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An understanding of the role of factors intrinsic to a species’ life history in structuring contemporary genetic variation is a fundamental, but understudied, aspect of evolutionary biology. Here, we assessed the influence of the propensity to outbreak in shaping worldwide genetic variation in Locusta migratoria, a cosmopolitan pest well known for its expression of density-dependent phase polyphenism. We scored 14 microsatellites in nine subspecies from 25 populations distributed over most of the species’ range in regions that vary in the historical frequency and extent of their outbreaks. We rejected the hypothesis that L. migratoria consists of two genetically distinct clusters adapted to habitats either rarely (nonoutbreaking) or cyclically (outbreaking) favourable to increases in population density. We also invalidated the current subspecific taxonomic classification based on morphometrics. Bayesian inferences indicated evidence of a homogenizing effect of outbreaks on L. migratoria population structure. Geographical and ecological barriers to gene flow in conjunction with historical events can also explain the observed patterns. By systematically assessing the effects of null alleles using computer simulations, we also provide a template for the analysis of microsatellite data sets characterized by a high prevalence of null alleles. [ABSTRACT FROM AUTHOR]

We develop a simple mathematical model to explain the lower than expected levels of infection of wild perennial ryegrass populations in France by endophytic Neotyphodium fungi (formerly named Acremonium). Indeed, seed-borne Neotyphodium endophytes are considered as mutualistic symbionts, because they increase survival, growth and flowering rates of their hosts, and should therefore be present at very high frequencies in all host populations. However, recent surveys have shown that 70% of wild populations of perennial ryegrass harbour such endophytes in France. Moreover, most infected populations exhibit a low level of infection. Our simple model, taking into account the life-cycles of the host and the fungus, shows that these patterns can be satisfactorily explained if the vertical transmission of the fungus is imperfect. Such imperfect transmission, though never measured in natural populations, is likely because of the reported mortality of the endophyte in stored seeds. This process, analogous to the mutation-selection balance of classical population genetics theor, may explain the observed patterns even better when we consider random fluctuations of selection coefficients over time and genetic drift. [ABSTRACT FROM AUTHOR]

The genetic structure of wild populations of Colletotrichum lindemuthianum was evaluated for virulence and molecular markers. Forty-five isolates were collected from five wild common bean populations located in their South-Andean centre of origin. The five pathogen populations were monomorphic in their ITS regions, but 45 polymorphic markers were identified using RAPDs. Polymorpbism for virulence was also observed; 15 pathotypes were characterized on an international set of 12 differentials. A molecular variance analysis (AMOVA) showed that a very large part of the total genetic variation was within populations. Statistical analysis showed that there was a weak though significant differentiation among the five populations for the RAPD and virulence markers. A positive and significant correlation was found between geographic distance and the distances from RAPD and virulence data, suggesting migration between adjacent populations along the Argentinian transect. Our results suggest that the Andean wild isolates of C. lindemuthianum do not reflect all the putative diversity found in the isolates from cultivated common bean. [ABSTRACT FROM AUTHOR]