gametophyte reduction

Nancy Harrison vulpia at
Sat Oct 2 09:20:50 EST 1999

In article <7t2j0r$q88$1 at>, 
ddudle at says...
>Hi All--
>After such helpful and interesting responses to my question regarding
>polyploidy, I have decided to try yet another question on y'all:
>What were the intermediate steps between the dominant, photosynthetic
>gametophyte and reduced sporophyte present in the non-vascular plants
>(bryophytes), and the dominant sporophyte that is present in all vascular
>plants (as far as I have been able to discern).  The oldest fossils of
>vascular plants that are most often referenced in textbooks and review
>articles, e.g. Cooksonia and Rhynia, are called sporophytes... 
>IT seems as if this was a very important development in the evolution of
>plants, and yet I have found very little reference as to HOW this change
>occurred.  Does anyone know of fossil evidence or living transitional
>forms that may give some insight into this major change in the focus of
>the alternation- of- generations in land plants?
>I have always been taught that "mosses have dominant gametophytes, and
>ferns have dominant sporophytes", but I have not questioned the "how or
>why" until I started writing lectures and preparing classes on this topic.
>Any hints or leads would be greatly appreciated (as well as speculations!)

What a good question. Here is a hypothesis (from an instructional
aide who has been struggling with this for years) - let's assume
everything had a dominant sporophyte in the plant kingdom. A small,
reduced, independent gametophyte like the fern's was selected for
in certain situations, flourished and grew bigger, It was able to
survive better in whatever habitat it was in, thus reducing the
sporophyte generation to smaller size and briefer life span.
  The problem, of course, is why a haploid form would be able
to become dominant and survive better than a diploid form.
Sheer dumb luck? Random chance?  Some mysterious limiting factor?
Bryologists, help! - NH
---Nancy Harrison, SRJC Life Sciences, Santa Rosa CA 95401 (with link to CNPS in Sonoma 

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