The arctic is a difficult region for life to colonise. Plants living here have to cope with massive swings in temperature and a short growing season. Surveys have found few species of plants growing here, but research by Lovisa Gustafsson and colleagues shows that some of the biodiversity in the arctic is hidden.
The study builds upon earlier work that found three species of Draba, or whitlow-grasses, may be more than three species. This unexpected diversity was due to reproductive isolation. While plants may look similar, they’re not reproducing with each other, and the hybrids they produce are sterile.
This speciation is a challenge to current evolutionary thought. As Grundt as colleagues write in their paper: “The Arctic flora has long been viewed as depauperate. Indeed, the decrease in biological diversity with increasing latitude is one of the oldest recognized patterns in ecology. Diversity has, however, typically been quantified as the number of morphological or ‘taxonomic’ species. Little is known about biological species diversity in the Arctic, because the recognition of biological species requires information on reproductive isolation.”
Grundt and colleagues propose that arctic Draba has developed reproductive isolation to create new species. A problem with the idea of reproductive isolation proposed by these authors is that it is rapid. Reproductive isolation takes time to develop. How much time? In this paper, Gustafsson and colleagues refer back to earlier work that finds it needs four million or even eight million years to develop. They point out this is a problem when estimates of the arctic tundra’s age are between two and three million years.
So is Draba a freak, or is something else happening in the arctic that we don’t know about?
To find out, Gustafsson and colleagues experimented with species collected across the circumpolar Arctic. They selected six species in four distantly related families. They then started crossing plants thought to be the same species gathered at various distances from each other. This produced over seven hundred plants that the team raised to maturity. They then tested to see if the hybrids were sterile. If the plants were of the same species, then you’d expect to have fertile offspring. Gustafsson and colleagues then took their study a step further than the earlier work by Grundt and colleagues. They looked at the hybrid seeds.
They referred back to other work that found that even when hybrids produced seeds, those seeds might not be viable, resulting in hybrid seed failure. It is known that hybrid seed failure can develop rapidly. This failure usually occurs when something interferes with the development of the endosperm, the part of the seed that provides the energy to grow.
The results were striking. Gustafsson and colleagues write, “We found that strong hybrid incompatibilities also have developed within five (of the six examined) Arctic species belonging to other genera of the Brassicaceae (Cardamine bellidifolia and Cochlearia groenlandica) as well as to other, distantly related plant families; Saxifragaceae (Saxifraga hyperborea), Ranunculaceae (Ranunculus pygmaeus) and Caryophyllaceae (Silene uralensis). Remarkably, most populations examined of each of these five taxonomic species as well as of the three examined by Grundt et al. (2006) have accumulated hybrid incompatibilities and can therefore be considered to represent incipient species. Future studies are needed to address potential morphological differentiation among these populations, but it is notable that no or only little intraspecific morphological variation has been reported within these species in the taxonomic literature…”
The findings are a bit of a surprise. There is thought to be a latitudinal gradient in species diversity. The tropics are considered hotspots where you find most of the diversity, and as you move to the poles, the number of species decreases. It’s an idea formed by not recognising some of the diversity in the arctic. Rather than the forces driving speciation in the tropics merely being stronger, they may be different.
One driver for this difference could be genetic bottlenecks. Having a small genetic pool could lead to the development of genetic incompatibilities between populations. This is supported by work that shows a decrease in genetic diversity away from Beringia as plants colonised the arctic after the last glaciation.
In tandem with this, unreliable pollinators mean that self-pollination can be the dominant reproductive strategy for plants. They cease, or at least dramatically reduce, the exchange of genes with other populations, helping create barriers between plants. The authors write that selfing was indeed an issue. “Our results are remarkable in that all of the five selfing species we successfully crossed showed widespread intraspecific hybrid sterility, whereas the single outcrossing species for which we were able to obtain crossing data, Silene acaulis, did not .”
“Our results showing widespread and rapid incipient speciation in Arctic plants stand in strong contrast to a view of the Arctic as an “evolutionary freezer” with low evolutionary rates and little species diversity due to extreme environmental constraints such as low temperature, a short growing season, and drought…,” write Gustafsson and colleagues. “Rather, our results add to a growing body of evidence suggesting that both plants and animals at high latitudes may show higher speciation rates, as well as higher extinction rates, compared to the tropics…”
Gustafsson, A.L.S., Gussarova, G., Borgen, L., Ikeda, H., Antonelli, A., Marie-Orleach, L., Rieseberg, L.H., Brochmann, C., 2021. Rapid Evolution of Postzygotic Reproductive Isolation is Widespread in Arctic Plant Lineages. Annals of Botany. https://doi.org/10.1093/aob/mcab128