On the Origin of Sponges

Heather Gosnell, Dory Bennett, Martin Davis, Leah Bilski

● Sponges are monophyletic● Sponges evolved 500 million years ago● "We are pretty confident it was after the sponges

split from trunk of the tree of life and sponges went one way and animals developed from the other, that nerves started to form," said Bernie Degnan of the University of Queensland. "What we found in sponges though were the building blocks for nerves, something we never expected to find."

Sponge Fun Facts

● “Collared” cells of choanoflagellates and sponges almost identical

● Choanoflagellates found in salt and fresh water● No fossil record to show common ancestor● Choanoflagellate rDNA and mitochondrial genome

comparisons with three sponge species indicate protista -> animalia connection (Lavrov et. al., 2005)

Syncytial theory - Metazoa evolved from multinucleated ciliate based on:

(Syncytial refers to protoplasm that contains numerous nuclei not separated from each other by plasma membrane)

• Many protozoans are multinucleated, including ciliates• Bilaterality in ciliates as in most metazoa• Such an ancestor might be similar to ciliated 'planula' larva

found in cnidarians (and some porifera and one ctenophore) and to acoel flatworms

Problems with Syncytial Theory• Radial symmetry occurs in "Radiates" (sponges, cnidarians,

ctenophores, and placozoans)• Little evidence of syncytial cells in most basal metazoans. Colonial Theory -Metazoa evolved from a colonial flagellate· A colonial sphere of cells, termed a blastaea, invaginated

to form cnidarian-like gut resulting in differentation of cell types.

http://scienceblogs.com/pharyngula/2006/05/08/stromatoveris/

Phylogeny of PlacozoaA: sometimes argued as the earliest branch of animalsB: “placozoan epithelial cells are connected by junctions of

extracellular proteins, a condition present in all animals other than sponges, suggested that placozoans may have diverged later in the history of animals”

C: “More recently, data from molecular sequences have indicated that placozoans might have diverged even later in the history of animals”

http://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html

References:www.livescience.com/2807-origin-nerves-traced-sponges.html

http://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html

http://scienceblogs.com/pharyngula/2006/05/08/stromatoveris/