In ecological studies, ants are commonly used as bio-indicators because they are able to respond to rapid changes in their environments. This also makes them a great system to use in urban studies given the rapid changes that urbanization poses on ecosystems. However, there studies that look at the evolutionary patterns of ants in response to urbanization are rare.
Recently, Khimoun et al., investigated how urban and forest populations in Temnothorax nylanderi, a forest leaf litter-dwelling, cavity-nesting, small-sized predatory ant responded to urbanization. They had a replicated design of three highly populated cities in France – Bordeaux (BorC), Lyon (LyoC) and Paris (ParC) – with a paired forest locales that were 30-50 km away from the cities and 12 colonies sampled per locale. The authors were interested in exploring (i) genetic diversity, population structure, and demography and (ii) signatures of selection between urban and forest locales. To account for the entire geographic distribution of these acorn ants, the authors added an additional two forest locations.
To identify genetic diversity and signatures of selection, genome-wide molecular markers were generated using a RADseq protocol that results in thousands of SNPs.To look at genetic diversity, observed and expected heterozygosity were calculated. To look at population structure, a principal component analysis was done as well as calculating pairwise FST. To look at potential signatures of selection, two complementary random forest variable selection procedures were implemented.
Genetic Diversity
Similar values for paired sites were observed for both observed and expected heterozygosity. The authors found that populations within each site, regardless of habitat type, were in equilibrium. There was no significant signature of urbanization influencing genetic diversity.
Population Structure
Both the PCA and the FST analyses suggest that there is no significant population structure between colonies either. In the PCA, in the first 20 axes, there was no separation by habitat (Figure 1). The FST values between populations were quite low and non-significant (global FST=0.028).
Demographic History
The demographic history was inferred using stairway plots and abc-skyline. For both of these analyses, there appeared to be a population decline, but this was consistent across all habitat types and happened ~35,000 years ago. This suggests that urbanization is not the factor that caused the decline.
Selection
There were 19 SNPs that showed signatures of selection to the urban environment. Of these 19 SNPs, there were 5 loci that aligned to known genes. Two of these loci were identified as a protein kinase, involved in molecular reactions. One locus was associated with a protein involved in mRNA processing. One locus was associated with an integral component of the membrane. The last locus was associated with genes correlated to caste phenotypes.
Conclusions
Urbanization did not significantly influence genetic diversity, population structure, or demographic patterns in the acorn ant. There was, however, a signal of adaptation to the urban environment. The signatures of selection here are associated with molecular components and do not yet seem to show extreme phenotypic changes. One of the loci under selection is associated with sociality, which is in agreement with other recent phenotypic studies. However, more research into changing phenotypes is needed.
I am interested in urban ecology and evolution. Urbanization can have drastic impacts on local ecosystems, both negative and positive, and creates a new environment for organisms to experience. To understand the impacts that urbanization plays on organisms, I look through the lense of population genetics and molecular evolution.
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This is very interesting, in the invasive Argentine ant (Linepithema humile) decreased antagonistic recognition between colonies is partly what facilitated their global spread. Check out this paper to go further down the ant hole (https://doi.org/10.1111/j.1365-294X.2008.04056.x)! Great write up!!