Regional Response of Galling Arthropods in a Trispecific System of Hybridizing Populus: A Community Approach

The drainage basin of the Oldman River in southern Alberta, Canada, supports three species of cottonwood that overlap and hybridize along an elevational gradient: Populus balsamifera, P. angustifolia (both in Section Tacamahaca) and Populus deltoides (Sect. Aigeiros).  Pure, intra- and intersectional zones were previously defined in this region using leaf morphology.

This tri-hybrid region represents an ideal playground for testing the Hybrid Bridge Hypothesis (Floate and Whitham 1983). Indeed, this ecological hypothesis argues that hybrids allow associated organisms (e.g., herbivores, mutualists, pathogens) to encounter and potentially adapt to the genome of another species in a series of intermediate genotypes that effectively act as “stepping stones” that bridge two genomes.

With the aim of testing this hypothesis, we randomly sampled trees in this region to obtain genetic information on 729 mature trees using species-specific markers. We then applied Bayesian methods in combination with simulated datasets to assign these trees to parental or hybrid categories (pure, F1 hybrids, back-cross, and tri-hybrids of all species combinations).  On the same trees, we also quantified their gall-forming arthropod communities for 10 different species. All intermediate hybrid classes between the two Populus species from the Tacamahaca section were detected, suggesting bi-directional hybridization. In contrast, intersectional hybridization appeared mostly unidirectional. We then analyzed how these hybridization patterns affected the distribution of arthropods. Answers to these questions are important because they address how hybridization in plants can affect associated species. They also emphasize how hybrid zones may be regarded as sources of biodiversity and novelty.