Moss gametophytes

Janice M. Glime jmglime at MTU.EDU
Thu Apr 11 19:58:46 EST 1996


Timothy Metz wrote:
> 
> I got stumped on a question in class today.  I was asked how does a
> developing moss gametophyte "know" whether to form an antheridium or an
> archegonium.  I know some mosses are monoecious and others are dioecious.
> What determines the type in unisexual gametophytes?  I assume all
> gametophyte plants that develop from the same protonema produce the same
> type of reproductive structure?
> Thanks for any help!

Bryologists have mostly adopted the terms monoicous and dioicous because
the plants are haploid.  However, that distinction is interesting in
answering your question.  First, the very first X and Y chromosomes known
in plants were found in the moss Mnium, probably now part of what is
called Plagiomnium.  Mnium and its offspring genera are known for their
evolution by doubling chromosome number.  Thus, plants with 10 chromosomes
in a set are dioicous, but those with 20 in the gametophyte set are
monoicous.  For dioicous plants, the separation of sex would occur at
meiosis so that the spore determines the sex to be male or female. 
However, there is no sexual dimorphism in the spores except in
Macromitrium.  It was once thought that Fontinalis had two sizes of
spores, and indeed it does in the capsule, but these represent abortions
during development, and very few of the smaller spores will germinate. 
They are not sexually different.  As far as I know, all plants from a
single protonema are of one sex, and this is evident if one looks at
colonies of Polytrichum with numerous splash cups together, and a
different clump with the tight tips of the females.  Some intermixing
occurs, but is most likely the result of vegetative spreading.
I don't think we have direct proof that all mosses rely on X and Y
chromosomes, but the fact that some are male and some are female is no
different from a man producing some gametes that are male and some that
are female (or Y and X).
You might get more information if you pose your question on
bryonet-l at mtu.edu.  If you want to join, let me know (I'm the manager).  If
you only want to ask the question, ask people to reply to the net with a
copy to you. 
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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