Re: How have gametophyte and sporophyte switched roles of host and parasite?

It's a bit difficult to gauge the level of my answer as I don't know how 
much botany you have had. I'll do my best to make it simple and as 
brief as possible, but it won't be easy. Of necessity, I will have to 
invoke some rather esoteric concepts without extensive explanation.

The term 'plants' is a bit nebulous. In the broad sense (sensu lato) 
plants include all of the algae, the fungi, and historically, even the 
bacteria. In a more strict definition (sensu strictu) plants are limited 
to the vascular plants (the angiosperms or flowering plants, the 
gymnosperms or non-flowering plants, and the ferns and their 
relatives) and the bryophytes (mosses, liverworts, and hornworts). 
This second, sensu strictu group is sometimes called the "land 
plants." The land plants and those protists formerly classified as the 
"green algae" or Chlorophyta are now collective referred to as the 
Viridaeplantae or Viridiplantae (the green plants).

It is the land plants (plants, sensu strictu) that I assume you are 
referring to.

Our knowledge of the early evolutionary history of plants is spotty. The 
land plants are thought to have evolved from certain members of the 
protist group formerly called the green algae or chlorophytes. The 
evidence for this assumption is fairly abundant and consists of 
biochemical and structural similarities of the chloroplasts; 
biochemical similarities in the storage and metabolism of 
carbohydrates; similarities in the formation of the embryo and its 
protective cells; similarities in the mechanism of cell division; and 
similar nucleotide (DNA and RNA) sequences, i.e. direct genetic 
similarities. That the land plants are related to the chlorophytes, 
especially to a certain group of the former Chlorophyta, the 
charophytes, is undeniable. Whether the land plants evolved from the 
charophytes is debatable, but currently not fruitfully so as no viable 
alternative hypothesis has been put forward.

A fairly good "family tree" for land plants may be seen here:
http://www.fmnh.helsinki.fi/users/haaramo/Plantae/Viridiplantae.htm

This is a fascinating site, by the way, and by all means enjoy it, but...  
take it with a grain of salt as it seems to be somewhat roguish in 
terms of its adherence to a strict scientific discipline. The family tree 
diagram (cladogram) shown there is taken from a reputable biology 
text.

Note that there are two main branches of the land plant tree; the 
bryophytes (the non-vascular plants). and the vascular plants. Both 
are believed to have evolved from the charophytes but there is too 
little fossil evidence to show how they relate to one another, if at all, 
other than the shared ancestry. 

Well, now we are getting to the point. Without going into laborious 
detail about reproductive biology, suffice it to say that these two land 
plant forms have quite different reproductive life cycles. Both 
alternate, however, between two generations, the sporophyte 
(spore-bearing stage) and the gametophyte (gamete-bearing stage).

The sporophytes are diploid (have cells with double sets of 
chromosomes in their nuclei) and the gametophytes are haploid 
(have cells with single sets of chromosomes in their nuclei). 
Sporophytes produce spores by meiotic cell divisions (cell divisions 
that reduce the diploid condition to haploid). Gametophytes produce 
gametes by mitotic cell divisions (cell divisions that replicate the 
chromosome complement of the nuclei). The gametes subsequently 
fuse to reestablish the diploid condition and thus found, through 
mitotic cell growth and development, the new sporophyte cell line in 
the form of the early plant embryo. Whew!

So much for the similarity between the reproductive life cycles of the 
vascular plants and the bryophytes. They differ remarkably.

In the bryophytes, the gametophyte dominates the life cycle. It is what 
you see when you look at a moss, for example. The sporophyte 
generation grows on the gametophyte generation in what could be 
termed a parasitic relationship if it weren't for the fact that they are the 
same species. It is a parasite only in the sense that a baby is 
parasitic on its mother. Sporophytes are visible as the little hairy, 
non-green things that are sometimes seen projecting out of the 
moss. See here:
 http://www.sbs.auckland.ac.nz/info/schools/nzplants/moss_sporophyte.htm

In the vascular plants, the sporophyte dominates the life cycle. It is 
what you see when you look and any plant, such as a tree, a vine, a 
grass, a flower, or a fern. In most vascular plants, the gametophyte is 
microscopic and grows (again, like a parasite) on the sporophyte 
body. It eventually becomes part of the seed in the flowering plants 
and gymnosperms (the seed plants). Take a look:
 http://io.uwinnipeg.ca/~simmons/ovule5.htm

In the ferns and other fern-like, non-seed plants, the gametophyte is 
tiny and free living (not dependent on the sporophyte) but is host to 
the sporophyte embryo that, while it is parasitic on the gametophyte 
at first, quickly grows to independence and (relatively) large size while 
the nurturing gametophyte withers beneath it. Here you go:


That's what a fern looks like when it's a baby (a sporeling). Here is a 
fern as a toddler:



Here is another sporeling (I can't get enough of these):



I think that last one is not a fern but a lycopod (club moss), perhaps 
Selaginella.

One thing that seems clear is that the charophyte ancestors of the 
land plants were dominated by the haploid (i.e., gametophyte) stage 
of the life cycle. Sporophytes, in the form of multicellular descendants 
of the zygote, do not exist in the extant representatives of this group. 
Consequently, both vascular and non-vascular plants have 
elaborated a sporophyte stage by mitotic intervention at the level of 
the zygote. 

So, now you have your answer. That's what is meant by the statement 
"that in the evolutionary history of plants, gametophyte and 
sporophyte have switched roles of host and parasite..." Aren't you 
glad you asked?