Geographical structure and adaptive population differentiation in herbivore defence genes in European aspen (Populus tremula L., Salicaceae).

When a phenotypic trait is subjected to spatially variable selection and local adaptation, the underlying genes controlling the trait are also expected to show strong patterns of genetic differentiation because alternative alleles are favoured in different geographical locations. Here, we study 71 single nucleotide polymorphisms (SNPs) from seven genes associated with inducible defence responses in a sample of Populus tremula collected from across Sweden. Four of these genes (PPO2, TI2, TI4 and TI5) show substantial population differentiation, and a principal component analyses conducted on the defence SNPs divides the Swedish population into three distinct clusters. Several defence SNPs show latitudinal clines, although these were not robust to multiple testing. However, five SNPs (located within TI4 and TI5) show strong longitudinal clines that remain significant after multiple test correction. Genetic geographical variation, supporting local adaptation, has earlier been confirmed in genes involved in the photoperiod pathway in P. tremula, but this is, to our knowledge, one of the first times that geographical variation has been found in genes involved in plant defence against antagonists.