Mutation and genetic drift

Mutation is the ultimate source of genetic variation, leading directly to changes in the DNA sequence of individual genes and thus creating new alleles in populations (McDonald and Linde, 2002). The loss of alleles over time, or genetic drift, can also generate new diseases through the selection of gene combinations that can adapt to a new ecosystem. The evolutionary potential of a small population is limited, but its adaptation capability to a new environment should not be underestimated. Yellow rust is a wheat disease known to occur in cool environments. It is caused by Puccinia striiformis, a biotrophic asexually reproducing fungal species harbouring new virulence strains resulting from mutation. A study on P. strii-formis diversity at global level has demonstrated the recent intercontinental spread of yellow rust (Hovmoller et al., 2008). New epidemics in North America may be driven by an increase in aggressiveness conferring the ability to cause disease more quickly and at temperatures once considered too warm for the fungus (Milus et al., 2009). Particular strains and their derivatives resulting from mutation were found at multiple sites in relatively warm or dry wheat growing areas where severe yellow rust epidemics have been observed in recent years. The generation time (latent period) was approximately 2 days shorter for 'new strains' compared with isolates of representative strains sampled before 2000 from multiple regions in North America and Europe (Hovmoller et al., 2008; Milus et al., 2009). The dramatic increase in spore production potential explains why a new and stronger strain can spread rapidly at a global scale, for example, by increasing the likelihood of 'rare events' occurring, such as long-range spore dispersal by wind or accidental spread (Wellings et al., 1987; Brown and Hovmoller, 2002; Hovmoller and Justesen, 2007; Hodson, 2009).

Table 4.2. Effects of climate changes and human activities on evolutionary forces leading to a modification of pathogen populations resulting in new pest and disease epidemics: examples from forestry, agroecosystems and food crops.

Evolutionary forces Pathogen

Disease

Affected Effects of climate changes and human crop/species activities

References

Mutation and genetic drift

Gene flow

Puccinia striiformis Yellow rust

Wheat

Cryphonectria Asian chestnut tree American chestnut parasitica blight

Ophiostoma novo- Dutch elm disease Elm ulmi

Puccinia graminis f. Stem and leaf rusts Oat sp. avenae, Puccinia coronata

Gene expression or functionality

avenae P. striiformis

Stem rust Yellow rust

Xanthomonas oryzae

Blumeria graminis Powdery mildew

Triticum turgidum ssp. dicoccoides Bacterial leaf blight Rice

Magnaporthe oryzae

Blast

Barley Rice

Intercontinental spread Adaptation to higher temperatures; reduction in generation time; increase in spore production potential Wind and accidental spread

Introduction of pathogen into new ecological niches Introduction of pathogen into new ecological niches Interactions at the agroecological interfaces between wild host and cultivated populations

Temperature sensitive resistance genes deactivated As a result of gene Yr36 HTAPRa is effective

Xa7 resistance gene influenced by temperature Mlo resistance gene disrupted by drought stress

Elevated atmospheric CO2 increases lesions possibly due to a reduction in leaf silicon content

Wellings et al. (1987), Brown and Hovm0ller (2002), Hovm0ller and Justesen (2007), Hodson et al. (2009)

Anagnostakis (1987) Brasier (1991) Burdon and Thrall (2008)

Maertens et al. (1967) Uauy et al. (2005) Garrett et al. (2006) Newton and Young (1996) Kobayashi et al. (2006)

Barley yellow dwarf Barley yellow dwarf Oats, barley and virus (BYDV) (BYD) wheat

Interspecific hybridization

New Phytophthora species

Alder tree

New Phytophthora species Pyrenophora tritici- Tan spot repentis

Primula,

Spathiphyllum Wheat

Sexual and asexual reproduction

Phytophthora infestans

Late blight

Potato a HTAPR, high temperature adult-plant resistance.

Elevated atmospheric C02 increases root biomass, photosynthesis and water-use efficiency, favouring the persistence of infected plants and virus reservoirs

New aggressive species emerging naturally from hybridization between Phytophthora cambivora-like and Phytophthora tragariae-like taxons

New natural hybrids from Phytophthora cactorum and Phytophthora nicotianae

Horizontal transfer of ToxA gene from Phaeosphaeria nodorum into the P. tritici-repentis genome

Introduction of a second mating type to new areas allowing sexual recombination leading to more aggressive isolates with high sporulation capacity and lower generation time in the absence of host resistance

Malmström and Field (1997)

Brasier et al. (1999), Brasier (2001)

Man in't Veldt etal. (1998)

Friesen etal. (2006), Stukenbrock and McDonald (2008)

Goodwin etal. (1994), McDonald and Linde (2002)

Table 4.3. Synopsis of type of events and succession of effects resulting from the influence of climate change on evolutionary forces modifying host-pathogen interactions and leading to new disease and pest epidemics.

Evolutionary forces

Type of change occurring Induced effect

Impact on host and pathogen

Outcome

Mutation and genetic drift Ultimate change at DNA level

Gene flow

Exchange between populations of alleles or individuals

Gene expression or Phenotypic changes functionality

Interspecific hybridization New species formed

Adaptation to new environmental conditions

Increased population diversity

Change in pathosystems Change in pathosystems

Sexual and asexual reproduction

New aggressive strains Change in pathosystems formed with high fitness

Reduction in generation time, higher spore production

Variation in host resistance; variation in pathogen virulence; new specific interactions

Interactions at the agroecological interfaces between wild host and cultivated populations

Host physiology and resistance modified

Shifts in the geographical distribution of hosts and pathogens

Horizontal gene transfer

Recombination leading to emergence of more adapted aggressive isolates with high sporulation and shorter generation time leading to reduction of host resistance

New epidemics resulting from dispersal or introduction to new areas including continents through rare events and human activity

New disease or pathogen emergence

Introduction of pathogen into new ecological niches

Susceptibility or resistance to disease increased

Dispersal of exotic pests or pathogens

Emergence of new diseases

Emergence of new outbreak and chemical treatments; efficacy reduced due to rapid fungicide resistance selection capacity

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