Some people practice a form of vegetariansim called
veganism. The definition of vegan can be different from person to person, but
generally it means that one does not consume or use animal products in which an animal was harmed to obtain them.
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Vegans
range from those who won’t eat animals or
products
that require animals to be harmed, to those
that
will not use any animal product whatsoever.
Plants
make up their entire diet. How shocked will
they
be to learn that research has uncovered that
plants
might well have feelings and sensations.
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Unfertilized eggs can’t develop into a chick. Because of
this, some vegans will eat chicken and quail eggs that are certified
unfertilized. Now for a potential problem – chickens, quails, turkeys, and
other birds are known to undergo parthenogenesis!
Unfertilized eggs might have a partially developed embryo inside them. What is
a vegan to do?!
The thought that birds might be able to undergo
parthenogenesis is not that strange. Reptiles are the ancestors of bird
species, and reptiles are famous for their number of parthenogenic species,
both obligate and facultative.
The difference between parthenogenesis in birds and in reptiles
is that the bird form rarely, very rarely, ends with a chick hatching from the
egg. Most die at some point in development - usually early.
The first species in which bird parthenogenesis was studied
was the Beltsville Small White (BSW) Turkey. It was recognized in the 1950’s that this breed
had some eggs that had started to develop even though they had not been mated
to a male.
The researchers then embarked on a long breeding project in
which they increased the number of parthenogenic embryos. By breeding females
that had a higher tendency to lay parthenogenic eggs
to males from mothers who were more likely to develop parthenogenic eggs,
the scientists developed the breed for parthenogens that developed longer
and longer.
In some cases (still less than 1%) the parthenogenic eggs
would hatch. And some of those turkeys matured fully and lived to a ripe old
age! The breed still exists and is still studied, so parthenogenic tom turkeys
are born to these females every once in a while.
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Zebra
finches are native to central Australia, but have
been
introduced into North America, Brazil, and
Portugal.
They are good for research because they can
breed
all year round, usually after strong rains, they
have
clutches of five-seven eggs, and they are fast maturing.
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After studies in turkeys were publicized, it was recognized
that parthenogenic development was also occurring in chickens. There was a
single report of parthenogenesis in a pigeon. Then in the late 2000’s,
parthenogenesis in both Chinese Painted Quails and Zebra Finches was
recognized. It is important to note that in these later named species, only one
parthenogen was noted to survive; that being a chicken in the 1970’s. At that
time, molecular methods of genome identification were not available, so we are
not sure if this was a true parthenogen.
The other point to note is that these are all domesticated,
captive birds. We don’t know if parthenogenesis takes place in birds in the
wild. Similar to the cases in other animals, we first recognized parthenogenesis in sharks and komodo dragons in zoos, because that is where
people could control if females were exposed to males. Many assumed that
parthenogenesis was caused by a lack of males and that they would not give
birth from unfertilized eggs in the wild. We now know that isn’t true for
komodos, and we have the report showing that pit vipers will undergo parthenogenesis in the wild, even if males are present. Who knows if this is
the case for birds.
To understand parthenogenesis in birds, it would help to
look at how eggs are produced; we’ll use the chicken as a model. Some weird
things can happen with chicken eggs and their process of production is
responsible for most of these oddities.
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This
isn’t the most pleasant of pictures, but it shows the
reproductive
tract well. The yellow orbs on the left are the
ova
with developing yolk in the ovary. The bigger ones will
be
released first. The magnum is larger and adds albumen;
the
isthmus is narrower and adds the membranes. Guess
what
the shell gland does. The finished eggs exit via the vent.
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If the human egg is fertilized, the embryo will implant into
the wall of the uterus and the placenta will develop. If not, the uterine environment
will flush itself out each month and the cycle will begin again. This is
different in chickens. Whether the egg is fertilized or not, the ovum (and
attached yolk) will be sent on along the oviduct and an egg will be formed.
Chicks are born with 13,000-14,000 ova and they produce no
more. Not all will be laid as eggs, but every 26 hours or so, a new ovum with
developed yolk (fatty nutrients for the developing chick) will be released from
the ovary. The timing of the release is actually controlled by the laying of
the egg. When an egg is laid, a new ovum will be released about 30-60 minutes
later.
Ovulation is also controlled by the amount of sunlight in
the day. Summer day lengths stimulate ovulation, so egg producers manipulate
the lights so the hens always think it is summer.
Of course, there is an exception to this. Chickens won’t
ovulate after about 3:00 pm! They must where watches. And the entire process
for laying an egg takes about 26 hours. This is longer than a day – duh - so
each day the chicken will ovulate about 2 hours later. This keeps up until she
would be due to ovulate after 3:00. In this case, she just won’t do it, and
will wait until the next morning to ovulate. As a result, a chicken will not lay an
egg once every six days or so.
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Double
yolks aren’t really that uncommon, occurring
in
about 1in 1000 eggs. However, they are usually
caught
in the production process and used for other
egg
products instead of putting them in your Styrofoam
box.
Quadruple yolks are much less common, but they
do
occur, and some breeds are more likely to give
them
than others.
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After the magnum is the isthmus.
This is where the egg is surrounded by the inner and outer membranes. The next
stop is the shell gland, and you can
guess what is added here. The calcium carbonate shell takes about 20 hours to
form around the egg, so this is where the egg spends the majority of its time.
Then it is laid by being squeezed out with muscle movement.
Like I mentioned above, weird things can happen during this
25-27 inch trek through the chicken. Sometimes two ova may be released at once.
These can both be surrounded by a single albumen and shell and come out as a
double yolk egg. There are instances where one or both yolks may be fertilized,
but the lack of space and nutrients usually leads to at least one of the chicks
dying in the shell, and usually both. The record is nine yolks in a single egg!
On a different note, when the hen is young no ovum may be released, and a small
piece of loose tissue could be mistaken for an ovum. In this case, it will be
wrapped in albumen, membranes and shell, and a yolkless egg will be produced. I
have a student who is seriously considering investigating a way to manipulate
chickens to give yolkless eggs all the time – could be a million dollar idea.
There was a recent story that illustrated one more weird
possibility. If the muscular movement shoots the egg backward instead of out,
it can happen that the developed egg will go through another round of the
process. It can also meet up with the next developing ovum. In this case, the
developed egg could be surrounded with more albumen, membrane, and then be
wrapped in another shell - an egg within an egg! Don’t believe me? Watch the video.
In parthenogenesis, the ovum + yolk will be diploid, the
result of endomitosis or fusion of two ova. They will be sent along the path of egg production, and once
laid, they look like regular eggs. The embryo will not develop beyond three
days or so, so they are hard to tell from unfertilized eggs or those eggs that
are fertilized and undergo spontaneous early embryonic death. You probably
wouldn’t know if you were eating one.
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The
Beltsville Small White turkey was developed in the
1930’s
and quickly became the most popular turkey
on
American plates. The name comes from the USDA
research
farm n Beltsville Maryland, where they were
developed.
However, in the 1940’s the broad breast
turkey
was bred and the BSW quickly faded, except for
in
research – they have high rates of parthenogenesis!
That
should keep them popular.
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In the BSW turkeys, breeding led to later development and
finally some live hatchings. This is now being tried in quails as well. Dr.
C.D. McDaniel at Mississippi State University is investigating the idea that
parthenogenic development actually reduces the hen’s ability to hatch
fertilized eggs.
After nine generations of cross breeding females and males
to increase parthenogenic development, McDaniel reported in late 2012 that quail that have more
parthenogenic events do indeed have fewer fertilized eggs that hatch and
develop to mature quails. Late embryonic death decreased, but early death
increased dramatically. This is a significant economic question, as it would
seem that lower rates of parthenogenesis will lead to greater production of
quails.
Next week, we will see that parthenogenesis is not always
the “choice” of the female. Sometimes, parthenogenesis can be forced on an
animal.
