Biology concepts – parthenogenesis, polyploidy, geographic
parthenogenesis
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Komodo
dragons are the largest lizards on Earth, reaching
more
than 10 ft (3 m) in length and upwards of 300 lb.s
(136
kg). It was believed that they used the toxic bacteria in
their
moths to infect the prey they bite, then wait for it to die.
But
later research shows they have a toxin in their saliva as well.
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Later that same year, a Komodo Dragon in the Chester Zoo in
England also laid a clutch of eggs, but she had never been house with a male!
What gives? In both cases, DNA tests showed that the offspring had only their
mother’s DNA – they were virgin births, technically called parthenogensis (parthenos
= virgin, and genesis = birth).
The first incident had been attributed to storage of sperm
from a past mating (many animals can do that), but the genetic tests proved
that both mothers had resorted to asexual reproduction when faced with a lack of
males.
A similar event occurred in 2008 in the Virginia Aquarium. A
female black tip shark gave birth to several baby sharks, and they all had
her DNA only. This made everyone go, “Hmmmm,” and then they started checking some
other reports of shark births to females that hadn’t been housed with males.
Like the Komodos, this had been reported, but they assumed they were cases of
stored sperm. Low and behold, a 2001 family of bonnethead sharks from the Omaha
Zoo showed that all the offspring had just their mother’s DNA as well.
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Hammerhead
shark come in different flavors. The bonnethead
has
a curved front appendage (cephalofoil), while the
hammerheads
have scalloped or straight edges. In the front
appendage
houses their eyes, as well as an electrical sensor
and
it also helps them to turn quickly.
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But a study published in December of 2012 showed that pit vipers,
specifically cottonmouth and copperhead snakes, can revert to asexual
reproduction and undergo parthenogenesis in the wild, even with males all around!
There are many known one-sex species of fish, reptiles, and
amphibians that only undergo parthenogenesis as a reproductive strategy;
finding a sexual species that will randomly switch to asexual in the wild had
not been seen before, especially not in a vertebrate. This was a daunting task, since following the snakes around and proving that they didn’t mate. And then
proving that the offspring (if you can catch them) have the same DNA as the mom
ain’t easy.
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Pit
vipers are a group of snakes that can sense prey and
predator
by their heat signature. The pit organ is an
infrared
heat sensor, controlled by a protein called
TRP1a,
a protein that is usually a chemical sensor
in
other animals.
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There are two links between polyploidy and parthenogenesis,
and they themselves are linked together. First is the issue of meiosis. We have
discussed before that polyploidy messes with meiosis. Homologous pairs of
chromosomes are hard to align when they don’t come in pairs (odd ploidys) or
when there are more than one pair of the same chromosome (tetraploidy and
higher even ploidys). The pairing gets mixed up with some left out, or more
than one segregating together in meiosis I. This doesn’t even take into account
how high ploidys seem to alter the production of centrosomes (centrioles + spindles), the apparatus
that pulls the chromosomes apart.
As a result, gametes are more likely to be defective, and
dosage problems (how much protein is made due to increased copies of a gene)
can render a polyploidy organism sexually immature. These difficulties make it
less likely that the organism will successfully reproduce if it has to rely on
sexual means of propagation.
Therefore, through genetic drift and natural selection of
the sex genes that were being used less, parthenogenesis appeared. With this
strategy, the problems of meiosis can be avoided by merely skipping that step
and making diploid (or higher ploidy) eggs. Being diploid, the eggs don’t need the
contribution of sperm DNA to be complete, they “just” need to be jump started
to develop into an embryo. That’s a big “just”, and we will talk about it in a
bit.
The second link between polyploidy and parthenogenesis has
to do with geography, and is often called the “rule of geographical parthenogenesis.”
As a model, let’s use Alaskan bachelors. Men that relocated to Alaska first find
gold and later to find oil were moving to a harsh environment. They were
spread out over large areas so that the population density was low. So what
were the chances that they were going to meet a nice girl, settle down, and
have a family out there in the wilds?
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In
this figure, the desert regions (yellow) and the ice
and
snow regions (blue) correspond to where the most
polyploidy
and parthenogenic animals are found. In
South
America, the ice/snow region is located high in the
mountains
– remember that elevation is similar to
movement
to extreme latitudes.
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Either way, parthenogenesis is going to become more common
in these habitats. One - they switch to parthenogenesis because they can’t find a
mate, or two - they switch to parthenogenesis because they have hybridized and are
now quite likely to be polyploid. Our model fails here, at least I hope it
does, because I don’t think the Alaskan bachelors did either; they didn’t have
babies on their own and I really hope they didn’t hybridize with a local species!
So geography is linked to polyploidy and it is linked to
parthenogenesis. Here’s a simpler way of phrasing this geography idea - there
is very little parthenogenesis and very few parthenogenic species in the
tropics, but as you travel further north or south you gain more of both. In the
polar and sub-polar regions, both polyploidy and parthenogenesis are much more
popular.
Another factor is elevation. Most people, other than
ecologists, don’t think much about it, but going up in elevation tends to mimic
moving further north or south of the equator. In fact, every 300
feet of elevation equals
one degree of latitude or 70 statute miles north. Elevation brings the same changes in
climate and habitat as do changes in latitude. So as you go up a mountainside,
you are likely to find more and more polyploid species and more parthenogenic
species.
So there is little parthenogenesis where it
is hot, and much less sex going on where and when it is cold. That is sort of the
opposite of humans; you ever wonder why more babies in the US are born in July through September?
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Platythyrea punctata is a ponerine ant of
Central and
southern
North America, as well as amny Caribbean
islands.
It has a nasty sting; it belongs to the same
group
as the very toxic bullet ant.
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The point is that islands are geographically isolated, so
finding mates that are genetically different will be difficult, and if an ant
isn’t going to gain the advantage of genetic diversity by sex, why go to the
cost and energy of having sex. Parthenogenesis allows them to populate much
faster and easier.
In general, insects that are parthenogenic
are almost exclusively polyploid. No study has been carried out to see if P. punctata on the
islands is polyploid, but they do have an abnormally high number of chromosomes
for ant (84). As with many species, polyploidy and parthenogenesis in insects
seem to be linked; those in tough areas do both because they need to.
Insect parthenogens and those in other taxa
also tend to be less mobile. Parthenogenic insects, for instance, are often
flightless. This makes moving around harder, and that means they are more
likely to not find mates (one reason to be parthenogenic) or to hybridize with
those they can find, and become polyploid (another reason for parthenogenesis).
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The
New Zealand mud snail, Potamopyrgus
antipodarum,
is
an invasive snail that can reach amazing densities in
temperate
waters, even though each individual is very
small.
It was first introduce to England in the 1850’s and
spread
to North America and the rest of Europe from
there.
Wasn’t identified in the USA until 1987, in Idaho. The
US dime is 18 mm across.
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What is more, a 2010 paper that was a collaboration of Indiana
University, University of Iowa, and the Swiss Federal Institute of Technology
(studying a New Zealand snail!) showed that the P. antipodarum has sexual and asexual reproductive strategies. It
is usually assumed that the diploids reproduce sexually and the triploids and
higher reproduce by parthenogenesis, but that is not what the researchers found
that many of the diploid, triploid and higher ploidy males are offspring of
asexual females, while some higher ploidy individuals likely come from sexual
reproduction. Leave it to nature to screw up a good pattern.
Now that we know that parthenogenesis is widespread occurs
in many different kinds of animals (and plants), let’s dive in a bit deeper. Even though it is reproduction without sex, it is still a battle of the sexes.
For
more information or classroom activities, see:
Parthenogenesis
–
Rule
of geographical parthenogenesis –
Polyploidy
and parthenogenesis -
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