The Genetics Behind Evolution

Highlight Annals of Botany cover
Highlight Annals of Botany cover

Over the years, hundreds of papers have been published describing the genetic changes that occur during evolution of plants, and the differences evident between individual species. An equally high number of papers has been published about the definition of species, their separation and naming. Many have joined discussions of where and why new species form – by reproductive barriers or island colonization for example. But only now are we realizing that there is a strong genetic component to these events: speciation does not ‘just happen’.

We plan most highlight issues of Annals of Botany a year of more in advance. But the seven papers for the September Highlight issue, “Genes in evolution”, was just because these papers were all submitted around the same time. This shows that the control of diversity and speciation is a really hot-topic, where advances unforseen a year ago are now taking place – and the implications are widespread from the consequences of changing climate on natural ecosystems, to the breeding and selection approaches for new and improved crops.

The review by Rieseberg and Blackman (2010) identifies no less than 41 genes that can lead to reproductive isolation of populations. Interestingly, the genes are very diverse, and include those which have prezygotic and postzygotic effects. Simple geographical isolation is not seen as the only widespread cause of speciation and the cessation of gene flow between populations; nevertheless, both larger and smaller features of geography lead to isolation of populations, and He et al. (2010) are able to show an example where the genetic structure of a species, Banksia hookeriana, is not solely dependent on the structure of a landscape, in this case where the population is located on sand dune crests physically separated by uninhabitable hollows. Occasional long-distance dispersal of seeds or movement of individuals from one population to another maintains connectivity within the species and prevents isolation. In contrast, Nomura et al. (2010) provide insight into the phylogeny and implications of habitat diversity. Farfugium (Asteraceae) is a monophyletic group associated with a wide range of habitats, including forest understorey (sciophytes), coastal crags (heliophytes) and riverbeds (rheophytes) in an archipelago in east Asia. They conclude that isolation on islands and subsequent parallel adaptation events followed migration over Quaternary land-bridges along the distribution range. Uninformative DNA sequence variation coupled with highly divergent morphologies suggest that adaptive diversification was rapid.
The other four papers make conclusions related to the genetic diversification and selection seen in crop plants. The Tehuacán Valley in Mexico provides a remarkable ‘natural laboratory’ for study of human selection effects on plants, because there are well over 100 native plant species where artificial selection is being practiced and these silvicultural and cultivated populations co-exist with wild populations. Parra et al. (2010) study the cactus Stenocereus puinosus and find that despite selection for larger and sweeter fruit, there are high levels of gene flow that have promoted morphological divergence and moderate genetic structure between wild and managed populations, while conserving genetic diversity. Hence, despite strong selection, there is no evident genetic bottleneck that might limit breeders of the fruit in future, unlike that seen in many other domesticated species.
As noted above, hybridization or polyploidy – speciation by whole-genome duplication – can lead to new, and reproductively isolated, species in a single event. Evidence of evolutionarily recent polyploidy events is seen in half of all plants, and contributes significantly to plant biodiversity. Shi et al. (2010) look at ancient events, or paleopolyploidy, that may be inferred from genomic data and their analysis of the nuclear genomes of kiwi fruit (Actinidia) and related Ericales shows evidence for at least two paleopolyploidy events. Their results provide evidence that gene-family methods are able to reliably uncover ancient polyploid speciation events.
Soybean, Glycine max, is an important tetraploid crop, but there are still gaps in its domestication history. Guo et al. (2010) carry out an extensive study of the diversity of soybean from its South China centre of origin using molecular markers, and propose a single origin with a moderately severe genetic bottleneck during domestication. Wild soybeans in this region have an unexploited and valuable gene pool for future breeding, but, unlike the situation in Stenocereus, it will require careful study and extensive crossing with selection to introduce this diversity into the germplasm pool available to plant breeders. The final paper in the Highlight presents an important example of how diversity from a wild species can be used in one of the world’s three most important grain crops. Rice, Oryza sativa, requires fertilization to set seed, but the temperatures experienced during hot weather (over 32–36 °C) induce sterility. Considering that such temperatures are rarely reached in the early morning, Ishimaru et al. (2010) introgressed an early-morning flowering (EMF) trait from wild rice, O. officinalis, where anthesis occurs soon after sunrise. Although the temperature effect on sterility itself was similar in the two rice species, the avoidance of high temperatures by a few hours caused by the early-morning flowering trait leads to significantly increased fertility in the line with introgression of the EMF trait.
Together, these seven papers add considerably to our understanding of genetics in a broad evolutionary context. The results are of significance for wild species, with implications for whole ecosystems, and indeed the species used as examples in these papers are those of coastal, island and duneland systems that are particularly threatened by changes including urban ‘development’, sea-level alterations, storm frequencies and temperature. The crop papers show how knowledge of the mechanisms of evolution and associated genes can impact on our exploitation of biodiversity in crops and their wild relatives.