Recently radiated groups pose a taxonomic challenge even with extensive molecular data, as they may be genetically differentiated only at small and/or patchy regions of the genome. Griffin and Hoffmann investigate species’ structure among the Poa that dominate the Australian alpine zone, which have radiated in the last 0.5–1.2 million years. Using a Bayesian approach to co-estimate nuclear and chloroplast gene trees with an overall dated tree, they find that most species are not genetically distinct, despite distinguishable phenotypes, which suggests recent adaptive divergence with ongoing inter-taxon gene flow.
Clonal spread influences genetic structure and diversity in plant populations as well as their realized outcrossing rate. Using microsatellite markers, Somme et al. investigate genetic diversity and the effect of clone distribution, structure and size on the mating of bee-pollinated marsh cinquefoil, Comarum palustre (Rosaceae), which is a rare, self-compatible species that grows in endangered European wetlands.
They find that clones are spatially clumped, with intermediate to no intermingling of the ramets, and large clones show lower outcrossing rates than small clones. Pollen dispersal mainly occurs within patches with very few pollination events occurring between patches of more than 25 m separation.These factors need to be taken into account in management strategies for ensuring population persistence.
There’s a handy article available from the American Journal of Botany that’s caught my eye: Is gene flow the most important evolutionary force in plants? by Norman C. Ellestrand. It opens with a strong statement.
Some scientists consider the word “evolution” to be more or less equivalent with “natural selection” or adaptation. They would, of course, be wrong.
Ellestrand states that biological evolution is the change in allele frequencies in a population over time, and that this is due to four evolutionary forces: mutation, selection, drift, and gene flow. Gene flow is important because even low levels of gene flow can have a large impact, counteracting the other evolutionary forces.
So what is gene flow?
The evolutionary relationships of present-day species are the result of genetic drift due geographical isolation, gene flow and mutations. Zou et al. use population genetic data to determine interspecific relationships, speciation patterns and gene flow between three Asian spruce species with a similar morphology, Picea wilsonii, P. neoveitchii and P. morrisonicola. Modelling of the data supports the hypothesis that P. morrisonicola derived from P. wilsonii within the more recent past, most probably indicating progenitor-derivative speciation with a distinct bottleneck, although further gene flow from the progenitor to the derivative continued. They conclude that the extent of mutation, introgression and lineage sorting taking place during interspecific divergence and demographic changes in the three species has varied greatly between the three genomes.
Riparian systems are prone to invasion by alien plant species, which may be facilitated by hydrochory, the transport of seeds by water. Love et al. study gene flow associated with hydrochoric dispersal of the invasive riparian plant Impatiens glandulifera (Himalayan balsam) in two contrasting river systems and find a significant increase in levels of genetic diversity downstream, consistent with the accumulation of propagules from upstream source populations. There is strong evidence for organisation of this diversity between different tributaries, reflecting the dendritic organisation of the river systems studied. The results indicate that hydrochory, rather than anthropogenic dispersal, is primarily responsible for the spread of I. glandulifera in these river systems.
Rapid development of biotechnology offers new opportunities to ensure our future food supply. Novel traits can be introduced into crops by transgene technology more efficiently than by conventional crop breeding. Since the first commercialization of a genetically modified (GM) crop in 1996, the global area of GM crops has grown steadily and reached 170·3 million hectares in 2012. Increasing numbers of GM crops with different traits are being produced and released into the environment. The introduction of new transgenes into crops has raised concerns about possible negative effects on the environment. Transgenes could move from crops into wild relatives via gene flow. Depending on the nature of the transgene and its product, such transgene flow may lead to unwanted ecological and evolutionary consequences in wild populations
The process of transgene flow from crops into wild relatives involves several steps: first, the formation of crop–wild hybrids with a transgene through hybridization between crops and wild populations; second, the establishment of the transgene in local wild populations through backcrossing with wild plants; third, the spread of the transgene across the whole metapopulation of the wild species via pollen and seed dispersal. The majority of previous studies have focused only on evaluating the first two steps of transgene introgression. A recent paper in Annals of Botany examines the role of metapopulation dynamics in transgene spread.
Wild populations close to crop fields are usually strongly affected by human disturbance. Habitat loss and fragmentation due to human disturbance may alter the level of gene flow among patches and the rate of patch turnover. If gene dispersal becomes limited under strong human disturbance, the distribution pattern of genetic diversity may change dramatically in the metapopulation. In this case, a newly emerged gene, such as a transgene in a local wild population, may not be able to spread through the metapopulation. Conversely, human-mediated dispersal may enhance connectivity among populations in areas where anthropogenic disturbance is high, which would lead to increased spread of an escaped transgene. However, it is difficult to study the effects of human disturbance and associated habitat changes on gene flow, because finding intact wild populations as controls is hard and the effects of other factors may interfere with those of human disturbance.
The authors compared historical and contemporary patterns of gene flow in a wild carrot metapopulation, testing the null hypothesis that human disturbance did not change gene flow in the metapopulation and that contemporary gene flow was similar to historical gene flow in wild carrots and aiming to answer the following questions:
- What is the rate of gene flow in the wild carrot metapopulation?
- Is contemporary gene flow equal to historical gene flow in the wild carrot metapopulation?
- How does the rate of gene flow affect the chance of transgene introgression into the wild carrot metapopulation?
They found that the contemporary gene flow was five times higher than the historical estimate, and the correlation between them was very low. Moreover, the contemporary gene flow in roadsides was twice that in a nature reserve, and the correlation between contemporary and historical estimates was much higher in the nature reserve. Mowing of roadsides may contribute to the increase in contemporary gene flow.
The potential risks of genetically modified crops must be identified before their commercialization. In this context, several studies have reported the transfer of transgenes from transgenic rice to red rice weed. However, gene flow also occurs in the opposite direction, resulting in transgenic seeds that have incorporated the traits of wild red rice. In a new study in AoB PLANTS, Serrat et al. found that this reverse flow was higher than direct gene flow, but that transgenic seeds carrying wild genes remained in the spike and were thus mostly removed at harvesting. Nevertheless, this phenomenon must be considered in fields used for elite seed production and in developing countries where there is a risk of increasing GM red rice weed infestation.
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.