Natural hybridization between two butterfly bushes in Tibet: dominance of F1 hybrids promotes strong reproductive isolation

Rongli Liao,Weibang Sun,Yongpeng Ma

doi:10.1186/s12870-021-02909-7

Abstract

BackgroundIt has been recognized that a certain amount of habitat disturbance is a facilitating factor for the occurrence of natural hybridization, yet to date we are unaware of any studies exploring hybridization and reproductive barriers in those plants preferentially occupying disturbed habitats. Buddleja plants (also called butterfly bush) generally do grow in disturbed habitats, and several species with hybrid origin have been proposed, based solely on morphological evidence.ResultsIn the present study, we test the hypothesis that B. × wardii is of natural hybridization origin in two sympatric populations of three taxa including B. × wardii and its parents (B. alternifolia and B. crispa) plus 4 referenced parental populations, using four nuclear genes and three chloroplast intergenic spacers, as well as with 10 morphological characters. Our results suggest that at both sites B. × wardii is likely to be a hybrid between B. alternifolia and B. crispa, and moreover, we confirm that most of the hybrids examined are F1s. That these plants are F1s is further supported by morphology, as no transgressive characters were detected. B. crispa was found to be the maternal parent in the Bahe (BH) population, from cpDNA evidence. However, in the Taji (TJ) population, the direction of hybridization was difficult to establish due to the shared cpDNA haplotypes between B. alternifolia and B. crispa, however we still predicted a similar unidirectional hybridization pattern due to results from cross-specific pollination treatments which supported the “SI × SC rule”.ConclusionsThe presence of mainly F1 hybrids can successfully impede gene flow and thus maintain species boundaries in parental species in a typical distribution of Buddleja, i.e. in disturbed habitats.

Highlights

It has been recognized that a certain amount of habitat disturbance is a facilitating factor for the occurrence of natural hybridization, yet to date we are unaware of any studies exploring hybridization and reproductive barriers in those plants preferentially occupying disturbed habitats
The leaf width (L/W) ratio was significantly larger in B. alternifolia than in the other two taxa (Table 1)
Of the seven floral characters, corolla tube width (TW) and anther height (AH) in B. × wardii were intermediate between the values of the two assumed parental species, whereas herkogamy (HE) did not differ significantly between the three taxa (Table 1)

Summary

Introduction

It has been recognized that a certain amount of habitat disturbance is a facilitating factor for the occurrence of natural hybridization, yet to date we are unaware of any studies exploring hybridization and reproductive barriers in those plants preferentially occupying disturbed habitats. To date we are unaware of any studies exploring hybridization and reproductive barriers in those plants preferentially occupying disturbed and/or newly alternated habitats. F1s have been often inaccurately described as new species, especially by taxonomists concerned with morphology. An example of this is Rhododendron agastum from Yunnan, China, which has long be treated as a good species [24]. Its hybrid origin has been confirmed, and population studies at the type locality suggest that most hybrids are F1s [13]

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