A study of the range of volatile organic sulfur compounds produced by brassica plants has highlighted limitations to the use of Carboxen/PDMS fibers for their analysis by solid-phase microextraction (SPME). These fibers are sometimes advocated for the analysis of sulfur gases, but a quantitative comparison of analytical data derived by SPME and by direct gas sampling of standard mixtures of volatile low molecular weight sulfur compounds at 0.01-10 mg/L has identified potential errors associated with their use. Higher molecular compounds displace lower molecular weight compounds as a consequence of competition for active sites on the fiber, and the relative proportions of the components adsorbed onto the fiber depend on their ratio in the headspace. As their relative concentrations change from sample to sample, the varying interactions result in irregular analytical responses, reflected in erratic calibration curves. Standards containing single components are not valid; only a standard containing all components found in the sample to be analyzed, and at the same relative concentrations, is appropriate. In practice, this may preclude the use of the fibers for quantitative analysis of multicomponent mixtures.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.