THE EVOLUTION OFPARASITISM INSCROPHULARIACEAE/OROBANCHACEAE: PLASTIDGENE SEQUENCES REFUTEAN EVOLUTIONARYTRANSITION SERIES'Nelson D. Young,2 Kim E. Steiner," andClaude W. dePamphilis1ABSTRACT Parasitic plants in Scrophulariaceae and Orobanchaceae have been traditionally depicted as forming a linear evo-lutionary series beginning with hemiparasitism and ending with holoparasitism. The genera Lathraea, Harveya, andHyobanuhe have been viewed as transitional links between the parasitic members of Scrophulariaceae and the strictlyholoparasitic habit of the traditional Orobanchaceae. Phylogenetic analyses of plastid rps2 and matK sequences wereperformed. The transitional genera are not transitional to the traditional Orobanchaceae, but represent multiple inde-pendent origins of holoparasitism. Within Scrophulariaceae, the two traditional subfamilies Rhinanthoideae and Antir-rhinoideae are defined by the arrangement of the corolla lobes during aestivation. However, neither of the two subfam-ilies is monophyletic in our analyses, suggesting that corolla lobe position is a homoplastic character. While thetraditional Orobanchaceae are monophyletic, tribes Buchnereae and Rhinantheae are clearly not, and genus Orobancheprobably is not. Clades of parasitic genera correspond well with biogeographic provinces. One strongly supported cladecontains the parasitic Scrophulariaceae, the traditional Orobanchaceae, and Lindenbergia. It is proposed that this cladebe defined as the Orobanchaceae. Parasitic angiosperms are found in 16 familiesand live in diverse habitats, ranging from tropicalforests to arctic islands (Musselman & Press,1995). Recent work on several parasitic groups hasexplored their anatomy, physiology, ecology, andmolecular biology and the control of economicallysignificant parasites (Press & Graves, 1995). Par-asites exhibit dramatic adaptations. Some lackleaves, stems, roots, and the ability to photosynthe-size. For many lineages of non-photosynthetic par-asites, study of their evolution is impeded by thevery fact that they are so specialized and so differ-ent from their nearest photosynthetic relatives. Inmany cases the identity of those relatives remainsunknown (Nickrent & Duff, 1996; Nickrent et al.,1998). The evolution of parasitism can best bestudied in a group where near relatives are known,and different degrees of parasitic specialization areencountered. The group with the greatest range inparasitic specialization is found within the Scro-phulariaceae/Orobanchaceae. This family pair in-cludes nonparasites, both facultative and obligatehemiparasites (photosynthetic plants that obtainwater, nutrients, or photosynthates from their host),and holoparasites (non-photosynthetic, obtaining ailphotosynthates from their host). Much has beenlearned about the anatomy and physiology of par-asitism in this group, especially in Striga Loureiroand Orobanche L., which are important pests ofcrop plants (Press & Graves, 1995). Reliable in-formation on phylogenetic relationships among thevarious nonparasites, hemiparasites, and holopar-asites would enable us to better understand the an-atomical, physiological, and genetic changes thatoccur during the evolution of parasitism and theloss of photosynthetic ability. I We thank J. Alison. the Missouri Botanical Garden. IL. Musselman, J. Hod�r, R. Olmstead, K. Kirkman, M. Weth-erwax, R. Wyatt, and J. Armstrong for help in obtaining some of the plants or DNAs used in this study. T. Atkinsonand W. Elisens for help with the sequencing, A. G. Moore for help on nomenclature, E. Kellogg, A. Wolfe. T. Barkman,and B. Smocovitis for helpful discussions of the text, and David Swofford for making PAUP. 4.0 available for betatesting. This research was supported by NSF grants DEB-91-20258 and DBI 96-04814 to CWD. 2 Department of Biology, Vanderbilt University, Nashville, Tennessee 37235, U.S.A., and Department of Biology,Trinity University, San Antonio, Texas 78212, U.S.A. (current address). SCompton Herbarium, National Botanical Institute. Kirstenbosch, Claremont 7735, Republic of South Africa. 4 Department of Biology, Vanderbilt University, Nashville, Tennessee 37235, U.S.A., and Department of Biology. ThePennsylvania State University, University Park. Pennsylvania 16802, U.S.A. (current address).ANN. MISSouRI BOT. GARD. 86: 876-893. 1999.
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