Species‐independent, geographical structuring of chloroplast DNA haplotypes in a montane herb Ipomopsis (Polemoniaceae)

Abstract

Two hypotheses have been proposed to explain the occurrence of hybrid zones between red‐flowered Ipomopsis aggregata and white‐flowered I. tenuituba. Either local adaptation to hummingbird and hawkmoth pollinators has given rise to sympatric (or parapatric) divergence of flower colour and morphology (primary intergradation at hybrid zones), or alternatively two previously allopatric species are coming into contact at several geographical areas of secondary intergradation. We examined restriction site patterns in nuclear DNA (nrDNA), chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) from populations of I. aggregata and I. tenuituba representing seven zones of sympatry. No variation was detected in a 350‐bp fragment of mtDNA and uninformative levels of variation were observed for nrDNA. We detected 22 potentially informative restriction site polymorphisms in cpDNA, all of which united geographical areas containing populations of both species. We detected no informative species‐specific markers. Studies of other species (e.g. oaks) have detected similar species‐independent geographical structure of cpDNA. However, in these cases secondary interegradation could be inferred from species‐specific nuclear alleles. The pattern in Ipomopsis is consistent with both primary intergradation (independent speciation in each area of sympatry) or secondary intergradation involving complete cytoplasmic replacement. Thus, additional data are needed to explain the origin of hybrid zones in Ipomopsis.