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Summary of previous lesson • Janzen-Connol hypothesis; explanation of why diseases lead to spatial heterogeneity • Diseases also lead to heterogeneity or changes through time – Driving succession – The Red Queen Hypothesis: selection pressure will increase number of resistant plant genotypes • Co-evolution: pathogen increase virulence in short term, but in long term balance between host and pathogen • Density dependance Disease and competition • Competition normally is conducive to increased rates of disease: limited resources weaken hosts, contagion is easier • Pathogens can actually cryptically drive competition, by disproportionally affecting one species and favoring another Janzen-Connol • Regeneration near parents more at risk of becoming infected by disease because of proximity to mother (Botryosphaeria, Phytophthora spp.). Maintains spatial heterogeneity in tropical forests • Effects are difficult to measure if there is little host diversity, not enough host-specificity on the pathogen side, and if periodic disturbances play an important role in the life of the ecosystem Diseases and succession • Soil feedbacks; normally it’s negative. Plants growing in their own soil repeatedly have higher mortality rate. This is the main reason for agricultural rotations and in natural systems ensures a trajectory towards maintaining diversity • Phellinus weirii takes out Douglas fir and hemlock leaving room for alder The red queen hypothesis • Coevolutionary arm race • Dependent on: – Generation time has a direct effect on rates of evolutionary change – Genetic variability available – Rates of outcrossing (Hardy-weinberg equilibrium) – Metapopulation structure Diseases as strong forces in plant evolution • Selection pressure • Co-evolutionary processes – Conceptual: processes potentially leading to a balance between different ecosystem components – How to measure it: parallel evolution of host and pathogen • Rapid generation time of pathogens. Reticulated evolution very likely. Pathogens will be selected for INCREASED virulence • In the short/medium term with long lived trees a pathogen is likely to increase its virulence • In long term, selection pressure should result in widespread resistance among the host More details on: • How to differentiate linear from reticulate evolution: comparative studies on topology of phylogenetic trees will show potential for horizontal transfers. Phylogenetic analysis neeeded to confirm horizontal transmission NJ Phylogenetic relationships within the Heterobasidion complex Het INSULARE Fir-Spruce True Fir EUROPE Spruce EUROPE True Fir NAMERICA Pine Europe Pine EUROPE Pine N.Am. Pine NAMERICA 0.05 substitutions/site NJ 11.10 SISG CA Geneaology of “S” DNA insertion into P ISG confirms horizontal transfer. 2.42 SISG CA BBd SISG WA F2 SISG MEX Time of “cross-over” uncertain NA S BBg SISG WA 14a2y SISG CA 15a5y M6 SISG CA 6.11 SISG CA 9.4 SISG CA AWR400 SPISG CA 9b4y SISG CA 15a1x M6 PISG CA 1M PISG MEX 9b2x PISG CA A152R FISG EU A62R SISG EU 890 bp CI>0.9 A90R SISG EU EU S A93R SISG EU J113 FISG EU J14 SISG EU J27 SISG EU J29 SISG EU 0.0005 substitutions/site EU F NA P Complexity of forest diseases • At the individual tree level: 3 dimensional • At the landscape level” host diversity, microclimates, etc. • At the temporal level Complexity of forest diseases • Primary vs. secondary • Introduced vs. native • Air-dispersed vs. splash-dispersed, vs. animal vectored • Root disease vs. stem. vs. wilt, foliar • Systemic or localized Stem canker on coast live oak Progression of cankers Older canker with dry seep Hypoxylon, a secondary sapwood decayer will appear Root disease center in true fir caused by H. annosum HOST-SPECIFICITY • • • • • Biological species Reproductively isolated Measurable differential: size of structures Gene-for-gene defense model Sympatric speciation: Heterobasidion, Armillaria, Sphaeropsis, Phellinus, Fusarium forma speciales NJ Phylogenetic relationships within the Heterobasidion complex Het INSULARE Fir-Spruce True Fir EUROPE Spruce EUROPE True Fir NAMERICA Pine Europe Pine EUROPE Pine N.Am. Pine NAMERICA 0.05 substitutions/site Recognition of self vs. non self • Intersterility genes: maintain species gene pool. Homogenic system • Mating genes: recognition of “other” to allow for recombination. Heterogenic system • Somatic compatibility: protection of the individual. Recognition of self vs. non self • What are the chances two different individuals will have the same set of VC alleles? • Probability calculation (multiply frequency of each allele) • More powerful the larger the number of loci • …and the larger the number of alleles per locus INTERSTERILITY • If a species has arisen, it must have some adaptive advantages that should not be watered down by mixing with other species • Will allow mating to happen only if individuals recognized as belonging to the same species • Plus alleles at one of 5 loci (S P V1 V2 V3) MATING • Two haploids need to fuse to form n+n • Sex needs to increase diversity: need different alleles for mating to occur • Selection for equal representation of many different mating alleles