The sedge genus Carex, the most diversified angiosperm genus of the northern temperate zone, is known for its holocentric chromosomes and karyotype variability. Escudero et al. provide the first comprehensive study of population-level patterns of molecular and cytogenetic differentiation in the genus. They demonstrate dispersal and genetic connectivity among populations of the North American Carex scoparia that differ in chromosome numbers, demonstrating that cytogenetically variable sedge species can still cohere genetically. This finding is important to our understanding of what constitutes a species in one of the world’s largest angiosperm genera.
Genetic connectivity is crucial in rapidly changing environments as it allows exchange and dispersal of adaptive genes among plant populations. Matter et al. study patterns of historic gene flow, flowering phenology and contemporary pollen flow in two common herbs, Ranunculus bulbosus and Trifolium montanum, along an altitudinal gradient of 1200–1800 m a.s.l. among alpine meadows in Switzerland, a habitat type thought to be particularly sensitive to climate change. They determine that pollen-flow along the gradient is extensive, explaining the very low genetic differentiation along the mountain slope. Congruent with this finding, they find that despite the delay in flowering caused by altitude, the overlap in flowering periods is large enough to allow for extensive pollen dispersal between populations.
Afromontane forest ecosystems share a high similarity of plant and animal biodiversity, although they occur mainly on isolated mountain massifs throughout the African continent. Kadu et al. use Prunus africana, one of the character trees of the ecosystem, as a model for understanding the biogeography of this vegetation zone and find strong genetic divergence amongst the five main Afromontane regions, which are most likely associated with Pleistocene changes in climatic conditions. Contrasting estimates of recent and historical gene flow indicate a shift of the main barrier to gene flow from the Lake Victoria basin to the Eastern Rift Valley, highlighting the dynamic environmental and evolutionary history of the region.
Responses of high-mountain plant species to global change are highly influenced by the genetic background of the species, including genetic diversity or gene flow between populations. García-Fernández et al. study the genetic patterns of Silene ciliata (Caryophyllaceae), a high-mountain cushion plant that shows local adaptation to altitude, by examining populations along two altitudinal gradients on separate mountains in central Spain. They find that genetic diversity is similar in all populations, and although substantial gene flow is found both along altitudinal gradients and horizontally within each elevation belt, greater values are obtained along altitudinal gradients.
For many Australian plants little is known about either their population genetics or the effects on mating systems of variation in pollen transfer distances. Forrest et al. manipulate pollination of Grevillea mucronulata to allow assessment of the reproductive success of crosses made within and among populations at varying distances and find evidence of both inbreeding and outbreeding depression. Pollen from populations at intermediate distances consistently produces superior outcomes for most aspects of fitness. Natural matings may currently be suboptimal and involve largely near-pollen transfer.
Mycorrhizal fungi are needed for germination and seedling establishment in orchids, so associations may be involved in determining hybridization between species. Jacquemyn et al. show that in three species of the genus Orchis (O. anthropophora, O. militaris and O. purpurea) seeds originating from hybrid crosses readily germinate in the field, and protocorms show overlap in mycorrhizal associations with their respective parents. Thus associations appear to contribute little to reproductive isolation in these species.
How far can a gene disperse? Historical and contemporary gene dispersal can be estimated from spatial genetic structure and paternity analysis, and Rong et al. (pp. 285–296) find that an estimate of gene flow in Daucus carota ssp. carota based on contemporary pollen dispersal is much larger than an estimate of historical flow. The results have implications for the ease with which transgene flow might occur from cultivated GM carrots to wild carrot populations.
The perennial Arabidopsis lyrata is able to propagate both sexually and asexually, and this is expected to influence spatial genetic structure in the species. A fine-scale molecular study by Lundemo et al. (pp. 243–254) shows how a large, alpine population in Norway overall functions as a panmictic unit, while at the same time individual plants in smaller patches exhibit considerable clonal growth.