Molecular evidence and ecological niche modeling reveal an extensive hybrid zone among three Bursera species (section Bullockia).

Eduardo Quintero Melecio,Antonio González Rodríguez,Andrés Lira Noriega,Yessica Rico,William J Etges

doi:10.1371/journal.pone.0260382

Abstract

The genus Bursera, includes ~100 shrub and trees species in tropical dry forests with its center of diversification and endemism in Mexico. Morphologically intermediate individuals have commonly been observed in Mexican Bursera in areas where closely related species coexist. These individuals are assumed to result from interspecific hybridization, but no molecular evidence has supported their hybrid origins. This study aimed to investigate the existence of interspecific hybridization among three Mexican Bursera species (Bullockia section: B. cuneata, B. palmeri and B. bipinnata) from nine populations based on DNA sequences (three nuclear and four chloroplast regions) and ecological niche modeling for three past and two future scenario projections. Results from the only two polymorphic nuclear regions (PEPC, ETS) supported the hybrid origin of morphologically intermediate individuals and revealed that B. cuneata and B. bipinnata are the parental species that are genetically closer to the putative hybrids. Ecological niche modeling accurately predicted the occurrence of putative hybrid populations and showed a potential hybrid zone extending in a larger area (74,000 km2) than previously thought. Paleo-reconstructions showed a potential hybrid zone existing from the Last Glacial Maximum (~ 21 kya) that has increased since the late Holocene to the present. Future ecological niche projections show an increment of suitability of the potential hybrid zone for 2050 and 2070 relative to the present. Hybrid zone changes responded mostly to an increase in elevational ranges. Our study provides the first insight of an extensive hybrid zone among three Mexican Bursera species based on molecular data and ecological niche modeling.

Highlights

Interspecific gene flow resulting in the formation of intermediate individuals across hybrid zones is a common phenomenon in nature with important consequences for evolution and conservation [1,2,3]
In this study we examined putative hybrid populations using nuclear and chloroplast DNA sequences coupled with ecological niche modeling to ask the following questions: (i) Are morphologically intermediate individuals of hybrid origin from B. cuneata, B. palmeri and B. bipinnata? (ii) To which parental Bursera species do putative hybrids show the closest genetic affinity? (iii) Do the putative hybrids occur in a similar ecological niche to the parental species? (iv) Can we spatially and environmentally predict the formation of the hybrid zone through ecological niche modeling? (v) What is the role of past and future climatic conditions on the formation and maintenance of this hybrid zone?
Putative hybrid sequences were intermixed among subclades of the three parental species in high proportion and as sister subclades of B. cuneata and B. bipinnata (S1 Fig)

Summary

Introduction

Interspecific gene flow resulting in the formation of intermediate individuals across hybrid zones is a common phenomenon in nature with important consequences for evolution and conservation [1,2,3]. Hybridization can reinforce the development of reproductive barriers, introduce potentially adaptive genetic variation into a population driving ecological divergence, and lead to the formation of new lineages [4,5,6]. The study of hybrid zones has been a prolific area of research since it is key for understanding the factors that promote reproductive isolation, ecological adaptation, and speciation in natural and human-modified habitats [3, 10, 11]. The genus Bursera encompasses a diverse group of nearly 100 species of deciduous and resinous shrub and trees occurring from the south-western USA to Peru, the Galapagos, Bahamas, and the Greater Antilles. Bursera species have economic and cultural importance as they are used for the extraction of aromatic resins for religious and medicinal purposes [17], for elaboration of wood handcrafts, and for ex-situ propagation in restoration programs or as living fences in vast extensions throughout tropical landscapes [18]

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