Morphological and genetic differentiation and reproductive isolation among closely related taxa in the Ipomoea series Batatas

Premise of the study: Identifying recently diverged taxa can be useful for studying the process of speciation. Ipomoea lacunosa and I. cordatotriloba, along with a putative homoploid hybrid, I. ×leucantha, are closely related taxa, which are promising for investigating the early stages of speciation. The objectives of this investigation were to determine how distinct these purported taxa are morphologically and genetically, and to assess the magnitude of reproductive isolation among the taxa.

Methods: We measured morphological characteristics and determined genotypes at four microsatellite loci in several populations of each of the taxa in North Carolina and South Carolina to quantify genetic and morphological differentiation. We also included a previously undescribed fourth taxon, which we term 'I. austinii'.

Key results: Our study revealed that all four taxa had distinct but overlapping geographical ranges, and had significantly distinct morphologies. Patterns of microsatellite variation and the results of crosses indicate that I. ×leucantha and I. austinii are morphologically and genetically distinct taxa. Each exhibits substantial reproductive isolation from the other three taxa. By contrast, microsatellite markers indicate that I. lacunosa and I. cordatotriloba exhibit little differentiation at neutral markers, despite substantial morphological differentiation, and exhibit some reproductive isolation.

Conclusion: I. ×leucantha and I. austinii should be considered separate species. Our results provide no evidence that either species originated through homoploid hybrid speciation. I. cordatotriloba and I. lacunosa should be considered incipient species, but may be experiencing considerable reciprocal gene flow.