Sexual and asexual reproduction

By affecting the distribution of gene diversity among individuals in a population, reproduction is a strong driver of evolution, particularly for pathogens undergoing regular recombination, but also for asexually reproducing pathogens because environmental conditions promote the selection of adapted individuals. Phytophthora infestans, the causal agent of potato late blight, has often caused major damage as the fungus has moved into new countries. The Irish famine, from 1844 to 1849, is a well-known example illustrating how the introduction of a new pathogen can affect food security in the absence of host resistance. Until the late 20th century, with the exception of Mexico, little genetic variation was found within and among pathogen populations dominated by a single mating. In the 1980s, the migration of a the second mating type from northern Mexico allowed sexual recombination in P. infestans populations and the appearance of increased aggressive isolates with a high sporulation rate capacity and lower generation time (Goodwin et al., 1994; McDonald and Linde, 2002). As new strains of P. infestans evolve, new outbreaks of the disease occur, which affects not only host resistance capacity but also chemical treatment efficacy as fungicide-resistant strains are selected more rapidly (Anderson et al., 2004).

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