Recent study compares the genetic diversity of three independent invasions.
If there is one species that deserves the adjective “common” in its name, it is definitely the Common Starling (Sturnus vulgaris). This noisy songbird can be found on every continent (except Antarctica). Starlings are native to the Palearctic but have been repeatedly introduced in other locations. In a previous blog post, I described how the Common Starling conquered Australia where it spread across the island after its release in the 1850s. Similar introductions occurred in North America as part of an American Acclimatization Society initiative to populate Central Park with the birds from Shakespeare’s plays. This initiative led to the release of 60 individuals in 1890 and an additional 40 in 1891. The introduction in South Africa was more modest with about 18 individuals being released around 1897. In each case, the starlings managed to get a foothold and establish stable populations. Their success has been attributed to their generalist diet and their ability to quickly change their migratory behavior.
These three independent introduction events (North America, Australia and South Africa) all started with small populations that quickly expanded. How has this impacted the genetic make-up of the current starling populations? A recent study in the journal Ecology and Evolution compared genetic diversity at the mitochondrial control region to answer this question.
Before we dive into the results of this study, we need to understand the concept of a haplotype. This commonly used term refers to a DNA sequence of genetic variants that are inherited as a whole. By comparing different haplotypes and identifying particular mutations, we can construct a haplotype network that visualizes the connections between the haplotypes. Imagine that we sequenced a short DNA sequence in four individuals:
- Individual 1: AAAA
- Individual 2: AAAA
- Individual 3: AATA
- Individual 4: AATC
We can clearly see that individuals 1 and 2 have the same haplotype (AAAA) while individual 3 has one mutation (T instead of A) and individual 4 has two mutations (TC instead of AA). The relationships between these four individuals can be depicted by circles (representing the haplotypes) connected by lines. The size of the circles indicates the number of individuals with a particular haplotype and the length of the line signifies the number of mutations separating two haplotypes. Hence, the haplotype network for our example looks like this. The different colors indicate the sampling locations: individuals 1, 2 and 3 come from one area (in blue), whereas individual 4 was found somewhere else (in green).
The researchers constructed a haplotype network using almost 1000 samples from North America, Australia, South Africa and the United Kingdom. They found a total of 64 unique haplotypes that revealed some interesting patterns. In the haplotype network, the South African samples form a separate cluster (in light gray) from the other locations. A more detailed look at the network shows that only one haplotype (H25) is shared by the other two non-native areas, namely North America and Australia. These results suggest that the different introduction events used Common Starlings from different parts of the United Kingdom. To pinpoint the exact source populations, more genetic data is needed.
Unsurprisingly, all three invasive populations showed a reduction in genetic diversity compared to the native populations from the UK, reflecting the genetic bottlenecks that occurred at the onset of the introductions. This finding indicates that a low level of genetic diversity of no insurmountable obstacle for the rapid spread of the Common Starling. However, not all invasive populations showed genetic signatures of this expansion. The researchers found genetic support for population expansion in both North America and Australia, but the analysis of South African data did not support a sudden expansion model. Possibly, the South African population is still in a lag-phase following the introduction. The population might be slowly expanding until it reaches a certain threshold that triggers a more explosive expansion. Or the spread of South African starlings is being slowed down by other processes, such as adverse climatic conditions or competition with native species. As always, more research is needed to figure this out. And while ornithologists keep studying these invasions, the Common Starlings will probably keep expanding.
Bodt, L. H., Rollins, L. A., & Zichello, J. M. (2020). Contrasting mitochondrial diversity of European starlings (Sturnus vulgaris) across three invasive continental distributions. Ecology and Evolution, 10(18), 10186-10195.
Featured image: Common Starling (Sturnus vulgaris) © Pierre Selim | Wikimedia Commons