Molecular Biology and Evolution, Vol 13, 233-243, Copyright © 1996 by Society for Molecular Biology and Evolution
E Boudreau and M Turmel
We have constructed a physical and gene map for the chloroplast DNA (cpDNA)
of the unicellular green alga Chlamydomonas gelatinosa, a close relative of
Chlamydomonas reinhardtii. At 285 kb, the C. gelatinosa cpDNA is 89 kb
larger than its C. reinhardtii counterpart. The alterations in the order of
77 genes on the cpDNAs of these green algae are attributable to nine
inversions and one event of expansion/contraction of the inverted repeat.
These rearrangements are much more extensive than those previously reported
between the cpDNAs of the closely related Chlamydomonas moewusii and
Chlamydomonas pitschmannii. Because the divergence level of the C.
gelatinosa and C. reinhardtii chloroplast-encoded large subunit rRNA gene
sequences is equivalent to that of the corresponding C. moewusii and C.
pitschmannii sequences, our results may suggest that, in the same period of
time, there have been more numerous rearrangements in the lineage
comprising C. gelatinosa and C. reinhardtii than in the lineage comprising
C. moewusii and C. pitschmannii. Alternatively, given that substitution
rates in chloroplast genes are not necessarily uniform across lineages, the
extensive rearrangements between the C. gelatinosa and C. reinhardtii
cpDNAs may reflect a longer divergence period for this pair of
Chlamydomonas species compared to that for the C. moewusii/C. pitschmannii
pair. We have also found that, like its C. reinhardtii homologue but unlike
its C. moewusii and C. pitschmannii counterparts, the C. gelatinosa cpDNA
features a large number of dispersed repeated sequences that are readily
detectable by Southern blot hybridization with homologous fragment probes.
Assuming that the two pairs of closely related Chlamydomonas species
diverged at about the same time, these data suggest that the susceptibility
of Chlamydomonas cpDNAs to rearrangements is correlated with the abundance
of repeated sequences. Preliminary characterization of a 345-bp C.
gelatinosa cpDNA region containing a repeated sequence by both DNA
sequencing and Southern blot analysis has revealed no sequence homology
between this region and the cpDNAs of C. reinhardtii and other
Chlamydomonas species.
ORIGINAL ARTICLE
Extensive gene rearrangements in the chloroplast DNAs of Chlamydomonas species featuring multiple dispersed repeats
Program in Evolutionary Biology, Canadian Institute for Advanced Research, Departement de biochimie, Faculte des sciences et de genie, Universite Laval.
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