(Crossposted from NeuroDojo)
Living things are made out of cells. Most people with even a passing familiarity with cells knows some of the parts that they have. A membrane to keep the outside out and the inside in. Some mitochondria for energy. Some endoplasmic reticulum to make your proteins. But the part of the cell that is the most familiar, the most famous, the big mac daddy of organelles, is the home of DNA, the center, the nucleus.
But now, my friends! Prepare to be amazed! Prepare to be astonished! Prepare to enter...
The world without the nucleus.
Well, not the world, exactly, but a nervous system in which most of the neurons have no nuclei. That nervous system belongs to the animal pictured in the upper left corner: Megaphragma mymaripenne, a microscopic wasp.
The other two cells in the picture above should be familiar to anyone who took any science in school: they’re a paramecium and an amoeba - and they’re shown at the same scale as the wasp. These wasps are tiny, tiny little animals.
Alexey Polilov has counted the nuclei in these wasps, both as adults and pupae. All of them. This is not as hard as it might sound, if you’re coming in with the expectation that most invertebrates have thousands, or tens of thousands, of neurons. Just one abdominal ganglion in crayfish holds about 600 neurons. But the total number of nuclei in the adult wasp was less than 400. And this wasp is capable of some complicated behaviour, not least of which is flying. I don’t know of anyone who thinks that powered flight is a simple behaviour that can be controlled only by a simple circuit with a handful of neurons. Flying is hard.
The lack of nuclei in the adult is not because they have so few neurons throughout their life, like C. elegans (302 neurons in wild-type adult). Rather, the wasps lose nuclei during development. The younger pupae have about 7,400 nuclei in their neurons, which sounds a reasonable number for such a tiny animal. But most of the nuclei are broken apart during the metamorphosis into the adult form. I know some other cells do not have a nucleus, like human red blood cells, but wonder if the mechanisms would be similar.
How can neurons without nuclei work physiologically? Polilov doesn’t provide an hypothesis, but he notes the adult wasps live only about 5 days, which is long given the size of the wasp. I suppose it’s possible that the adult life span is short enough that the nucleus can make all the proteins the neuron needs to function for five days during the pupal stage.
Polilov suggests that the size of the neurons limits how small you can make an animal. These wasps devote proportionately more of their body to their nervous system than larger insects: about 6% for Megaphragma compared to 1% or less for a honeybee. Despite the title of this paper, Polilov only examines the one species of miniature wasp in this paper. Whether or not other miniature arthropods would show the same kind of nuclear abandonment remains to be seen.
Polilov A. 2011. The smallest insects evolve anucleate neurons. Arthropod Structure & Development: in press. DOI: 10.1016/j.asd.2011.09.001
Protester picture from here.