Strong Selection Against Early Generation Hybrids in Joshua Tree Hybrid Zone Not Explained by Pollinators Alone.

Anne M Royer,Christopher Irwin Smith,Jackson Waite-Himmelwright

doi:10.3389/fpls.2020.00640

Anne M Royer, Christopher Irwin Smith + Show 1 more

Open Access

https://doi.org/10.3389/fpls.2020.00640

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Abstract

Coevolution frequently plays an important role in diversification, but the role of obligate pollination mutualisms in the maintenance of hybrid zones has rarely been investigated. Like most members of the genus Yucca, the two species of Joshua tree (Yucca brevifolia and Yucca jaegeriana) are involved in a tightly coevolved mutualism with yucca moths. There is strong evidence of a history of coevolution between Joshua trees and their moth pollinators. We use a geographic clines approach in the Joshua tree hybrid zone to ask if selection by the moths may currently contribute to maintaining separation between these species. We compare genomic, phenotypic, and pollinator frequency clines to test whether pollinators maintain the hybrid zone or follow it as passive participants. The results reveal dramatic overlapping genomic and pollinator clines, consistent with a narrow hybrid zone maintained by strong selection. Wider phenotypic clines and a chloroplast genomic cline displaced opposite the expected direction suggest that pollinators are not the main source of selection maintaining the hybrid zone. Rather, it seems that high levels of reproductive isolation, likely acting through multiple barriers and involving many parts of the genome, keep the hybrid zone narrow.

Highlights

Understanding the forces that drive speciation has been a question of interest since the field of evolutionary biology began, and continues to generate complex questions, including the role of intrinsic and ecological factors (Coyne and Orr, 2004; Sobel et al, 2010; Nosil, 2012)
Note that the simulations assume that markers recombine freely (r = 0.5), so if some of these loci are tightly linked, we would be underestimating the number of generations of hybridization required to generate hybrid classes beyond the F1. We found both pollinator species co-occurring in the center of the hybrid zone, over a 4.2 km wide band ranging from ∼1.1 km west of the center to ∼3.1 km east of the center, with 90% of trees with both species observed on them located between 0.4 km west and 2.4 km east
The strong geographic structure in the location of different tree identities leads to a strong correlation between tree location in the hybrid zone and tree Qscore (Pearson’s product-moment correlation = 0.76, t = 7.23, p = 1.22∗10−8, df = 38)

Summary

Introduction

Understanding the forces that drive speciation has been a question of interest since the field of evolutionary biology began, and continues to generate complex questions, including the role of intrinsic and ecological factors (Coyne and Orr, 2004; Sobel et al, 2010; Nosil, 2012). Interactions between species are interesting because of the potential for the species to both evolve in response to each other (Janzen, 1980), offering opportunities for direct and diffuse coevolution Such coevolution can contribute to explaining phenomena involving speciation including adaptive radiations (Ehrlich and Raven, 1964; Kiester et al, 1984; Smith and Benkman, 2007; Althoff et al, 2014; Hembry et al, 2014; Marquis et al, 2016) and the latitudinal biodiversity gradient (Mittelbach et al, 2007). In spite of a large body of literature on interspecific interactions and speciation, there are relatively few studies examining obligate pollination mutualisms in hybrid zones (but see Leebens-Mack et al, 1998; Moe and Weiblen, 2012)

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