Contrasting patterns of local adaptation along climatic gradients between a sympatric parasitic and autotrophic tree species

Abstract

Sympatric tree species are subject to similar climatic drivers, posing a question as to whether they display comparable adaptive responses. However, no study has explicitly examined local adaptation of co‐occurring parasitic and autotrophic plant species to the abiotic environment. Here we test the hypotheses that a generalist parasitic tree would display a weaker signal of selection and that genomic variation would associate with fewer climatic variables (particularly precipitation) but have similar spatial patterns to a sympatric autotrophic tree species. To test these hypotheses, we collected samples from 17 sites across the range of two tree species, the hemiparasite Nuytsia floribunda (n = 264) and sympatric autotroph Melaleuca rhaphiophylla (n = 272). We obtained 5,531 high‐quality genome‐wide single nucleotide polymorphisms (SNPs) for M. rhaphiophylla and 6,727 SNPs for N. floribunda using DArTseq genome scan technology. Population differentiation and environmental association approaches were used to identify signals of selection. Generalized dissimilarly modelling was used to detect climatic and spatial patterns of local adaptation across climatic gradients. Overall, 322 SNPs were identified as putatively adaptive for the autotroph, while only 57 SNPs were identified for the parasitic species. We found genomic variation to associate with different sets of bioclimatic variables for each species, with precipitation relatively less important for the parasite. Spatial patterns of predicted adaptive variability were different and indicate that co‐occurring species with disparate life history traits may not respond equally to selective pressures (i.e., temperature and precipitation). Together, these findings provide insight into local adaptation of sympatric parasitic and autotrophic tree species to abiotic environments.

Association of putatively adaptive genetic variation with climatic variables differs between a parasite and its host

“…Our findings on the association of putatively adaptive variation with climatic variables in a mistletoe–host system were similar to those of a previous study on a generalist parasite and co‐occurring autotroph (Walters et al, 2020 ). For both parasitic plants, there was a stronger association of putatively adaptive variation to temperature variables, in comparison to precipitation variables for host species.…”

“…GDM analysis showed putatively adaptive variation in the host to be associated with precipitation, which is consistent with findings of other studies on autotrophic plants (e.g. Manel et al, 2012;Shryock et al, 2017;Steane et al, 2017;Walters et al, 2020).…”

“…Procrustes residuals measure the absolute difference in patterns of predicted genetic compositions between putatively neutral and putatively adaptive data sets for each species. Further, to allow direct comparison between species, we scaled residuals by the largest and smallest value observed across both species following the method of Walters et al ( 2020 ). Finally, residuals were mapped geographically to identify areas with the largest differences in genetic composition patterns between SNP data sets.…”

“…A recent study by Walters et al ( 2020 ) found associations between genome‐wide variation and climatic variables to be different between a parasitic plant and a sympatric autotrophic species. However, the study species ( Nuytsia floribunda ) was a generalist parasite that has numerous host species (Calladine et al, 2000 ), compared to specialist parasites that utilize a single, or few, often related, host species.…”

“…A recent study by Walters et al (2020) found associations between genome-wide variation and climatic variables to be different between a parasitic plant and a sympatric autotrophic species.…”