Phylogeny&Evolution in Holoparasitic Plants: Orobanche

Research project P 14352 Phylogeny & Evolution in Holoparasitic Plants: Orobanche Manfred A. FISCHER 06.03.2000 Few angiosperms have pursued a parasitic life-style, and even fewer have become holoparasitic (i.e., completely nutritionally dependent on a host plant). In past decades, interest in this biologically fascinating and informative group of organisms has strongly increased, particularly because some parasites are pests that can cause severe crop damage, yielding losses up to 100%. Despite the economic importance of parasitic plants, little is known about general evolutionary aspects and interrelations among taxa with different life-strategies. Orobanche sect. Trionychon, a well-circumscribed taxonomic section within holoparasitic Orobanchaceae ("the broom rapes"), contains a variety of different life-strategies that appear to reflect different evolutionary pathways. For example, species in the group include taxa with both narrow vs. wide host ranges, those growing on short-lived as well as long-lived (perennial) host plants, and those inhabiting natural vs. man-disturbed habitats. Because of this diversity within section Trionychon, it can serve as a good model for revealing evolutionary patterns within holoparasitic plants in general. As a first step in the proposed research, a reliable phylogenetically based taxonomic system within Orobanche sect. Trionychon must be established to provide the foundation for studies on evolutionary aspects. Different morphological, karyological (chromosome numbers, C-values) and molecular markers (DNA-sequencing; AFLPs within very closely related taxa) will be utilized. Combination of such different methods will permit accurate assessment of relationships among taxa (particularly challenging in the closely related Orobanche ramosa-group) as well as with representative outgroups. As a second step, evolutionary patterns within Orobanche sect. Trionychon will be investigated. This is of special interest, because the section comprises species with contrasting life-strategies. Based on a more predictive taxonomy, it will be possible to establish hypotheses about interrelations between taxa in aspects such as the origin of weedy species, closest non-weedy relatives of weedy taxa, and hence also about possible preadaptations for weediness. It will also be possible to reveal biogeographic characteristics of Orobanche sect. Trionychon, such as centers of origin, endemism, and patterns of migration. The proposed research will contribute not only to the knowledge of a group of specific parasitic taxa, which is the source of some of the most economically important weedy pests, but also to an understanding of general patterns and strategies within holoparasitic plants as a whole.

In the course of this project for the first time a hypothesis on the phylogenetic relationships within the holoparasitic genus Orobanche (broomrape) has been established. It is based on molecular, karyological, cytological and morphological data, most of which have been obtained within this project. This hypothesis has fundamental consequences for the taxonomic treatment of this group. The genus Orobanche in its most widely used circumscription is not monophyletic, but falls into two clearly separated phylogenetic lineages, which themselves can be further divided into smaller subgroups, each well defined molecularly, karologically, cytologically, and morphologically. Using previous taxonomic concepts, Orobanche will be split into several smaller genera, e.g., the two groups restricted to the New World will be treated as separate genera in the forthcoming treatment of the Flora of North America. Apart from the taxonomic consequences, the phylogenetic hypothesis can be used to investigate evolutionary trends in this biologically interesting group. For instance, members of one of the two main groups appear to have similar genome sizes, while those of the other group exhibit more pronounced differences, indicating differences in the evolution of genome size in these phylogenetic groups. Further studies will show, if these can be at least partly attributed to different activities of mobile genetic elements like retrotransposons. Most members of the genus Orobanche have narrow host ranges and are (at least potentially) perennial. Based on the phylogenetic relationships, this is the ancestral condition in Orobanche. There are, however, a few species growing on many different hosts among those also short-lived ones, which must be (at least facultatively) annuals themselves. The latter include those Orobanche species, which occur in agricultural systems and can cause severe crop losses especially in Mediterranean areas. Analysis of the two important life traits host range and life history in a phylogenetic context reveals that these characters evolve in a correlated fashion, suggesting a causal connection between these two traits, which, however, is not yet understood. A further interesting outcome of this study is the clear evidence for horizontal gene transfer between members of those subgroups of the Orobanche- and the Phelipanche-group, which also include the economically important taxa. If horizontal gene transfer is corroborated, this would have strong impact on any control measures for these species.