gametophyte reduction

Janice M. Glime jmglime at MTU.EDU
Tue Oct 5 17:29:49 EST 1999


Dear Plant-edders,
  There is one more bit I can add to this discussion that may shed some
light on the origins of sporophytes.  This is the theory that the
expression of the genome was strongly under the influence of the
surrounding tissue.  In most of the algae, the zygote is shed into the
water or if retained, it is in decomposing parent tissue.  By the time
development resumes, there is no parental (i.e. gametophyte) influence on
the zygote.  Thus, many of the algae have zygotic meiosis.  
  Compare such algae as Ulothrix, Spirogyra, Chlamydomonas, and Oedogonium
with Rhodophyta such as Polysiphonia.  In Polysiphonia, the zygote is
retained on the gametophyte and gametophyte tissue expands around the
zygote to form a cystocarp.  Polysiphonia subsequently divides its zygote
in situ, forming carposporangia and pinching off carpospores.  (Sounds
like a fungus!)  These carpospores then leave the cystocarp and develop
into a sporophyte alga, but this sporophyte looks like a gametophyte.
Polysiphonia has been cited as an example to support the homology theory
of the origin of alternation of generations.
  This example appears somewhat isolated and the support it lends is
rather weak, but some very interesting experiments were carried out by
Arnon on bryophytes (if my memory serves me well).  He took gametophyte
tissue and put it into an archegonium and got a sporophyte from it! 
Furthermore, bryophytes typically develop gametophytes from isolated bits
of tissue of stems, leaves, rhizoids (all gametophyte parts) and from
setae and capsules (both sporophyte), and develop new protonemata and
ultimately gametophytes.  All of this suggests that the change to a
sporophyte was strongly influenced by the retention of the zygote within
the reproductive structure to form an embryo.  When the parent tissue is
absent, the sporophyte plant fails to develop as a different entity, but
when it is present, the sporophyte is a different generation.  This same
early influence may be necessary to develop the structural support needed
to support an independent sporophyte, but now I begin to digress from
evidence to maybe's and it is time to quit. 
Janice
***********************************
 Janice M. Glime, Professor  
 Department of Biological Sciences
 Michigan Technological University
 Houghton, MI 49931-1295
 jmglime at mtu.edu
 906-487-2546
 FAX 906-487-3167 
***********************************




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