The chloroplast genome of Phalaenopsis aphrodite (Orchidaceae): comparative analysis of evolutionary rate with that of grasses and its phylogenetic implications.
Whether the Amborella/Amborella-Nymphaeales or the grass lineage diverged first within the angiosperms has recently been debated. Central to this issue has been focused on the artifacts that might result from sampling only grasses within the monocots. We therefore sequenced the entire chloroplast genome (cpDNA) of Phalaenopsis aphrodite, Taiwan moth orchid. The cpDNA is a circular molecule of 148,964 bp with a comparatively short single-copy region (11,543 bp) due to the unusual loss and truncation/scattered deletion of certain ndh subunits. An open reading frame, orf91, located in the complementary strand of the rrn23 was reported for the first time. A comparison of nucleotide substitutions between P. aphrodite and the grasses indicates that only the plastid expression genes have a strong positive correlation between nonsynonymous (Ka) and synonymous (Ks) substitutions per site, providing evidence for a generation time effect, mainly across these genes. Among the intron-containing protein-coding genes of the sampled monocots, the Ks of the genes are significantly correlated to transitional substitutions of their introns. We compiled a concatenated 61 protein-coding gene alignment for the available 20 cpDNAs of vascular plants and analyzed the data set using Bayesian inference, maximum parsimony, and neighbor-joining (NJ) methods. The analyses yielded robust support for the Amborella/Amborella-Nymphaeales-basal hypothesis and for the orchid and grasses together being a monophyletic group nested within the remaining angiosperms. However, the NJ analysis using Ka, the first two codon positions, or amino acid sequences, respectively, supports the monocots-basal hypothesis. We demonstrated that these conflicting angiosperm phylogenies are most probably linked to the transitional sites at all codon positions, especially at the third one where the strong base-composition bias and saturation effect take place.
Chang
Ching-Chun
CC
Institute of Biotechnology, National Cheng Kung University, Tainan, Taiwan.
Lin
Hsien-Chia
HC
Lin
I-Pin
IP
Chow
Teh-Yuan
TY
Chen
Hong-Hwa
HH
Chen
Wen-Huei
WH
Cheng
Chia-Hsiung
CH
Lin
Chung-Yen
CY
Liu
Shu-Mei
SM
Chang
Chien-Chang
CC
Chaw
Shu-Miaw
SM
D018742:DNA, Chloroplast / Q000235:genetics*; D018076:DNA, Complementary / Q000235:genetics; D018745:Genome, Plant / Q000235:genetics*; D029595:Orchidaceae / Q000235:genetics*; D010802:Phylogeny
D018742:DNA, Chloroplast; D018076:DNA, Complementary