ABSTRACT
Urbanization leads to complex environmental changes and poses multiple challenges to organisms. Amphibians are highly susceptible to the effects of urbanization, with land use conversion, habitat destruction, and degradation ranked as the most significant threats. Consequently, amphibians are declining in urban areas, in both population numbers and abundance, however, the effect of urbanization on population genetic parameters remains unclear. Here, we studied the genomic response to urbanization in two widespread European species, the common toad Bufo bufo (26 localities, 480 individuals), and the smooth newt Lissotriton vulgaris (30 localities, 516 individuals) in three geographic regions: southern and northern Poland and southern Norway. We assessed genome-wide SNP variation using RADseq (ca. 42 and 552 thousand SNPs in toads and newts, respectively) and adaptively relevant major histocompatibility complex (MHC) class I and II genes. The results linked most of the genetic differentiation in both marker types to regional (latitudinal) effects, which also correspond to historical biogeography. Further, we did not find any association between genetic differentiation and level of urbanization at local scales for either species. However, urban smooth newts, but not toads, have lower levels of within-population genome-wide diversity, suggesting higher susceptibility to the negative effects of urbanization. A decreasing level of genetic diversity linked to increasing urbanization was also found for MHC II in smooth newts, while the relationship between MHC class I diversity and urbanization differed between geographic regions. We did not find any effects of urbanization on MHC diversity in the toad populations. Although two genetic environment association analyses of genome-wide data, LFMM and BayPass, revealed numerous (219 in B. bufo and 7040 in L. vulgaris) SNPs statistically associated with urbanization, we found a marked lack of repeatability between geographic regions, suggesting a complex and multifaceted response to natural selection elicited by life in the city.
ABSTRACT
Phenotypic plasticity is a major factor contributing to variation of organisms in nature, yet its evolutionary significance is insufficiently understood. One example system where plasticity might have played an important role in an adaptive radiation is the threespine stickleback (Gasterosteus aculeatus), a fish that has diversified after invading freshwater lakes repeatedly from the marine habitat. The parallel phenotypic changes that occurred in this radiation were extremely rapid. This study evaluates phenotypic plasticity in stickleback body shape in response to salinity in fish stemming from a wild freshwater population. Using a split-clutch design, we detected surprisingly large phenotypically plastic changes in body shape after one generation. Fish raised in salt water developed shallower bodies and longer jaws, and these changes were consistent and parallel across families. Although this work highlights the effect of phenotypic plasticity, we also find indications that constraints may play a role in biasing the direction of possible phenotypic change. The slopes of the allometric relationship of individual linear traits did not change across treatments, indicating that plastic change does not affect the covariation of traits with overall size. We conclude that stickleback have a large capacity for plastic phenotypic change in response to salinity and that plasticity and evolutionary constraints have likely contributed to the phenotypic diversification of these fish.
