Evolution in parasitic plants

Chris Thorogood

Parasitic plants are an ecologically and economically important group. Approximately 4,000 (~ 1 %) species of flowering plants are parasitic and rely on other plants for nutrient acquisition, yet our understanding of the evolution of parasitism in plants is rudimentary. The evolutionary transition to parasitism has been associated with extreme reductions in morphology, for example many parasitic plants lack all trace of chlorophyll, leaves, stems and roots, and have a fungus-like appearance. Until recent advances in molecular techniques such as DNA sequencing, these reductions in morphology deprived scientists of useful features for classifying this curious group of plants and identifying their closest photosynthetic ancestors. For example Rafflesia, which produces the largest flower on earth, has recently been identified to have evolved from the tiny-flowered Euphorbia family!
Parasitic plants show considerable variation in host specificity, for example, dodders (Cuscuta spp.) can potentially infect plants from hundreds of taxonomically diverse families, while Beechdrops (Epifagus virginiana) in North America parasitizes a single species of beech tree. Intensive research into plant-feeding insects has shown that “shifts” from one host plant to another can lead to speciation – the formation of new species. However, little has been done to determine whether host shifts may act as a driver of speciation parasitic plants, which are a poorly understood group of angiosperms (flowering plants). Our research over the past three years has focussed on the hypothesis that populations of the parasitic plant broomrape (Orobanche) that preferentially parasitize different host plants, may be in the process of forming new species.
Previous work we have carried out has shown that populations of common broomrape (Orobanche minor) growing on either Red Clovers (Trifolium pratense) or Sea Carrots (Daucus carota ssp. gummifer) are genetically distinct. The different ecologies of clovers and carrots (calcareous grassland and south-facing sea cliffs respectively) may have prevented the movement of gene flow between these populations, and effectively, isolated them. In a further study, we sampled plants of O. minor from numerous populations from around Europe, and sequenced variable DNA regions of to look for wide-scale patterns of “host-driven genetic divergence” – i.e. changes in DNA sequence associated with parasitism of particular hosts. These results gave us interesting insights into the evolution of populations of O. minor. It appears that continental populations have been repeatedly introduced in Great Britain, and that these alien plants have hybridised with our native populations which parasitize Sea Carrots in southern England. This mixing of foreign and native populations may explain the troubled taxonomic history of O. minor in Great Britain, where populations have frequently been incorrectly identified (for example Sitwell, 1984 Shell Guides to Britain’s Threatened Wildlife describes a photograph of O. minor as O. picridis). Ultimately our goal is to identify how host specificity may drive speciation in Orobanche, which may be an underestimated driving force in the evolution of parasitic plants as a whole.

For a summary of the key features distinguishing the various varieties of O. minor in Great Britain, consult the following website:

www.bsbi.org.uk/html/orobanche_minor.html

Other references:

Rumsey FJ, Jury SL (1991) An account of Orobanche L. in Britain
and Ireland. Watsonia, 18, 257–295.

Rumsey FJ (1994) Orobanche minor var. maritima. In: Scarce Plants in Britain (eds Stewart A, Pearman DA, Preston CD). Joint Nature Conservation Committee., Peterborough, UK.

Rumsey FJ (2007) A reconsideration of Orobanche maritima Pugsley (Orobanchaceae) and related taxa in southern England and the Channel Islands. Watsonia, 26, 473–476.

Thorogood CJ, Rumsey FJ, Harris SA, Hiscock, SJ (2008) Host-driven divergence in the parasitic plant Orobanche minor sm. (Orobanchaceae). Molecular Ecology, 17, 4289–4303.