Differences in pyrenoid morphology are correlated with differences in the rbcL genes of members of the Chloromonas lineage (volvocales,chlorophyceae) |
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Authors: | Nozaki Hisayoshi Onishi Keisuke Morita Eiko |
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Institution: | (1) Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, JP;(2) National Institute of Infectious Diseases, Toyama 1-23-1, Shinjuku-ku, Tokyo 162-8640, Japan, JP |
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Abstract: | Chloromonas is distinguished from Chlamydomonas primarily by the absence of pyrenoids, which are structures that are present
in the chloroplasts of most algae and are composed primarily of the CO2-fixing enzyme Rubisco. In this study we compared sequences
of the rbcL (Rubisco large subunit-encoding) genes of pyrenoid-less Chloromonas species with those of closely related pyrenoid-containing
Chlamydomonas species in the "Chloromonas lineage" and with those of 45 other green algae. We found that the proteins encoded
by the rbcL genes had a much higher level of amino acid substitution in members of the Chloromonas lineage than they did in
other algae. This kind of elevated substitution rate was not observed, however, in the deduced proteins encoded by two other
chloroplast genes that we analyzed: atpB and psaB. The rates of synonymous and nonsynonymous nucleotide substitutions in the
rbcL genes indicate that the rapid evolution of these genes in members of the Chloromonas lineage is not due to relaxed selection
(as it preasumably is in parasitic land plants). A phylogenetic tree based on rbcL nucleotide sequences nested two Chlamydomonas
species as a "pyrenoid-regained" clade within a monophyletic Chloromonas "pyrenoid-lost" clade. Character-state optimization
with this tree suggested that the loss and the regain of pyrenoids were accompanied by eight synapomorphic amino acid replacements
in the Rubisco large subunit, four of which are positioned in the region involved in its dimerization. However, both the atpB
and the psaB sequence data gave robust support for a rather different set of phylogenetic relationships in which neither the
"pyrenoid-lost" nor the "pyrenoid-regained" clade was resolved. The appearance of such clades in the rbcL-based tree may be
an artifact of convergent evolutionary changes that have occurred in a region of the large subunit that determines whether
Rubisco molecules will aggregate to form a visible pyrenoid. |
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