Genetic structure of the poplar rust fungus Melampsora larici-populina: Evidence for isolation by distance in Europe and recent founder effects overseas

INRA, Université Bordeaux I, UMR1202 BioGeCo, F-33883 Villenave d’Ornon, France
Infection Genetics and Evolution (Impact Factor: 3.02). 04/2008; 8(5):577-587. DOI: 10.1016/j.meegid.2008.04.005
Source: PubMed


Dispersal has a great impact on the genetic structure of populations, but remains difficult to estimate by direct measures. In particular, gradual and stochastic dispersal are often difficult to assess and to distinguish, although they have different evolutionary consequences. Plant pathogens, especially rust fungi, are suspected to display both dispersal modes, though on different spatial scales. In this study, we inferred dispersal capacities of the poplar rust fungus Melampsora larici-populina by examining the genetic diversity and structure of 13 populations from eight European and two overseas countries in the Northern hemisphere. M. larici-populina was sampled from both cultivated hybrid poplars and on the wild host, Populus nigra. The populations were analyzed with 11 microsatellite and 8 virulence markers. Although isolates displayed different virulence profiles according to the host plant, neutral markers revealed little population differentiation with respect to the type of host. This suggests an absence of reproductive isolation between populations sampled from cultivated and wild poplars. Conversely, studying the relationship between geographic and genetic structure allowed us to distinguish between isolation by distance (IBD) patterns and long distance dispersal (LDD) events. The European populations exhibited a significant IBD pattern, suggesting a regular and gradual dispersal of the pathogen over this spatial scale. Nonetheless, the genetic differentiation between these populations was low, suggesting an important gene flow on a continental scale. The two overseas populations from Iceland and Canada were shown to result from rare LDD events, and exhibited signatures of strong founder effects. Furthermore, the high genetic differentiation between both populations suggested that these two recent introductions were independent. This study illustrated how the proper use of population genetics methods can enable contrasted dispersal modes to be revealed.

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    • "Thus, populations with small effective sizes are likely to have reduced capacity for adaptation and, in the special case of pathogens, a low probability of overcoming chemical controls or host resistance. Crop pathogens or parasites can also be found in wild hosts (Barrès et al. 2008; Barrett et al. 2009; Gracianne et al. 2014), particularly because wild hosts are the initial host species before domestication or because parasites can also subsequently shift to other cultivated plants (Stukenbrock and McDonald 2008). Once in a cultivated environment, the geographical distribution, host associations, and disease dynamics of pests are mainly influenced by human activities (Morgan et al. 2012; Bousset and Chèvre 2013). "
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    • "MLP in Europe displays variability in pathogenicity and five pathotypes (E1 to E5) have been recognized, distinguished on the basis of eight virulence types (Pinon & Frey 2005). All virulence types have been observed in western Europe, although in central Europe, the main types to date have been virulence 3 and 4, and to a lesser extent types 2 and 5 (Barrès et al. 2008). The geographic distribution of types is likely due to the cultivation of different cultivars compared to those grown in western Europe. "

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    • "F ST versus Q ST comparisons have suggested that this pattern is indeed an adaptation, rather than merely the consequence of genetic drift (Ali et al. 2010). There may initially be several proximal causes for a loss of sexual reproduction: (i) a lack of appropriate mates where asexual propagation is possible, as in the case of Phytophthora infestans, P. cinnamomi, P. ramorum and O. novo-ulmi, for which a single mating type was present for long periods in introduced ranges (Goodwin et al. 1994; Brasier 2001; Dobrowolski et al. 2003; Hardham 2005); (ii) a hybridization event, for example, in the human pathogen Cryptococcus neoformans (Lin et al. 2007); and (iii) a lack of the alternate host on which sex occurs in the case of complex life cycles, for example, in the rusts Puccinia striiformis f. sp.tritici and Melampsora larici-populina (Mboup et al. 2007, 2009; Barr es et al. 2008). There may also be diverse evolutionary causes for the selection against sexual reproduction (Bazin et al. 2014 "
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