Oomycete Evolution

Background: Oomycetes (in German Eipilze or Algenpilze) have meanwhile been recognized as a special group of algae, the heterokonts. The unicellular form of these organisms harbours two flagella, one is large and is used to drive the cell, the other is short and serves for steering. Whereas the Brown Algae and the Diatoms follow a free-living, autotrophic lifestyle, the Oomycete ancestors had lost their plastids and first lived as saprophytes (feeding on dead material). From there, the step to eating dying material is not far and later several groups developed necrotrophy, actively killing the host cell and then feeding on it. In the final phase of Oomycete evolution, biotrophy was invented several times independently. The effectors, necessary to modulate host immunity, are organized in specific regions of the genome, where repetitive sequences accumulate. Meanwhile, it has been found that the effectors were acquired from other parasititc organisms (fungi, bacteria) by horizontal gene flow. During the course of this specific evolution, the host range changes, of course, because effectors have to be specific. Some oomycetes are now specialized for only certain families, some even are bound to individual host species.

Task: Due to the economic impact of this group of plant pathogens, molecular analysis has been intense. For several organisms of this group, entire genomes have been sequenced and assembled, which now allows to understand the molecular mechanisms behind the evolutionary change. A timeline model for Oomycete evolution has been constructed (Matari et al. 2014), which allows to compare Oomycete evolution with evolution of the host plants and to give dates for the specific events driving this coevolution. Do a bibliographic search for the 11 Oomycete species given in that paper and find out their host range. Then construct a tree for these host species and compare the two trees. How do you interpret the result?

Advice: You can infer the phylogenetic relationship of the host plants based on public information, for instance the Tree of Life project by the Smithsonian.

Material:

• background on this topic you can find here (protected by password, because it contains unpublished material)
• the construction of the evolutionary timeline by a molecular clock by Matari et al. 2014 (accessible through the KIT library)
• a review on Oomycete evolution describing the mechanisms and the horizontal gene flow by Jiang et al. 2012 (accessible through the KIT library)