This post is part of our series covering the large number of urban evolution manuscripts that were published in August in the special issue “The Evolution of City Life” in the Proceedings of the Royal Society B.
Urban environments are often composed of novel communities of native and introduced species. In several cities in Germany, non-native lineages of the common wall lizard (Podarcis muralis) have been introduced. This provided Joscha Beninde (postdoctoral researcher, UCLA) and coauthors an opportunity to investigate the role of invasion history and hybridization in the success of these urban lizards. In their paper, “Admixture of hybrid swarms of native and introduced lizards in cities is determined by the cityscape structure and invasion history“, Beninde and team sampled 826 lizards across four German cities to look at fine-scale genetic patterns in these admixed native and introduced populations. In particular, they were interested in understanding how native and non-native lineages were mixing, how gene flow was impacted by urban landscape features, and if admixture contributed to gene flow.
Admixture
Depending on the fitness of hybrids, when native and non-native lineages extensively hybridize various outcomes can arise including adaptive introgression (the incorporation of novel genes into native populations), stable hybrid zones (a genetic cline between unique lineages), admixture (hybridization), and potentially even hybrid speciation. Beninde and co-authors sought to understand the genetic history of the introduced lineages as well as if/how they were mixing with native lineages.They did this by sampling different cities that varied in their introduction history: two with native only lineages and two with non-native lineages mixed with native lineages (or so they thought).
To their surprise, they detected non-native lineages (only 55% native haplotypes) in one of the cities (Saarbrücken) for which they assumed had native only lineages. In the other cities: they found 100% native haplotypes in Trier as expected and both native and introduced lineages in Mannheim (10% native haplotypes) and Freiburg (34% native haplotypes). In cities that had non-native lineages, the native and non-native lineages formed clusters of hybrid swarms composed from 3-4 different lineages. Moreover, Beninde found that the hybrid swarms within cities were each composed of unique combinations of non-native lineages, and in Saarbrücken and Freiburg the hybrid swarms admixed with each other leading to completely new genetic compositions.
Figure 2 from Beninde et al. (2018) – admixture in each of the four cities sampled with yellow representing native mitochondrial lineages and all other colors non-native.Gene Flow
The authors were also interested in understanding what landscape features hindered or facilitated gene flow in each city. Other studies have found various landscape features to be correlated with movement in cities: green-space facilitates movements in small mammals, while rivers, roads, and railways are corridors for some species and barriers for others. These variable effects across species led the authors to also ask if landscape features had variable effects on gene flow within a species in different cities and between populations differing in genetic admixture.
They found that landscape features had different effects on gene flow in different cities. Across all cities, waterways were the strongest barriers to gene flow for all lizards. Other features, such as buildings and vegetation, impacted gene flow significantly in some cities but not others. Even more interesting, the landscape features appear to have variable effects not only across cities, but within cities depending on the genetic lineages: railroads facilitated gene flow for admixed populations but reduced gene flow in the native populations!
Figure 3 from Beninde et al. (2018) – different landscape features facilitated gene flow in different cities. In Trier (TR), water, buildings, and tree cover impeded gene flow. In Saarbrücken (SB), vegetation facilitated gene flow. In Mannheim (MA), water was a barrier to gene flow. In Freiburg (FR), railroads and rocky/gravel substrates facilitated gene flow while roads impeded gene flow.What does this tell us about urban lizards?
Beninde’s results are interesting for several reasons. To start, the fact that they detected non-native lineages in a city for which they believed had not had any introductions suggests that these non-native introductions are probably more widespread than researchers realize. This made me wonder how common this is in other species and what this means for urban evolutionary research if there is substantial undetected admixture in urban populations. I also think it is fascinating that landscape features have differential effects between cities and also between populations depending on their history of admixture. Features that are barriers or facilitators of gene flow in one population are not necessarily barriers/facilitators in others! Finally, that multiple lineages are admixing to create populations with completely novel genetic compositions is intriguing. If this elevated genetic diversity includes an increase in diversity of adaptive traits, then hybrid swarms may be uniquely suited to rapidly adapt to the environmental changes associated with urbanization.
Note: Header image is by Nino Barbieri, Wikimedia Commons
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