Tuesday, April 24, 2012

Parthenogenesis and the Dandelion

If you've been following along, you know that flowers are the sex organs of plants, specifically adapted to combine half of the DNA of a male plant (in pollen) with half the DNA of a female plant (in ovules) to produce a new plant, packaged in layers of a seed and wrapped in a fruit.  Sexual reproduction allows for each parent to pass on some of his/her genes to make an offspring that is combined from those genes.  It's why we all can see a little of each of our parents when we look in the mirror, and plants would too, if they used mirrors.  Unless they are dandelions.

Dandelions and Lake Michigan
Common dandelions, belonging to the species Taraxacum officinale, mostly do not reproduce sexually, though a few dandelions in Southern and Central Europe do.  The dandelions in North America are all clones of a few original European dandelions.  The method of cloning, or asexual reproduction, used by dandelions is called parthenogenesis.  In parthenogenesis, offspring are produced that look like normal offspring - starting out as embryos and growing to adults (versus asexual reproduction by fragmentation where the adult parent breaks in two, and viola, you have two new organisms).  For plants, parthenogenesis usually means producing ovules that have a complete copy of the mom plant's DNA, called apomixis.  The apomictically-produced ovules develop into seeds that are genetically identical to the mom plant, and there is no pollination involved.  That's female liberation (though there is a plant that does male apomixis!).

Dandelions decorating a lawn.
It takes a while to get used to the idea that plants have sex, but once you think about it, it makes perfect sense.  Sexual reproduction allows for continuity of traits being passed on from generation to generation along with genetic variation to survive in diverse habitats.  After you're used to 'normal' plant reproduction, parthenogenesis seems positively bizarre. Why would a plant want to give up on genetic diversity? Actually, lots of plants reproduce asexually by parthenogenesis (blackberries, onions, grasses and more), and there are some specific advantages.

The main advantage to asexual reproduction is the ability to capitalize on a successful genotype.  If humans could do this, we might have hundreds of little Steve Jobs growing up to make our future world a better place.  Instead, he left us a few offspring, but they each only have half his genes - and they may or may not have gotten the good ones.  For dandelions, their successful growth strategy can be reproduced in perpetuity because they are identical copies, with only random mutations providing genetic diversity.  Dandelions may have less need for genetic diversity, since the DNA they have allows them to grow differently depending on their growth conditions.  This is what makes them such successful and widespread weeds.

One last issue for today:  dandelions grow without being pollinated, but most dandelions do produce pollen (Did your childhood friends used to rub dandelions on your face?  Remember the yellow dust they left behind?).  Botanists scratch their heads a bit on this issue.  Dandelions that don't produce pollen can make more seeds, since they are not wasting their energy.  It may just be that because pollen production is genetically determined and dandelions don't evolve very quickly, the pollen production may just be an evolutionary leftover.  Also, there are some sexually-reproducing dandelions in the world, so if dandelions maintain the ability to pollinate potential dandelion mates, they could just happen upon a better combination of genes than the ones they already have.  I don't know that dandelions really need to become more successful at growing...though I love them, we have enough already!

Monday, April 2, 2012

Massive Pollen Overload: Hello Spring Allergies

Brace yourself.  These pictures are of terrifying plant structures that cause much anguish to their human victims, and the overall post is frankly somewhat disturbing.  Be brave.  For the first picture, imagine some ominous, slow, suspenseful music, because a threat is just beginning to emerge from its lair.  Yes, the picture below shows flowers from a wind-pollinated tree emerging out of a bud.  The tiny green globs in the picture below will open and shower the world with..........pollen!!! No!!!!  These innocent-looking little guys are going to make you miserable for the next few weeks.

Leaf and flowers emerge from this hornbeam maple tree bud.

"Can those really be flowers?" you ask. "They're so small and boring-looking." Well, not all spring flowers are beautiful and showy.  When flowers are conspicuous, you can rest assured that they are not causing your spring allergies.  Beautiful, sweet-smelling flowers are attempting to attract insect pollinators, and insect-transported pollen sticks to the flower, then to insect legs, and it does not blow in the wind.  Tiny, green, anonymous-looking tree flowers are usually wind-pollinated, which means their pollen is dusty, copious, and perfect for floating along on a breeze to any location, including your sinuses.

"Why do plants make all that pollen?  What's the purpose??!!"  You're asking a lot of questions today. Unfortunately, if you are reading this and sniffling due to a nose full of pollen, you might find my answer to be a little disconcerting.  Pollen is the plant equivalent of sperm.  So, yes, your sinuses are clogged with plant sperm.  Pollen is produced by the male parts of flowers, and it combines with the ovule in the female part of flowers to produce a fertilized cell that will develop into a new offspring plant.  In this picture, you can see the female parts of tiny winter hazel flowers reaching into the air to snag pollen grains to make some new baby hazel seeds that will grow into new hazel trees.

Winter hazel flowers with stigmas reaching out to catch wind-borne pollen.
If you were to look at grains of pollen under the microscope, they wouldn't look much like sperm.  Pollen has varied shapes, depending on the tree.  Here is a book of scanning electron microscope images of different types of pollen - amazing stuff!  My favorite is pine pollen, shaped like Mickey Mouse's head (Google it).

Pollen does a very, very, very strange thing when it fertilizes plant ovules.  When pollen lands on a female flower structure, it divides into three sperm cells, with actual flagellae.  The sperm swim down a channel in the female structure of the flower.  One sperm fertilizes the ovule, as we would expect based on what we learned about human anatomy in 7th grade.  The other two sperms combine with another cell near the ovule to make a substance called endosperm.  The endosperm is genetically the combination of two parents, but it is not really an offspring.  Endosperm is the structure inside the seed that stores food for the new growing plant.  For example, in a corn seed the endosperm is the starch in the corn kernel (yes, popcorn is exploded endosperm, and the little nubs in popcorn are toasted corn embryos....mmmmm!). Now you know why I put three "verys" in the first sentence of this paragraph.

New leaves and flowers hanging in clusters called catkins on a red oak tree.

Oak trees (pictures above and below) are my favorite trees, so don't think I'm picking on them.  They are pretty bad at making giant clouds of pollen.  Pines are even more intense.  There are a few days in spring in Georgia that you really don't want to be outside because the pine trees seem to spew pollen like snow-making machines spew snow.  If you catch a tree as it's releasing pollen and shake one of it's branches, you can make a nice, yellow cloud in the air.

When pollen lands inside your nose, the membranes in your nose recognize it as a foreign object to be removed.  Your body leaps into action with sneezes, mucus production, and swelling (which can cause headaches) in order to get rid of the pollen.  This immune response can make you tired and uncomfortable.  Fortunately trees only make pollen for a short period of time.  The benefits of having lots of trees near you (shade, habitat, aesthetics, food, property values, reduced heat bills, etc.) vastly outweigh the annoyance of allergies.  If your spring allergies are really bad, stay inside and be sure to wash your hair and clothes after you go outside to keep the pollen away from your nose.  And just wait around a few days for a spring rain shower, and the pollen will be gone.

New leaves and catkins on a white oak.