Lecture--15 September 2003 Asexu

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					Lecture--15 September 2003

Asexual reproduction
      Binary fission
              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

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