The utility of the morphological variation of pollen for resolving the evolutionary history ofBillia(subfam. Hippocastanoideae, Sapindaceae)

Abstract

AbstractIn this study, we examined the utility of pollen morphology for resolving questions about the evolutionary history ofBillia, which is a poorly known genus of Neotropical trees.Billiahas been traditionally circumscribed with two species and treated as sister toAesculusL. However, the number of species inBilliais uncertain, because the genus exhibits abundant morphological diversity but little discontinuous variation. Therefore,Billiamay be monotypic and highly polymorphic, or it may have two species with blurred boundaries due to incipient speciation and/or hybridization. Moreover, one recent molecular phylogenetic study showsBillianested withinAesculus. Our work sought to address the following questions: (i) Are there discontinuities in the pollen ofBilliathat may suggest species boundaries? (ii) Does the pollen ofBilliashow evidence for inter‐specific hybridization? (iii) Do the exine morphology and size of pollen inBilliadiffer from those inAesculus? Our results from scanning electron microscopy showed that pollen exine morphology is not taxonomically informative inBilliabut that there are significant differences in pollen size between red‐ and white‐flowered individuals. Thus, our pollen data support the utility of flower color inBilliafor species delimitation. Our assessments of pollen viability do not support hybridization in the genus, but cannot be used to rule it out. Finally, pollen exine morphology may lend some support to an evolutionary origin ofBilliawithin eastern North AmericanAesculus. In contrast, data on pollen size suggest thatBilliamay belong in a topological position outside ofAesculus.