Genetic differences in the elevational limits of native and introduced Lactuca serriola populations

Aim Differences in phenological timing might explain why populations of the annual Lactuca serriola reach higher elevational limits in a part of its introduced range than in its native range. I investigated (1) whether this difference in elevational limits has a genetic basis, (2) the importance of clinal genetic differentiation and phenotypic plasticity in phenology as responses to elevation in L. serriola, and whether these responses differ between regions, and (3) whether the realized temperature niche of L. serriola differs between the two regions. Location Plant material was collected in Canton Valais, Switzerland (native range) and the Wallowa Mountains, Oregon, USA (introduced range). The field experiment was conducted in Canton Grisons, Switzerland. Methods Plants from 20 populations collected along elevational gradients were grown in eight common gardens established at 200-m elevational intervals (600-2000 m a.s.l.). The timing of phenological transitions was monitored and analysed with mixed-effects models to determine differences in (1) elevational limits, and (2) clinal genetic differentiation and phenotypic plasticity as responses to elevation for plants from each region. The limits of the species along five temperature gradients were derived from generalized linear models using published occurrence data to quantify regional differences in the realized temperature niche. Results The limit of seed set (1400 m a.s.l.) was the same for plants of both regions. However, the limit of flowering, probably a better reflection of elevational limits in this study, was 400 m higher for plants from the introduced region due to their faster development. Native populations showed clines in development time with elevation consistent with expectations. However, these were weaker in introduced populations, the responses of which were rather characterized by phenotypic plasticity. Thus, although introduced populations grow at considerably cooler sites than in the native region, this is unlikely to have resulted from direct selection for tolerance of high-elevation conditions. Main conclusions This study supports a genetic basis for differences in the elevational limits of L. serriola populations between two parts of its native and introduced range. Although it is not yet clear whether these differences evolved in the introduced range, these findings highlight the potential of alien species for gaining insights into niche evolution.