Duplicate genes and the root of angiosperms, with an example using phytochrome sequences

Abstract

The root of the angiosperm tree has not yet been established. Major morphological and molecular differences between angiosperms and other seed plants have introduced ambiguities and possibly spurious results. Because it is unlikely that extant species more closely related to angiosperms will be discovered, and because relevant fossils will almost certainly not yield molecular data, the use of duplicate genes for rooting purposes may provide the best hope of a solution. Simultaneous analysis of the genes resulting from a gene duplication event along the branch subtending angiosperms would yield an unrooted network, wherein two congruent gene trees should be connected by a single branch. In these circumstances the best rooted species tree is the one that corresponds to the two gene trees when the network is rooted along the connecting branch. In general, this approach can be viewed as choosing among rooted species trees by minimizing hypothesized events such as gene duplication, gene loss, lineage sorting, and lateral transfer. Of those gene families that are potentially relevant to the angiosperm problem, phytochrome genes warrant special attention. Phylogenetic analysis of a sample of complete phytochrome (PHY) sequences implies that an initial duplication event preceded (or occurred early within) the radiation of seed plants and that each of the two resulting copies duplicated again. In one of these cases, leading to the PHYA and PHYC lineages, duplication appears to have occurred before the diversification of angiosperms. Duplicate gene trees are congruent in these broad analyses, but the sample of sequences is too limited to provide much insight into the rooting question. Preliminary analyses of partial PHYA and PHYC sequences from several presumably basal angiosperm lineages are promising, but more data are needed to critically evaluate the power of these genes to resolve the angiosperm radiation.