Biology concepts – dioecy, heterodichogamy, diphasy, plant
breeding systems, evolution, botany
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Bananas
grow from cuttings of the previous banana
plant.
Each plant produces just one bunch and is then
cut
down. None of the bananas grown by Chiquita or
Dole
are known to wear pajamas. Maybe the kids’
television
program was a way of showing how
genetically
similar we are.
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DNA sequence can include of a lot of A’s, C’s, T’s, and G’s
that are not in coding regions. The genes are sequences of DNA that code for a
specific product, not just a run of nucleotides. The genes of the banana and
man may be different in sequence (slightly to moderately), but they each code
for a protein that does the same job. Overall, it’s easier to find DNA that
matches outside of genes because there is so much of it (humans have about 2%
of their DNA invested in genes).
Still, 25% similarity in genes is pretty amazing, though some people are truly bananas and should be better matches. In reality, we aren’t that different from
any organism that descended from those
first algae that moved on to land.
Our molecular biology is very similar to plants. Consider
all the genes responsible for making DNA, proteins, modifying all the different
biomolecules, and making energy in the form of ATP. In those ways, we are almost
identical to plants. True, most plants do some things we can’t, like
photosynthesis. And we do some things they can’t, like calculus and making
videos of cats playing the piano, but our biochemistry is remarkably similar.
One way in which plants and animals are often different is
that about 99% of animal species have two separate groups of individuals – guys
and gals. Sure, there are the exceptions, and we have talked about some, but
for the most part animal species have separate sexes.
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These
are some of the ways plants can be dioecious.
The
heterodichogamy shown is the simplest form. It
can
also occur with duodichogamy (two switches on
one
plant) or can involve just one set of individuals
while
the other stays a single sex all the time.
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But before we get to the ways of being dioecious, we better
look at one mysterious exception, called subdioecy.
It’s hard to define, because it’s hard to tell if it is real or not. Subdioecy
is really just what scientists call it when they can’t tell what they’re
looking at. Most succinctly, subdioecy is when a dioecious plant has flowers
that aren’t clearly male or female. And that may be because the flower hasn’t
decided what it wants to be, speaking on an evolutionary scale, of course.
Flowers that are in the middle of becoming bisexual after being
unisexual for a long time, or those hermaphrodites that are just starting to
lose male or female structures will be hard to define as male, female, or
bisexual. The structures might be there, but be small, they may not look like
the typical structures, or they may be there but be non-functional.
All these would be reasons to classify a plant as
subdioecious. However, if a species keeps functional hermaphrodites and its unisexual plants in a stable system
(over time), then that’s another exception that we can talk about next week.
Three sexes, you say?
But for now, let’s move on to the more definable exceptions,
the first of which is the situation when there are clear-cut female and male
plants.
Dioecy – About 4%
of flowering plants have individual male and female plants, but the incidence
is higher on island systems than on continents. Hmmm. We talked a couple of weeks ago about how areas that are relatively resource poor, ie. dirt with fewer nutrients or areas of less water or higher stress, seem to promote development of maleness. It turns out that they promote dioecy as well.
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Tropical
islands and palm trees; they go hand in
hand.
Island conditions are most suited to dioecious
plants
so there are more there than on continents.
All
the date pam species are dioecious, but the
coconut
is an exception, it’s monoecious.
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Whatever the reason, it seems that dioecious plants compete
better in nutrient poor or stressed soils. If this is true, then you would
also expect to find a higher than normal percentage of dioecious plants in
tropical rain forests and along sandy beaches near salt water (high salt is a
stressful environment). Does it work out that way?
Yep, many studies have discussed a higher percentage of
dioecious plants on island systems. A 2005 paper extended this to a portion of SE Brail that is resource poor because it is rain forest (most of
the nutrients are tied up in large trees) and because it is sandy soil near the
beaches. In this locale, fully 32% of species are dioecious – that’s a huge
number compared with a normal range of 4-6%.
The reason may be that by dividing the duties of the plants
- males producing pollen and females producing ovules and then seeds – the
resource needs of any one plant is lower. Some of each will survive and keep the
species going. Once again, it’s related to males doing better in stressed situations.
Androdioecous – This
breeding system is related to that situation above. In androdioecy, there are
male plants and hermaphroditic plants. The unisexual males can
colonize more territory and produce a lot of pollen to ensure that the bisexual
flowers get pollinated. On the other hand, a single hermaphrodite can produce
several plants on its own, so it is good for colonizing new territory.
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Dwarf
Ginseng (Panax trifolius) has two
different
types
of flowers, but three different plants. One plant
has
no flowers, so I can’t show them two you. The
flowers
on the male plant have one short style, while
those
on the hermaphrodite plant have three or four
longer
styles.
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Gynodioecy – I’m
guessing you can figure out what this system must be like. Female plants and
hermaphroditic plants coexist in one population. This system is more advantaged
when resources are plentiful, so in areas of fertile soil, these species will
gain a better foothold and out compete gynodioecious species.
In fact, gynodioecy is much more common than androdioecy.
This may result from a pollen saving strategy, especially in areas where
nitrogen is limited (pollen uses a lot of nitrogen). More likely, it is
preferred because it gives more chances for seed production (more females). But
whatever the case, both gynodioecy and androdioecy are considered to be
important systems because they probably represent a transition between
monoecious and dioecious systems.
This begs the questions as to which was first, monoecy or
dioecy. Several recent reviews talk about the evolution of dioecy from
monoecious plants, with stop-overs in gynomonoecy, andromonoecy, gynodioecy or
andromonoecy. A 2012 review shows that we have good evidence for the first step
– monoecy to gynodioecy, but little evidence of the second step – gynodioecy to
dioecious.
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Z. jujubai is a tropical plant that is
heterodichogamous.
It’s
fruits are dried and eaten as treats. This is where the
name
for jujube and jujyfruit candies came from – mostly
from
the colors and texture, not that any of the plant is in
them.
Go ahead, ask your parents, they’ll remember
jujubes
and jujyfruits.
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Heterodichogamy –
This breeding system is the dioecious version of dichogamay we talked about
last week. Here, the males turn into females and the females plants become
male, but the two populations are out of phase. The timing is crucial so that
cross pollination is accomplished.
An example is the Zizyphus
jujuba. This tree has individuals that are male in morning and female in
afternoon. Another set of individuals in the same population are male in late afternoon
and female in morning. So there is always a female to accept the pollen being made
by males and the possibility of self-pollination is reduced almost entirely.
A more perplexing example of heterodichogamous breeding
system is found in a 2014 study of Platycarya
strobilacea (family Juglandaceae).
Most plants in this family of walnut trees are wind pollinated, but in this one
case, pollination occurs via a thrip insect.
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Monty
Python acted diphasic. Sometimes
they were
male,
sometimes they were female. It all depended on
what
the script called for. Many species of maple tree
are
diphasic. One year an individual may be male, the
next
year maybe female – it all depends on the
environment.
But as the picture shows above, when
happy,
they usually opt for female.
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It may be that the dichogamy in dioecious plants occurs in
just one of the populations of individuals. Now the names start getting long. Heterodichogamous androdioecious – one population is male while the other is first male then female.
Some maple trees were thought to exist in this system, but a 2007 study by
Renner cast doubt on the earlier observations and reclassified many as
dichogamous or duodichogamous monecious (see last weeks post). On the other
hand, there are no recorded instances of heterodichogamous
gynodioecious (females and females that turn male). Do you have any ideas
why?
Next week, a final flower breeding exception. Not one, not two, but
three stable sexes in one population of flowering plants. Trioecy is the reason we have dyed silk and tropical fruit salad.
Matallana, G., Wendt, T., Araujo, D., & Scarano, F. (2005). High abundance of dioecious plants in a tropical coastal vegetation American Journal of Botany, 92 (9), 1513-1519 DOI: 10.3732/ajb.92.9.1513
For more information
classroom activities, see:
Dioecy –
see last week’s list of links
(here)
Evolution of dioecy –
