Wednesday, February 16, 2005

Smallest multicellular organism

I've been searching for the smallest multicellular organism and there do not seem to be any adult creatures with fewer than a thousand cells. For some reason there seems to be no evolutionary advantage for being say an organism of two cells or three hundred cells. This seems to also hold true for colonies of cells like sponges or algae. No one seems to have an explanation for why this would be true.

The diversity among organisms on the order of a few thousand cells is immense. On the one hand we have the nematode worm C. elegans which has 959 somatic cells with a nervous system of about 300 neurons. It has muscles and a metabolic system that operates surprisingly like humans. It reproduces sexually with sperm and egg. It's genome has 100 million base pairs encoding an estimated 17,800 genes.

On the other hand we have Trichoplax adhaerens which is a candidate for the simplest multicellular organism. It is the only species in the phylum placozoa. Trichoplax is comprised of a few thousand cells that differentiate into four types. It has no neural or muscular systems. It basically looks and acts like a large amoeba. It reproduces by binary fission or sometimes by budding although sexual reproduction may be involved like yeast. It has the smallest genome of any known animal at 50 million base pairs which is only a factor of two smaller than the nematode.

Both animals are about the same size - a few millimetres in length - and both have roughly the same number of cells but they have employed drastically different strategies for survival. So it seems that the constraint on minimum number of cells in an animal is not one of limited strategies. Perhaps is is a result of a constraint of molecular biology or cellular physiology.

7 comments:

Anonymous said...

What wonderful posts.

Anne

Anonymous said...

This evening, however, I am given over to Don Quixote :)

Anne

Anonymous said...

I'm not sure what you mean by smooth. However, on that thought, if you only look at the genome and followed the phylogenetic tree, my bet is that the transition from species to species looks much more seamless at the genome level. We'll only know for sure when we sequence many more organisms. Thus speciation per se may not be as fundamental a process as is commonly believed. Organisms basically shuffle their genetic material either through sex or gene transfer with some background mutation on top. If this continues long enough, you could say speciation took place although there was not a single event when this happened.

Wow! How splendid you are....

Anonymous said...

Anne :) I must learn to sign, I must I must.

Anonymous said...

I was under the impression that when to breeding groups are isolated for a period of time and diverge geneticaly that only when a member of one group can no longer breed with a member of the other group and produce offspring who are capable of reproduction themselves that this determines that they are from different species

Carson Chow said...

Hi,

Yes, that is the operational definition of speciation but relating that directly to changes in the genome may be difficult. There likely is not a universal level of deviation within the genome where hybrid offspring will not be able to reproduce. I don't have any references to cite but I think in some instances, two morphologically defined species can actually hybridize successfully.

Todd said...

Rhombozoa have under 50 cells. (http://en.wikipedia.org/wiki/Rhombozoa)