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!

Jellies at Shedd

We're in the middle of getting 6-8 inches of snow here in Chicago.  We've been putting off going to the museums until the winter weather really kicked in, and today was the perfect day to have an indoor adventure.  We spent a couple of hours at Shedd Aquarium, and we could have spent several more there.  My favorite exhibit was of the animals formerly known as jellyfish, currently known as jellies since they really don't have anything to do with fish other than the fact that they live in water.  I could watch them for hours - they are mesmerizing and fascinating.  Another museum-goer called them living lava lamps.

Northeast Pacific Sea Nettle, Chrysaora fruscescens

 I've written about jellies before, so I'll take a different approach here.  I'd like to point out some interesting anatomical structures and explain a little bit of their life cycle.  Let's start with anatomy.

The jellies above are swimming upside down.  The main body is called the bell.  There are two types of structures that trail behind the bell: tentacles and oral arms.  Tentacles are darker in the Pacific Sea Nettle, and they extend off of the edge of the bell.  Oral arms are usually whitish, four in number, and extend from the center of the bell.  Both appendages are covered in microscopic stingers.  The tentacles are for initial stinging of food and defense.  The oral arms are for more stinging and for moving food toward the center of the bell for digestion.

Jellies have only one orifice on their bodies, which has to serve for the in and out orifice of the digestive, reproductive and respiratory systems.  Food goes in, gets dissolved, is carried out through the rest of the body, and whatever isn't digestible is 'spit' back out.  Reproductive organs, visible as the four ring-shaped white structures in the moon jellies below, connect to the single orifice.  Males release sperm into the ocean water.  Females can release eggs into the ocean water, hopefully to find a sperm somewhere, or more commonly, they take in sperm, allow fertilization, and even allow development of the next generation inside their body cavities. 

Moon Jelly, Aurelia aurita

Jellies have a ring of muscle one-cell-thick around the edge of the bell that they can contract and relax to move.  Also around the edge of the bell, it is common for jellies to have light-detecting tissue for knowing when to swim up to the surface of the ocean.

Good view of the two types of appendages attached under the bell.

The jelly life cycle is pretty head-scratching.  The pictures on this page show all adult forms, but they have juvenile forms that look completely different - like butterflies and caterpillars look different.  Fertilized jelly eggs grow into larvae called planulae.  Planulae are almost microscopic and swim free in the ocean.  Some planulae can sting, and those invisible stings that sometimes get trapped inside your bathing suit could be planulae or they could be broken-off adult stingers.  Planulae swim to a hard surface, like a pebble or a rock, and they stick and grow into a polyp.  A polyp is a cup-shaped, usually transparent animal with tentacles in a ring around the top of the cup.  The bottom of the cup is stuck fast to a surface.  Polyps can grow mini-polyps off the side of themselves, which can fall off and become independent organisms (this is asexual reproduction by fragmentation).  Eventually, polyps grow a beret-like structure, let go of their substrate, and become an adult medusa to roam the ocean, sting at will, and reproduce the next generation.  Here is a nice diagram of the jellyfish life cycle.

Flower Hat Jelly, Olindias sp.

I'll end with a picture of an extremely strange jelly, the flower hat jelly.  It keeps its tentacles tucked up underneath its bell, and it extends them quickly when they are needed.  It is a benthic species, spending most of its time on the sea floor.  Those tentacles that extend off the top of the bell are mysterious, but I do know they are called exumbrella tentacles.  They might be lures to attract prey, or they could be stinging tentacles for self-defense.  If you are searching for a creature to study to make a name for yourself as a scientist, this might be a good subject, because little is known about it.

If you'll be in Chicago sometime in the next year, you're invited to come with me to the Shedd Aquarium - we have a membership!