Mariette Nicolas

Thesis started december 1st 2012, defended in january 8^th 2016

Fund : CIFRE project,  financed by ACVNPT (Association des Créateurs de Variétés Nouvelles de Pomme de Terre) , the association of French potato breeding companies

Direction : Didier Andrivon & Josselin Montarry

Abstract:

West European populations of Phytophthora infestans, the oomycete causing late blight in potato (Solanum tuberosum), are characterized by a strongly clonal structure and rapid replacement of dominant lineages. This work thus aimed to identify the ecological, phenotypic and evolutionary determinants of the invasive character of clonal lineages of populations of this parasite. To this end, we analysed the phenotypic and genotypic population dynamics over two time scales, a long one (nearly a decade) and a shorter one consisting in a longitudinal tracking over two consecutive epidemics including the inter-epidemic phase. This monitoring was supplemented by the analysis of adaptive responses within these populations with respect to the main life-history traits of the parasite. Our results tend to reject the hypothesis, often advanced, that the invasive ability is linked to a higher aggressiveness of the new clonal lineages compared to the previous ones. Moreover, the short-term study revealed complex scenario, involving the presence of two main clonal lineages (6_A1 and 13_A2) with opposite reproductive strategies: while 6_A1 isolates produced many, small sporangia, 13_A2 isolates produced a fewer but bigger sporangia. The coexistence within a single population of these two strategies could result from the trade-off between these spore size and spore number, that we also demonstrated. Finally, we showed differential response between clones to temperature, as well as clear local adaptation patterns, including among geographically distant populations of a same clonal lineage (13_A2). Our results highlight that different adaptive factors can impact the same biological traits of P. infestans (size and number of sporangia, latent period, lesion growth) and that it is crucial to think about the consequences of these concomitant selective pressures in a general context associating climate change and deployment of resistant potato varieties.