Macro- and micro-geographical genetic variation in early-fitness traits in populations of maritime pine (Pinus pinaster)

[Background and Aims] Assessing adaptive genetic variation and its spatial distribution is crucial to conserve forest genetic resources and manage species’ adaptive potential. Macro-environmental gradients commonly exert divergent selective pressures that enhance adaptive genetic divergence among populations. Steep micro-environmental variation might also result in adaptive divergence at finer spatial scales, even under high gene flow, but it is unclear how often this is the case. Here, we assess genetic variation in early-fitness traits among distant and nearby maritime pine (Pinus pinaster) populations, to investigate climatic factors associated with trait divergence, and to examine trait integration during seedling establishment.

[Methods] Open pollinated seeds were collected from seven population pairs across the European species distribution, with paired populations spatially close (between <1 and 21 km) but environmentally divergent. Seeds were sown in semi-natural conditions at three environmentally contrasting sites, where we monitored seedling emergence, growth and survival.

[Key Results] At large spatial scales, we found significant genetic divergence among populations in all studied traits, with certain traits exhibiting an association with temperature and precipitation gradients. Significant trait divergence was also detected between pairs of nearby populations. In addition, we found consistent trait correlations across experimental sites; notably, heavier seeds and earlier seedling emergence were both associated with higher seedling survival and fitness over two years in all experimental conditions.

[Conclusions] We identified mean annual temperature and precipitation seasonality as potential drivers of P. pinaster population divergence in the studied early-life traits. Populations genetically diverge also at local spatial scales, potentially suggesting that divergent natural selection can override gene flow along local-scale ecological gradients. These results suggest the species exhibits substantial adaptive potential that has allowed it to survive and evolve under contrasting environmental conditions.

This work was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 676876 (GenTree project). A.S.-M. was supported by a PhD grant from the Subdirección General de Investigación y Tecnología of the Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (FPI-SGIT2016-01) and a contract within the Project ‘Adaptive BREEDING for productive, sustainable and resilient FORESTs under climate change’, grant agreement no. 773383.

Peer reviewed