brown or green algal origins of Metaphyta?

[Ken Kinman]

Curtis and other Taxacomers,
      Although botany is my weak point (as opposed to bacteriology and
zoology), I would like to take a shot at playing "devil's advocate" here (so
to speak).  I personally think Miklausen's view that various metaphytes
arose "polyphyletically" from brown algae is rather doubtful (although
possible).  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.
      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).  The ability of such organelles to exchange
genes with the nucleus of the host can further confound genetic evidence.
    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.
      The loss of the tinsel flagellum is simple enough, so that evidence
seems weak.  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.
    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.  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).  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, that would strengthen the
traditional view in my mind, and that is why I asked for "strong" evidence
in my query.
     I will have to look more closely at the evidence of cell wall
composition, which looks like it "could be" strong evidence for the
traditional view (but maybe not).  Meanwhile, I look forward to other
botanists expressing their views (and other lines of evidence).
                       -------Ken Kinman

>From: Curtis Clark <jcclark at CSUPOMONA.EDU>
>Reply-To: Curtis Clark <jcclark at CSUPOMONA.EDU>
>To: TAXACOM at USOBI.ORG
>Subject: Re: brown or green algal origins of Metaphyta?
>Date: Wed, 18 Aug 1999 20:46:54 -0700
>
>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.
>
>
>----------------------------------------------------------------
>Curtis Clark                  http://www.csupomona.edu/~jcclark/
>Biological Sciences Department             Voice: (909) 869-4062
>California State Polytechnic University      FAX: (909) 869-4078
>Pomona CA 91768-4032  USA                  jcclark at csupomona.edu


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