Molecular Evolution Research in the Wolfe Lab
Castilleja spp.Hyobanche rubra Sarcodes sanguinea
Background information on holoparasitic plants
Parasitic plants lacking chlorophyll (holoparasites) do not have a functional photosynthetic apparatus and are entirely dependent on their host for reduced carbon. Because holoparasites are not self-reliant for photosynthates, the nuclear and plastid genomes are under relaxed functional constraint for photosynthesis. One potential result of reduced selection for photosynthesis is that the genes involved in photosynthetic pigment biosynthetic pathways, light harvesting complexes, photoelectron transport chains, the manufacture of energy molecules, and carbon dioxide fixation may become nonfunctional.
The plastid genomes of several holoparasitic higher plant genera have recently been examined for genome size and/or gene content (e.g., Cuscuta (Cuscutaceae); Conopholis (Orobanchaceae); Orobanche (Orobanchaceae)). In general, the size of the plastid genome is greatly reduced in holoparasitic plants compared to their closest autotrophic relatives and many of the bioenergetic genes are deleted, sufficiently altered to be classified as pseudogenes or are presumably nonfunctional. Similarly, the heterotrophic euglenoid, Astasia longa, has a plastid genome half the size of its closest photosynthetic relative, Euglena gracilis, and most of the genes for the photosynthetic apparatus are absent.
Although there are at least seven independent lineages of nonphotosynthetic parasitic plants, little is known about the molecular evolution of genes coding for the photosynthetic apparatus. Two angiosperm plant families have been the main focus of recent research - Orobanchaceae and Cuscutaceae. For example, the entire plastid genome of Epifagus has been sequenced. Sequences for the plastid gene expression apparatus have also been examined in great detail for Conopholis and for some photosynthetic, chlororespiratory, and gene expression apparatus genes in Cuscuta. In contrast, the only plastid DNA sequences deposited in GenBank for Lathraea (Scrophulariaceae) and Orobanche are for genes involved with plastid gene expression and/or translation. In Epifagus, all photosynthetic and purported chlororespiratory (ndh) genes are either deleted from the plastid genome or are pseudogenes. In Cuscuta reflexa, the ndh genes are absent, but most of the photosynthetic genes are present and expressed in the plastid at reduced levels. Clearly, there is a serious lack of information available about photosynthetic genes in natural photosynthetic mutants. Furthermore, no research has been conducted on nuclear-encoded photosynthetic genes or biosynthetic pathways of photosynthetic pigments in parasitic plants.
Research in progress using holoparasitic plants as model organisms
The main focus in the lab has been to elucidate the molecular changes in the plastid genome associated with the loss of photosynthesis. To that end, several collaborative research projects have been completed with Claude dePamphilis while I was a postdoc at Vanderbilt University. The major project was to examine the changes in the RuBisCo large subunit encoded by the plastid gene rbcL. We used rbcL to construct a phylogeny of the nonparasitic and parasitic Scrophulariales and have surveyed the predicted changes in the protein using several methods (Nickrent et al. 1998; Wolfe and dePamphilis 1998). The major conclusions from this research was that: 1) parasitism has arisen once in the Scrophulariales; 2) loss of photosynthetic ability has occurred independently many times in Scrophulariales; 3) open reading frames for rbcL are maintained after the loss of photosynthesis in most lineages of holoparasitic Scrophulariales; and 4) rbcL pseudogene formation also occurred independently in holoparasitic Scrophulariales.
Projects currently in progress in the lab include population-level surveys of the plastid genome and photosynthetic genes in Boschniakia and Orobanche (Orobanchaceae), and Hyobanche (Scrophulariaceae). Check this site periodically for updates on these projects.
For additional information on parasitic plants, follow the link to the
Parasitic plant connection.
Background information on mycoheterotrophic plants
Mycoheterotrophic plants (MHPs) are nonphotosynthetic parasites using fungal intermediaries to withdraw nutrients from other plants. MHPs are found in ca ten lineages of flowering plants, primarily in monocot groups. Mycoheterotrophic plants (MHPs) of Ericaceae (dicots) include two independent lineages that have been transferred back and forth between Pyrolaceae and Monotropaceae. Unlike holoparasitic plants (e.g., Orobanchaceae), there is no direct contact of the parasite with its host plant. Instead, MHPs are attached to hosts indirectly through fungal intermediates and are considered to be epiparasitic. All taxa formerly included in Monotropaceae are nonphotosynthetic, whereas almost all species in the former Pyrolaceae are photosynthetic. Exceptional taxa of this latter group are leafless varieties and/or species of Pyrola (e.g., P. aphylla).
Recent phylogenetic reconstructions based on morphology and nuclear ribosomal DNA (nrDNA) sequences have shown that Pyrolaceae and Monotropaceae are part of Ericaceae. Results from morphological and 18S nrDNA analyses depict all MHPs as a monophyletic group, whereas 28S nrDNA results suggest three independent origins of mycoheterotrophy in Ericaceae.
Research in progress using mycoheterotrophic plants as model organisms
The major project in the lab is an investigation of parasitic plants of the Ericaceae (Pyrola, Monotropa, Allotropa, Pterospora, Sarcodes, Hypopitys, Hemitomes, Pityopus, Monotropsis, and Pleuricospora). Pyrola includes some nonphotosynthetic members, whereas all of the other genera traditionally circumscribed by the Monotropaceae are nonphotosynthetic. Included in this investigation is a phylogenetic reconstruction of the species of Pyrola and a survey of the plastid and nuclear-encoded photosynthetic genes present or absent in each genus.
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Last updated September 25, 1998.