Nuclear Phylogeny :
Plants (metaphytes) are the only multicellular eukaryotes for which a
specific evolutionary link with a unicellular group has been established,
i.e., with the green algae (Chlorophyta). This phylogenetic relationship
is evident at the level of the nuclear genome (and, to a lesser extent,
at the level of the plastid genome).
Huss VAR & Sogin ML (1990) analyzed the phylogenetic position of some
Chlorella species within the Chlorococcales, basing their analysis on
complete small subunit rRNA sequences.
In their tree, Chlorella protothecoides is the closest relative to
Prototheca wickerhamii (J Mol Evol 31:432-442; see Fig. 2).
Mitochondrial Phylogeny :
At the outset of this study, only one green algal mtDNA (that of Chlamydomonas reinhardtii,
a member of the Chlorophyceae, class Volvocales) had been characterized
(Gray & Boer, 1988).
The C. reinhardtii mitochondrial genome is strikingly unlike
that of land plants.
In fact, there is little to suggest
that C. reinhardtii and land plants shared a common mitochondrial
ancestor as recently as they shared a common nuclear or plastid one.
Thus, it was (and is) of interest to determine the nature and range of structural
and functional variation in mtDNA within the green algae, to ascertain
whether it is possible to make an evolutionary connection with plant
mtDNA (and, if we cannot, to understand what that means).
The chlorophyte Prototheca wickerhamii (a colorless Chlorella, also
a member of the Chlorophyceae but a different class, Chlorococcales)
has been studied recently by U. Kück's group. Distance matrix
analysis based on mitochondrial small subunit rRNA sequences indicates that Prototheca wickerhamii is more closely related
to plants than is Chlamydomonas reinhardtii
(Wolff & Kück, 1990).
Therefore, it appeared that P. wickerhamii would be a good candidate for complete mtDNA sequencing, and that such information could provide insight into the evolutionary linkages we are seeking between chlorophyte
and metaphyte mitochondria.
Phylogenetic analysis based on the cox1 gene does indeed show that
P. wickerhamii branches at the base of the land plant lineage and thus
constitutes the earliest diverging taxon of the green clade that has been studied thus far at the level of mtDNA
(Wolff et al. 1993)
mtDNAs (FMGP)* | Other
complete mtDNAs | Gene
names | Mitochondrial Genomics ]