Rapid speciation following recent host shift in the plant pathogenic fungus Rhynchosporium Tiziana Vonlanthen, Laurin Müller 27.10.15 1
Second paper: Origin and Domestication of the Fungal Wheat Pathogen Mycosphaerella graminicola via Sympatric Speciation Fungal pathogen Mycosphaerella graminicola. Causing septoria leaf blotch on wheat. Hypothesized that M. Graminicola emerged as a new pathogen during the process of wheat domestication. Isolates of this wild grass pathogen from 5 locations in Iran were compared with 123 M. Graminicola isolates from the Middle East, Europe, and North America. 6 DNA loci 27.10.15 2
Second paper: Origin and Domestication of the Fungal Wheat Pathogen Mycosphaerella graminicola via Sympatric Speciation The divergence of the wheat-adapted pathogen M. Graminicola from an ancestral population infecting wild grasses in the Middle East occurred approximately 10,500 years ago. Wheat-adapted Mycosphaerella population is likely to have occurred as a sympatric process. The data indicate that the pathogen host shift did not occur as one single event but probably happened through a series of introgressions of isolates from uncultivated grasses into the wheat-infecting population. 27.10.15 3
Domestication hypothesis Plant pathogens are usually assumed to have originated on the direct ancestors of their modern hosts and to have coevolved with them during domestication at the place of domestication. 27.10.15 4
Introduction Fungal pathogen Rhynchosporium secalis Diseases (scald) caused on barley, rye, triticale and other grasses Reference:https://www.anbg.gov.au/cpbr/program/sc/images/barleysc.jpg Domestication of barley ~10 000 years B.P. (before present) R. secalis colonized barley 5 000 to 2 500 years B.P. à Domestication hypothesis? 27.10.15 5
DNA extraction, Amplification and Sequencing Material & Methods Phylogenetic analyses Coalescent analyses Divergence time estimates and demographic analyses Pathogenicity Assays 27.10.15 6
Material & Methods 316 R. secalis isolates from 21 countries representing 5 continents. Collected from 9 different hosts: Barley Rye Triticale Six wild grasses 27.10.15 7
Material & Methods DNA extraction, Amplification and Sequencing DNA sequence regions (Loci): α-tubulin β-tubulin EF-1α ITS 27.10.15 8
Material & Methods phylogenetic analyses Two tree-building methods Maximum parsimony (MP) Bayesian maximum likelihood (BML) For all four loci phylogenetic trees of both methods (MP and BML) were performed. In addition a combined version of three loci was conducted. 27.10.15 9
Results phylogenetic analyses Three dinstict lineages A, B and C were found for all four loci. 27.10.15 10
Material & Methods coalescent analyses Determine the ancestral history of R. secalis Mutations of the population Relative time of divergence 27.10.15 11
Results coalescent analyses All three lineages coalesced to a single common ancestor. Most of the mutations seperating the three lineages emerged recently. 27.10.2015 12
Material & Methods divergence time estimates and demographic analysis Estimating posterior distribution of the most common recent ancestor Population size through time 27.10.15 13
Results - divergence time estimates and demographic analysis The posterior mean estimates of the TMRCA of all three lineages A C were between 1281 to 3627 years B.P. The diversification within the three lineages started during a similar time frame for all three lineages with posterior means of the TMRCA between 459 and 1139 years B.P. Decline of the population size of all three lineage almost simultaneously (~1200 to 3600 years ago) Maximum decline was reached ~250 to 500 years ago followed by a rapid expansion. 27.10.15 14
Material & Methods Pathogenicity Assays Definition pathogenicity:...the ability of a fungal isolate to infect a host species. Can isolates of R. secalis from one phylogenetic lineage (A,B or C) infect hosts of the other lineages? Inoculation of different hosts with different isolates of R. secalis 27.10.15 15
Results Pathogenicity Assays No cross pathogenicity on hosts of isolates from different phylogenetic lineages was detected. 27.10.15 16
Discussion Does these results now fit to the domestication hypothesis? R. secalis could have shifted from an ancestor on wild grasses and then on barley and rye during domestication in the fertile crescent 10 000 years ago. 27.10.15 17
Discussion Split of three lineages occurred recently (between 1200 and 3600 years B.P.) Time estimates fits with introduction of barley and rye cultivation in northern Europe (3000 to 5000 years B.P.) Mutations were relatively recent Lineages evolved in parallel (no lineage is ancestral to another) Genetic diversity was highest in northern Europe and not in the fertile crescent (previous study) à No, domestication hypothesis can be falsified! 27.10.15 18
Discussion More likely is that R. secalis underwent a host shift and so emerged as a barley and rye disease in northern europe 1200-3600 years ago. origin of host origin of the pathogen Brunner et al. 2007 27.10.15 19
Discussion Demographic growth reconstructions point to a recent recovery of the pathogen population from the bottleneck that was experienced after the host shift. After the bottleneck population size increased rapid due to gobal expansion and industrialization of agriculture and due to the Green Revolution. 27.10.15 20
Discussion Due to the clear distinction between the three lineages A, B and C they propose to define them as three seperate species. 1. absence of gene flow 2. lineages assigned to unique haplotypes according to their host 3. each lineages builds a monophyletic group 4. independent evolution of three lineages 5. host-association confirmed with pathogenicity tests 27.10.15 21
Discussion Nevertheless, agriculture played a crucial role in the emergence of scald Through agriculture the host and the pathogen were brought together The intensification of agriculture allowed the pathogen to adapt and diverge into new species very fast 27.10.15 22
Thank you for your attention! Questions? 27.10.15 23