Phylogenetic utility of the glycerol-3-phosphate acyltransferase gene: evolution and implications in Paeonia...

The nuclear-encoded chloroplast-expressed glycerol-3-phosphate acyltransferase (GPAT) gene has been found to be single-copy in a number of angiosperm families. In this study we investigated the phylogenetic utility of the GPAT gene at the interspecific level using the genus Paeonia (Paeoniaceae) as an example. An approximately 2.3- to 2.6-kb fragment of the GPAT gene, containing a large intron of more than 2 kb, was amplified, cloned, and sequenced from 19 accessions representing 13 Paeonia species. The GPAT gene phylogeny inferred by parsimony analysis supported interspecific relationships that were previously unresolved, suggesting that large introns of low-copy nuclear genes are particularly informative in the resolution of close relationships at low taxonomic levels. Whereas the GPAT phylogeny is largely congruent with the previous phylogenetic hypothesis of Paeonia, it shows a significant discordance involving the paraphyly of section Paeonia. Given evidence of an ancient duplication and the subsequent silencing of one GPAT locus in P. anomala, this discordance is most likely the result of paralogy. Two distinct genomic clones containing partial GPAT genes were isolated from P. anomala. The GPAT sequence from one clone corresponded to the functional copy of the gene, and the second genomic clone was determined to contain a GPAT pseudogene. The insertion of a retrotransposon in an intron of this pseudogene may have been responsible for the silencing of this GPAT locus in P. anomala. This study suggests that, although it is unlikely that universal nuclear gene markers free from paralogy are usually available, low-copy nuclear genes can be very useful in plant phylogenetic reconstruction, especially at low taxonomic levels, as long as the evolutionary dynamics of the genes are carefully examined.