Phylogeny and divergence times in Pinaceae: evidence from three genomes

Abstract

In Pinaceae, the chloroplast, mitochondrial, and nuclear genomes are paternally, maternally, and biparentally inherited, respectively. Examining congruence and incongruence of gene phylogenies among the three genomes should provide insights into phylogenetic relationships within the family. Here we studied intergeneric relationships of Pinaceae using sequences of the chloroplast matK gene, the mitochondrial nad5 gene, and the low-copy nuclear gene 4CL. The 4CL gene may exist as a single copy in some species of Pinaceae, but it constitutes a small gene family with two or three members in others. Duplication and deletion of the 4CL gene occurred at a tempo such that paralogous loci are maintained within but not between genera. Exons of the 4CL gene have diverged approximately twice as fast as the matK gene and five times more rapidly than the nad5 gene. The partition-homogeneity test indicates that the three data sets are homogeneous. A combined analysis of the three gene sequences generated a well-resolved and strongly supported phylogeny. The combined phylogeny, which is topologically congruent with the three individual gene trees based on the Templeton test, is likely to represent the organismal phylogeny of Pinaceae. This phylogeny agrees to a certain extent with previous phylogenetic hypotheses based on morphological, anatomical, and immunological data. Disagreement between the previous hypotheses and the three-genome phylogeny suggests that morphology of both vegetative and reproductive organs has undergone convergent evolution within the pine family. The strongly supported monophyly of Nothotsuga longibracteata, Tsuga mertensiana, and Tsuga canadensis on all three gene phylogenies provides evidence against previous hypotheses of intergeneric hybrid origins of N. longibracteata and T. mertensiana. Divergence times of the genera were estimated based on sequence divergence of the matK gene, and they correspond well with the fossil record.