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

Phylogenetic relationships within the Heterobasidion complex Fir-Spruce Pine Europe Pine N.Am.

Geneaology of “S” DNA insertion into P ISG confirms horizontal transfer. Time of “cross-over” uncertain 890 bp CI>0.9 NA S NA P EU S EU F

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

Phylogenetic relationships within the Heterobasidion complex Fir-Spruce Pine Europe Pine N.Am.

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