brown or green algal origins of Metaphyta?

[Curtis Clark]

At 08:56 AM 8/19/99 -0700, Ken Kinman wrote:
> However, this does not deter me from exploring other possible
>scenarios, such as Metaphyta arising holophyletically (strictly
>monophyletically) from a brown alga that captured a green chloroplast and
>lost the heterokont-type chloroplast in the transition to land.
>Evolutionary history can often be quite "messy", so I try not to constrain
>myself too much.

Ah, so we're back to "just-so stories". Did it at the same time capture the
nuclear genes of a chlorophyte?

>      Any evidence that deals with chloroplasts would not be considered
>"strong" evidence in my view.  Loss and acquisition of choroplasts has
>probably occurred many times (including the capture of the heterokont
>chloroplast from other algae).

Chloroplasts have a phylogeny. Their hosts have a phylogeny. When the
phylogenies match, it makes the most sense that the chloroplasts were at
that point symbionts. When the phylogenies don't match, it makes the most
sense that the chloroplasts were free-living, or else were laterally
captured.

I, too, am of the opinion that chloroplasts were acquired in separate
lineages. But some of the speakers at the IBC suggested that chloroplasts
are monophyletic: some recent gene trees suggest that the chlorophyte/land
plant clade is sister to the Rhodophyta, and those are the only groups that
seem to have primary chloroplasts (that come from free-living prokaryotes
rather than captured from other eukaryotes). I have strong doubts about
this, but it does show that the issue is subject to methodology (the search
for congruent phylogenies).

> The ability of such organelles to exchange
>genes with the nucleus of the host can further confound genetic evidence.

Yes and no. It has been a while, for example, since anyone has suggested
that the nuclear rbcS of land plants and the plastid rbcS of some green
algae are not homologues.

>    Perhaps the best candidate for "strong" evidence you presented is the
>composition of the cell walls, although I would have to explore how likely
>such differences could be due to the transition to land.

I'd more want to know how they formed. Both freshwater and marine
chlorophytes have similar cell walls to terrestrial land plants, and both
are different from marine phaeophytes. Hmmm.

>      The loss of the tinsel flagellum is simple enough, so that evidence
>seems weak.

Diatoms AFAIK lost it, too, but they didn't gain a whiplash in its place.
The multilamellar structure of the basal body that charophytes share with
land plants is a rather unique structure.

>  Cell-plate formation and the phragmoplast might be stronger
>evidence, but Miklausen (Brown Algal Origins of Land Plants, page 67)
>claims
>that cell-plate formation in the brown algae closely resembles that in
>higher plants.

Could be. I'd want to see the electron micrographs.

>    Although I am not a botanist, what I have gotten from Miklausen's book
>so far is that the similarities between brown algae and land plants (some
>of
>which you listed below) are perhaps more significant than we realize, and
>we
>need to take a strong second look at the origins of Metaphyta.

Second and perhaps third looks were taken decades ago. That doesn't mean we
shouldn't look again, but we shouldn't be surprised if we don't find
anything.

  The fact
>that these similarities do not occur in all brown algae isn't surprising
or
>particularly relevant in my view (just that most of them should occur
>together in some brown algae).

More important is that the similarities don't occur in all land plants.
Does Miklausen suggest that liverworts and mosses are "degenerate vascular
plants"?

  I grant you that Metaphyte chloroplasts
>probably had green algal origins, but I am still open to the possibility
>that the host plant might be something else.  If there is strong evidence
>that is congruent with chloroplast evidence,

Nuclear gene phylogenies, the multilamellar basal body, other things that I
have heard but don't have at hand or mind.

> Meanwhile, I look forward to other
>botanists expressing their views (and other lines of evidence).

Me, too. Are there some real phycologists out there?

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Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
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