hermaphrodites.
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The
king flower of the apple blossom is the key to
getting
good apples. It has to be pollinated for the
cluster
to all produce apples. The king flower opens
first,
is pretty, and smells good.
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Monoecious imperfect
– In the simplest form, monoecious imperfect
plants have some male flowers and some female flowers, but they both bloom on the same plant. The flowers themselves aren’t hermaphroditic, but the
plant is, since it has both male and female structures.
A single American chestnut tree will have both male and
female flowers at the same time. Some long catkins (an arrangement of small flowers on a single stem) have only male flowers while others have male flowers at the tips and
female flowers at the base.
Male and female flowers on separate individuals at the same
time like with the chestnut is one form of monoecious imperfect, but there are others as well.
Gynomonoecious or
andromonoecious – These are a mixing
of perfect and imperfect flowers on the same plant. Gyno- means female, so
these plants have imperfect female flowers AND perfect flowers at the same
time. Andro- means male so, you guessed it, they have male flowers and perfect
flowers.
A 2003 study of four Solanum (a large genus that includes potatoes and tomatoes) species showed that the number of male flowers compared to the number of
hermaphroditic flowers can vary greatly. Some species were about 7% male
flowers (weakly andromonoecious), while others were 69% male flowers (strongly
andromonoecious). Weak species would change the number of male flowers produced
according to how much fruit was produced, but strong species made the same
number of male flowers no matter what.
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The
American chestnut was a towering species until
the
late 1800’s when a nursery owner imported some
Japanese
chestnuts that had a fungal parasite. The
American
version had no immunity, and we lost these
huge
trees in short order.
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Problems with Monoecy
- With either perfect flowers or imperfect flowers one a single plant, it’s
possible for the pollen of an individual to fertilize an egg on the same
individual – self-pollination.
However, self-pollination isn’t always a good thing. Self-pollination produces
clones of the parent that provides both the pollen and egg. Pollen from one
plant fertilizing the egg of a second individual is called cross-pollination.
Cross-pollination promotes genetic diversity. Clones tend to
build up genetic mistakes, while cross-pollination help to spread genes through
the population and makes the species more likely to possess genes that might help them withstand
changes in the environment. Therefore, many plants take steps to prevent self-pollination
and promote cross-pollination.
Heterostyly (hetero = different, and style = part of the female reproductive
organ) prevents “selfing” in many animal pollinated plants. In this case, a
certain species will have two morphs (shapes) of flowers. One will have a long anther (pollen producer) and a short style (where the pollen is deposited and grows down to egg). The other will have short anthers and long styles (see picture and caption to below).
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Morph
1 and Morph 2 are the same species, but different
individual
plants. The large insect pollinator can easily
get
pollen from A anthers (right) and deliver it to A pistils (left), but
how
would it get pollen to the B pistil (right)? The reverse is true
for
the smaller pollinator. Therefore, Morph 1 can pollinate
Morph
2 and vice versa, but neither can pollinate their
own
morph. This is heterostyly.
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However, that same pollinator would be well-designed to
deliver its pollen load to a flower with a long style, the kind found on the
other morph of individuals of the species. This would promote cross-pollination. The strategy is equally successful for those pollinators best prepared to
gather pollen from short anthers.
It is not known whether the move to heterostyly in some plants
has been driven by genetics to avoid inbreeding or by pollinators and the need
for efficient fertilization. A 2006 study in Narcissus flowers looked at both genetics
and pollinator efficiency in breeding and concluded that the pollinator driven
evolution was supported to a greater degree. This agrees with the pollinator hypothesis that Darwin
proposed almost 150 years ago. He was pretty smart.
In other cases, the position of the flowers may discourage self-pollination. For instance, some wind pollinated fir trees have female
cones up high and male cones down low. The pollen from the male flowers might
travel on the wind and gain altitude to fertilize female flowers on adjacent
trees, but it is extremely unlikely that the pollen would be blown straight up to
fertilize the female flowers of the same tree.
The most common mechanism to prevent selfing is self-incompatibility.
There are two main mechanisms of self-incompatibility; they both work at the
genetic level to make sure that the pollen of a particular individual will not
successfully fertilize an egg of the same plant.
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The
blue pollen has one rearrangement of the
compatibility
genes (S3, S4), while the red pollen has a
different
rearrangement (S1, S2). If the ovule genome has
S1,
S2, then S1, S2 pollen landing on the stigma will be
destroyed.
S3, S4 pollen won’t be recognized and can grow
pollen
tubes to fertilize the S1, S2 egg. Self pollen is
incompatible
with the same egg; this promotes
cross-pollination.
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So these are ways to prevent self pollination in perfect and imperfect monoecious plants. But monoecy can get weird on its own in an attempt to prevent selfing:
Dichogamous monoecy
– This breeding system probably evolved as a way to prevent self-pollination in monoecious plants.
The pollen and ovule mature at different times. This is equivalent to having a
flower (plant) that can change its sex in just a short period of time, and
these count as additional monoecious breeding schemes. Some animals can do
this, but the change takes place over the period of a lifetime. Here were
talking about in the period of a few hours.
If an individual plant can change its sex over a short
period of time within one growing season, then it is called dichogamous (dicho = in two, apart, and gamous
= gametes), also called sequential
hermaphroditism or temporal dioecy.
But which comes first male or female? If the plant first produces male flowers,
then it is termed protandrous (proto = first). Protogynous is name for those that are female first. This is a
great way to prevent self-fertilization and there are a couple of ways plants can
employ dichogamy.
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This
chart will help explain the different monoecious
breeding
systems. Each large circle is a population of
plants
of the species. The circle with cross means female
flower,
the circle with arrow means male flower, and the
circle
with both means perfect flower. The line arrow
with
a “t” means a change as time passes. For dichogamy,
the
flowers may be perfect or imperfect, but they function
as
male or female at each time point.
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In perfect protandrous plants, the pollen matures and is carried away (ind, insects, animals, etc) before the ovules mature on the same flower - so no selfing. In perfect protogynous, the opposite is true. The early maturing eggs must procure pollen from individuals who have already had mature eggs and have changed to produce pollen.
The other
dichogamous possibility for monoecious plants is when they have imperfect
flowers. This means that the plant would make flowers of one sex first, and
then grow separate flowers of the other sex later on. The separate flowers are
still on the same plant, but self-pollination isn’t possible because they
aren’t there at the same time.
Corn is an imperfect,
protandrous plant. This is why country kids detassel in the summer. The tassel
is the male flower. If you remove it (de-tassel), it will prevent possible
selfing when the female flowers come out. You can create hybrids by planting a
few rows of a specific breed of male corn at the end of the rows.
It’s much harder to find an example of an imperfect, protogynous
plant. In general, doesn’t it seem silly for a species to produce their female
flowers first? They need pollen from the males in order to be fertilized, but
if they’re all female, who provides the pollen? It doesn’t seem logical.
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The
left image is the female flower of corn, every silk is a
flower
that can be pollinated. Each one that is will produce
a
corn kernel on the ear. The right image is the male
flower,
the tassel. This get pulled off by teenagers trying to
make
money for that prom dress or new stereo.
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But why no protogynous imperfect
plants? The wasted energy in making purely female flowers very early when
little pollen is present probably dooms this breeding system. At least with
perfect protogynous, they get some benefit by dispersing pollen later from the
same flower. No extra energy is consumed in producing an entirely different flower.
Duodichogamy - This
system can help with the timing issue above; it’s dichogamy taken a bit
further. Instead of being one sex then the other, they go back and forth and
back and forth – like Mystique of the X-Men. In Bridelia retusa, a
tropical tree from India, the switching between male and female occurs several times within a single
week! To my mind, this is like sexual dimorphism in antlered males of some species - the antlers just keep getting bigger and bigger. Where will it
all end - how many times will this plant change sex?
Next week - another mechanism for preventing
self-pollination is to separate the reproductive parts to different plants.
These are the dioecious plants.
For
more information or classroom activities, see:
Monoecy
–
self-pollination
and cross pollination –
