SummaryWith frequent host shifts involving the colonization of new hosts across large geographical ranges, crop pests are good models for examining the mechanisms of rapid colonization. The microbial partners of pest insects may be involved or affected by colonization, which has been little studied so far. We investigated the demographic history of the rosy apple aphid, Dysaphis plantaginea, a major pest of the cultivated apple (Malus domestica) in Europe, North Africa and North America, as well as the diversity of its endosymbiotic bacterial community. We genotyped a comprehensive sample of 714 colonies from Europe, Morocco and the US using mitochondrial (CytB and CO1), bacterial (16s rRNA and TrnpB), and 30 microsatellite markers. We detected five populations spread across the US, Morocco, Western and Eastern Europe, and Spain. Populations showed weak genetic differentiation and high genetic diversity, except the Moroccan and the North American that are likely the result of recent colonization events. Coalescent-based inferences releaved high levels of gene flow among populations during the colonization, but did not allow determining the sequence of colonization of Europe, America and Morroco by D. plantaginea, likely because of the weak genetic differentiation and the occurrence of gene flow among populations. Finally, we found that D. plantaginea rarely hosts any other endosymbiotic bacteria than its obligate nutritional symbiont Buchnera aphidicola. This suggests that secondary endosymbionts did not play any role in the rapid spread of the rosy apple aphid. These findings have fundamental importance for understanding pest colonization processes and implications for sustainable pest control programs.

Background and Aims Studying the relationship between phenotypic and genetic variation in populations distributed across environmental gradients can help us understand the ecological and evolutionary processes involved in population divergence. We investigated the patterns of genetic and phenotypic diversity in the European crabapple, Malus sylvestris, a wild relative of the cultivated apple (Malus domestica) that occurs naturally across Europe in areas subjected to different climate conditions, to test for divergence among populations. Methods Growth rates and traits related to carbon uptake in seedlings collected across Europe were measured under controlled conditions and associated with the genetic status of the seedlings, which was assessed using 13 microsatellite loci and Bayesian clustering method. Isolation-by-distance, -by-climate, and -by-adaptation patterns, which can explain genetic and phenotypic differentiation among M. sylvestris populations, were also tested. Key Results A total of 11.6% of seedlings were introgressed by M. domestica, indicating that crop-wild gene flow is ongoing in Europe. The remaining seedlings (88.4%) belonged to seven M. sylvestris populations. Significant phenotypic trait variation among M. sylvestris populations was observed. We did not observe significant isolation-by-adaptation; however, the significant association between genetic variation and the climate during the last glacial maximum suggests that there has been local adaptation of M. sylvestris to past climates. Conclusions This study provides insight into the phenotypic and genetic differentiation among populations of a wild relative of the cultivated apple. This may help us better utilize its diversity and provide options for mitigating the impact of climate change on the cultivated apple through breeding.