Parasitic Flowering Plants: How Did They Evolve?

The origins of haustoria in parasitic flowering plants are presently unknown. It is commonly assumed that these penetrating organs are modified roots that evolved via the accumulation of selected mutations. This paper discusses a pathogen model for the origin of haustoria, suggesting that they were first produced in response to microbial parasitism and subsequently modified for the acquisition of water and nutrients from autotrophic hosts. The model is based on circumstantial data generated on the premise that autotrophic populations may have required new genetic potential and a strong directional pressure to evolve toward heterotrophy. Although controversial, the predictions of the pathogen model are clear and testable and provide a research-generating alternative to the nontestable classical viewpoint.