brown or green algal origins of Metaphyta?

[Curtis Clark]

At 03:32 PM 8/18/99 -0700, Ken Kinman wrote:
>    I would be most interested in anyone's ideas on what are the strongest
>lines of evidence that refute brown algal origins of metaphytes (and
>support
>the traditional green algal origins).

I'd be interested in published summaries as well, because my information
comes from a variety of sources ranging from basic botany texts to papers
given week before last at the International Botanical Congress.

Land plants share with the charophyte green algae the form of the
chloroplast, the form of the flagellar basal body, and phragmoplast cell
division, along with a number of other traits I have forgotten.
Phylogenetic analyses, both parsimony and I believe maximum likelihood, of
a number of nuclear as well as chloroplast genes support this relationship.

The charophytes+land plants have the same sort of chloroplasts as other
green algae. All their flagella are whiplash. They all have cell walls of
cellulose plus hemicellulose and in some cases pectin. Again, phylogenetic
analyses involving a number of genes support this.

Brown algae have a chloroplast with different pigments, a different
arrangement of thylakoids, and features that suggest that it was stolen
from another eukaryote (although probably prior to the evolution of the
brown algae) rather than acquired directly as a prokaryotic symbiont. The
chloroplast is shared with diatoms, xanthophytes, and some dinoflagellates.
Brown algae lack phragmoplast division. Their cells walls have alginic
acids instead of hemicelluloses and pectins. One of their two flagella is
tinsel, a feature that they share with xanthophytes and oomycota.
Phylogenetic analysis allies them with xanthophytes, diatoms, and oomycota
in a group called the heterokonts or stramenopiles.

Brown algae are AFAIK entirely marine, whereas with the exception of some
flowering plants the charophyte+land plant clade is largely or entirely
associated with fresh water.

IMO the only similarities between brown algae and land plants are
parenchymatous growth (which is not present in all brown algae), apical
meristems (again not in all brown algae), "leaf"/"stem" organization
(again, not in all brown algae), and phloem (present in some brown algae,
and strikingly similar to the phloem of flowering plants, but different
from pteridophyte or conifer phloem, the leptoids of mosses, or the absence
of conductive tissue in liverworts).

I think the phloem of brown algae and flowering plants constitutes the most
remarkable case of convergent evolution in multicellular organisms. But it
is just that--convergence.


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