SPECIES-INDEPENDENT, GEOGRAPHICAL STRUCTURING OF CHLOROPLAST DNA HAPLOTYPES IN A MONTANE HERB IPOMOPSIS (POLEMONIACEAE)

Two hypotheses have been proposed to explain the occurrence of hybrid zones between red-flowered Ipomopsis aggregata and white-flowered I. t enuituba. Either local adaptation to hummingbird and hawkmoth pollinat ors has given rise to sympatric (or parapatric) divergence of flower c olour and morphology: (primary intergradation at hybrid zones), or alt ernatively two previously allopatric species are coming into contact a t several geographical areas of secondary intergradation. We examined restriction site patterns in nuclear DNA (nrDNA), chloroplast DNA (cpD NA) and mitochondrial DNA (mtDNA) from populations of I. aggregata and I. tenuituba representing seven zones of sympatry. No variation was d etected in a 350-bp fragment of mtDNA and uninformative levels of vari ation were observed for nrDNA. We detected 22 potentially informative restriction site polymorphisms in cpDNA, all of which united geographi cal areas containing populations of both species. We detected no infor mative species-specific markers. Studies of other species (e.g. oaks) have detected similar species-independent geographical structure of cp DNA. However, in these cases secondary interegradation could be inferr ed 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.