from the article “Molecular Evidence for the Early Evolution of Photosynthesis” (Science 289: 1724-1730, 2000) by Jin Xiong William M. Fischer, Kazuhito Inoue, Masaaki Nakahara, and Carl E. Bauer
The problem on the evolutionary path of photosynthesis arose because of a lack of photosynthesis gene sequence information in the entire photosynthetic domain. Previous studies have relied on the use of nonphotosynthetic genes that gave rise to conflicting analyses, hence, implying that the extrapolation of data is invalid. Also, only a single set of genes is used for the analyses, thus, failing to construct a phylogeny that comprises all five photosynthetic bacterial lineages.
In this study, Xiong et al. (2000) attempted to resolve the quandary by performing comprehensive gene sequencing and phylogenetic analyses. Green sulfur bacterium, Chlorobium tepidum, and green nonsulfur bacterium, Chloroflexus aurantiacus, were selected from which sequences of photosynthetic genes were obtained. Phylogenetic analysis was central to the method. The disparity on the results was minimized by employing neighbor-joining (NJ), and maximum likelihood (ML) methods and quite a lot of considerations critical to the study have been taken into account. They have conducted analyses on phylogenetic trees with and without chosen outgroups, and remarkably, showed no alteration of the ungroup topology. In the analyses of nucleotide sequences, they have exluded the third codon position, and focused only on the first and second codon positions so as to avoid compositional bias. Indeed, the study can be said to be very thorough and precise; attempted to and has actually successfully minimized errors, probably owing to the fact that the objective is essentially to conduct a detailed phylogenetic study in order to resolve the controversy in the earlier evolutionary path of photosynthesis; and has significantly arrived at sound conclusions.
They have confirmed that green sulfur and green nonsulfur bacteria are sister groups. Also, heliobacteria are closest to the last common ancestor. Remarkably, their study also revealed a contradicting result to the infamous Granick hypothesis which states the precedence of chlorophyll a on evolution over bacteriochlorophylls.
The plausibility of the results presented in this study is ascribed to the wide range of evidences that have spanned a number of biological disciplines – molecular biology, biochemistry, systematics, and evolution.
Interestingly, the selection of the organisms from which the sequences were obtained turned out to be not just merely because they were representatives of the two main photosynthetic lineages but that, ironically, only a few photosynthesis genes have been previously sequenced.
Commentary paper for my Bio150 class (1st sem, SY 2007-2008)
The problem on the evolutionary path of photosynthesis arose because of a lack of photosynthesis gene sequence information in the entire photosynthetic domain. Previous studies have relied on the use of nonphotosynthetic genes that gave rise to conflicting analyses, hence, implying that the extrapolation of data is invalid. Also, only a single set of genes is used for the analyses, thus, failing to construct a phylogeny that comprises all five photosynthetic bacterial lineages.
In this study, Xiong et al. (2000) attempted to resolve the quandary by performing comprehensive gene sequencing and phylogenetic analyses. Green sulfur bacterium, Chlorobium tepidum, and green nonsulfur bacterium, Chloroflexus aurantiacus, were selected from which sequences of photosynthetic genes were obtained. Phylogenetic analysis was central to the method. The disparity on the results was minimized by employing neighbor-joining (NJ), and maximum likelihood (ML) methods and quite a lot of considerations critical to the study have been taken into account. They have conducted analyses on phylogenetic trees with and without chosen outgroups, and remarkably, showed no alteration of the ungroup topology. In the analyses of nucleotide sequences, they have exluded the third codon position, and focused only on the first and second codon positions so as to avoid compositional bias. Indeed, the study can be said to be very thorough and precise; attempted to and has actually successfully minimized errors, probably owing to the fact that the objective is essentially to conduct a detailed phylogenetic study in order to resolve the controversy in the earlier evolutionary path of photosynthesis; and has significantly arrived at sound conclusions.
They have confirmed that green sulfur and green nonsulfur bacteria are sister groups. Also, heliobacteria are closest to the last common ancestor. Remarkably, their study also revealed a contradicting result to the infamous Granick hypothesis which states the precedence of chlorophyll a on evolution over bacteriochlorophylls.
The plausibility of the results presented in this study is ascribed to the wide range of evidences that have spanned a number of biological disciplines – molecular biology, biochemistry, systematics, and evolution.
Interestingly, the selection of the organisms from which the sequences were obtained turned out to be not just merely because they were representatives of the two main photosynthetic lineages but that, ironically, only a few photosynthesis genes have been previously sequenced.
Commentary paper for my Bio150 class (1st sem, SY 2007-2008)
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