I am coming
to believe that plants are more complex than animals, even more complex than
females. Female plants must be the most difficult things on Earth to understand!
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Complete
flowers have both anthers for pollen and
pistils for egg
fertilization. Incomplete flowers occur
on dioecious plants,
and have either the pistil
(gynoecious) or the anther
(androecious). Dioecious
plants cannot self pollinate, unless
they have
both types of incomplete flowers, like coast
redwoods
(see last picture).
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This isn’t
even the tip of the tip of the iceberg when it comes to diversity in plant
reproduction. There are also different ways to produce seeds. The gymnosperms have unenclosed seeds
(gymno = naked, and sperm = seed). Gymnosperms include the conifers (cone producers), the cycads that we talked a little about a few weeks ago, and the gnetum plants. Gnetum plants live close to the equator
around the globe and include the Ephedra
species. It is from these plants that we get ephedrine and pseudoephedrine that
work to relieve allergy and cold congestion.
The other
type of seed plants is the angiosperms
(angio = hidden). These are the flowering plants that have seeds encased in
fruits or other structures that help to protect them and to encourage their
dispersal.
One way that
the gymnosperms and angiosperms differ is in how they inherit their
plastid organelles. But even here there is a lot of overlap and exceptions;
plants just keep getting more complex.
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Gymnosperms
have there seeds exposed on the scales
of
the cones, while angiosperms have the protected
inside
the fruit (except for strawberries).
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But cpDNA is
much more passive, they do not have to do a huge amount of work to get to the
ovule of the pollinated plant. The pollen tube grows down to the ovule and
delivers the sperm cells right to the egg. There must be some other reason, but
I don’t know what it might be.
However,
there seem to be more exceptions in angiosperm inheritance of cpDNA than there is in animal mtDNA. A few families of plants, like alfalfa (Medicago sativa) and kiwi fruit vine (Actinidia deliciosa), have a strict
paternal inheritance of cpDNA. This is odd since, the angiosperms have a couple of
mechanisms for keeping the plastids out of the male gametes.
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Every
plant species has a distinct pollen shape, which
is
why you can be allergic to some plants and not
others.
But each pollen grain has the vegetative cell
that
becomes the sperms cells and the tube cell. The
tube
usually grows from the side that rests on the
fertilized
stigma.
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In many
species, when the generative nucleus divides to form sperm, the plastids are
partitioned off, and are not included in the sperm cells. This works to ensure
maternal inheritance. In other species, the sperm cells may include plastids,
but these quickly degenerate and are not delivered to the ovule. Somehow, the
alfalfa plants have overcome these mechanisms and even invented a new one to
eliminate or exclude the plastids from the ovule, giving strict paternal
inheritance.
Going beyond
the alfalfa and kiwi fruit ability to preserve their paternal plastids is the
fact that a full 20% of angiosperms can show (but don’t have to show),
bipaternal inheritance of cpDNA. This is called potential bipaternal plastid inheritance (PBPI) and is controlled by a male
gametic trait, called of all things - PBPI trait! Therefore, the fairly strict
maternal inheritance of mtDNA in animals (blue mussels excepted) is not matched
by cpDNA in angiosperms.
But it gets
weirder. The angiosperm exception is normal for the gymnosperm. Gymnosperms
tend to have paternal inheritance patterns for cpDNA. This difference is
important to note, since scientists often try to use cpDNA inheritance patterns
to track seed movements around the world and through evolutionary time, just
like human populations are often tracked using mitochondrial ancestry and
inheritance.
But this
must be frustrating, because there are also exceptions in the paternal
inheritance pattern in gymosperms. The Chinese fir (Cunninghamia lanceolata), which
isn’t a fir, is native to Asia but was brought to America in the 1800’s.
Remember that before molecular biology, most taxonomic classifications were made
on just the morphology (shape and look) of an organism, and its grouping and
name were based on how it compared to other organisms. Names often get stuck in
the language and are hard to change, so many of the misnomers persist.
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Godzilla,
or Gojira, always seemed surprised when
the
other monster grabbed his tail. Here it happens
to
be a giant wolfman. Everybody cashed in on the
werewolf
brand; I am surprised Abbott and Costello
aren’t
in that picture somewhere.
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But back to
the Chinese fir. This gymnosperm is a conifer that can grown 150 feet tall, but
flaunts its individuality by having a maternal inheritance pattern for cpDNA – much more
angiosperm-like behavior than gymnosperm. And this is even odder because the Chinese
fir is an older gymnosperm, a much more distant relative to the angiosperms
than many gymnosperms that have a strict paternal cpDNA inheritance.
Gymnosperms
that show maternal cpDNA inheritance are rare, or just less studied, so one
might assume that paternal cpDNA inheritance is fairly strict – wrong. Many
gymnosperms have bipaternal inheritance patterns of plastids, so the mechanism
might be different from angiosperms, but is no more consistent than that of the
flowering plants.
Finally,
there is the issue of crossbreeding. In this animal mtDNA and plant cpDNA seem
to be similar. Whatever the dominant form of inheritance is seen in natural
breedings, the numbers get screwed up when cross breeding occurs. We saw that
paternal inheritance of mtDNA in mice was much likely in the mating of
different species (interspecific breeding).
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The
passionflower vine can grow to be 10 meters high
and
is the source of the passion fruit that I enjoy so
much.
The fruit protects the fertilized seeds that
probably
have paternal cpDNA, since most of the
varieties
we eat are hybrids of different species.
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The
similarity between mitochondrial and plastid inheritance in hybrids
brings up another issue – what about mitochondrial inheritance patterns in
plants?
It turns out
that most plants that have been studied for mtDNA inheritance have a maternal
inheritance pattern, just like animals. Amazingly, this includes the gymnosperms,
most of which have paternal inheritance of cpDNA. But even some plants with
maternal cpDNA patterns can pass on paternal mitochondria. An example of this
is the banana - tomorrow morning you can feel like a rebel for garnishing your
cornflakes with such an outlaw fruit.
However, the
reason would be different. Remember that sperm have their mitochondria in their
tails, and in most animals, this is not included in what enters the egg or is
degraded just after entering. But few plants have flagellar sperm (like the cycads
we talked about before). The sperm mtDNA is not exposed to anymore oxygen
radical damage than the ovule mtDNA, yet there is most often uniparental,
maternal inheritance.
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Coastal
redwoods can reach up to 110 meters
(360
ft) tall, but their roots may only go 6 ft.
underground.
What's holding this tree in place?
It
has two different types of leaves, and has male
and
female branches and flowers, but all its
mitochondria
and chloroplasts come from one
place,
its father.
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So
mitochondria and chloroplasts both work in energy production, both evolved
through endosymbiosis, both have single, circular chromosomes (with
exceptions), and both have uniparental inheritance patterns (with exceptions).
Next week, let’s look a behavior that is different in these two organelles.
Zhang Q, & Sodmergen (2010). Why does biparental plastid inheritance revive in angiosperms? Journal of plant research, 123 (2), 201-6 PMID: 20052516Bendich AJ (2013). DNA abandonment and the mechanisms of uniparental inheritance of mitochondria and chloroplasts. Chromosome research : an international journal on the molecular, supramolecular and evolutionary aspects of chromosome biology, 21 (3), 287-96 PMID: 23681660
For
more information or classroom activities on monoecious/dioecious plants,
angiosperms, gymnosperms, or plastid inheritance, see:
Monoecious/dioecious
–
Angiosperms
–
Gymnosperms
–
Plastid
inheritance -
