Lecture--15 September 2003
Longitudinal-- Paramecia show longitudinal fission
Transverse--Euglena has transverse
Differences in mode may effect final shape is cells don’t separate. Things that use
transverse fission may end up as long strings. Things that use longitudinal fission
may end up as flat sheets. There are advantages to either mode.
Have no movement in and of themselves—example of Malarial organism, which is
transmitted form host to host by mosquito. Also viruses
Cilia—small, hair-like devices found on things such as paramecia
Flagella—longer, larger hair-like devices found on things such as euglenoids
Pseudopodia-extensions of the cell boundary.
Syncitial—term describing nuclear division, but no cell division. The resultant cell mass is called
a syncitium. Situation is found in Kingdom Fungi, but also found in some of the protists and
Origins of multicellularity—Two hypotheses
1. From a flagellate/sarcodine precursor
2. From a ciliate precursor
Let’s look at the first hypothesis
First, we need to know the characteristics of the Porifera (some images of sponges and sponge
No real tissues, but have choanocytes in interior and epidermal cells covering the
outside. Have spicules of either organic or inorganic matter that provides support
for the bag of cells. Cells can be dissassociated by subject the specimen to a
chelating agent ( see the discussion on EDTA, agent that “immobilizes” Calcium).
The resulting cells can be filtered to get rid of spicules, etc. and placed in a petri
dish. They will then swarm together and reform the sponge colony.
Looking for a precursor we choose the protozoan group, the choanoflagellates. Choanoflagellates
(the ones in this diagram are solitary) are free-living colonial sarcomastighophorans that use a
collar/flagellar apparatus for food getting.
A quick look at the Volvicines.
Volvox is a photosynthetic organism that is a colony with many similar individuals in a spherical
configuration. The creature shows some division of labor amongst the individuals that make up the
colony. The creature also buds off new colonies into the center of the sphere. These are released
when the adult disintegrates. The individual cells are biflagellated with the flagella facing outward.
How do the above groups fit our understanding of sponge evolution? First off, the volvicines did
not evolve into sponges. They are only photosynthetic algae. We mention them because of the
“division of labor” and the shape they take. The sponges actually produce a larval stage that looks
like a miniature volvox in that the collar cells are faciing outwards. What we assume is that the
choanoflagellatesevolved into the sponges by first becoming a “volvox-like” creature that then
moved its collar cells from facing outwards to facing inwards. Look up image of amphiblastula in
Projects looking for connections between the sponges and the rest of the invertebrates:
Mrs Nicole Boury-Esnault, Centre National de la Recherche Scientifique (CNRS)
Centre d'Océanologie de Marseille