Morphological and genetic divergence in European postglacial nine-spind stickleback (Pungitius pungitius) populations

Background/Question/Methods

An important objective of ecology is to understand the processes that govern phenotypic variation in natural populations. We assessed patterns of morphological and genetic divergence among coastal and inland lake populations of nine-spined stickleback in northern Sweden. Coastal populations are either from the Baltic coast (n = 5) or from nearby coastal lakes (n = 3) that became isolated from the Baltic Sea (< 100 years before present, ybp). Inland populations are from freshwater lakes that became isolated from the Baltic approximately 10,000 ybp; either single species lakes without predators (n = 5), or lakes with a recent history of predation (n = 5) from stocking of salmonid predators (~50 ybp). We measured 11 morphological traits on ~30 fish from each location and used a Principal Components Analysis (PCA) to investigate patterns of morphological divergence between the four habitat types. Additionally, we investigated genetic divergence using F statistics and patterns of genetic diversity using standard methods. Patterns of genetic and morphological diversity were estimated using within-group P_ST (a proxy for Q_ST) from PCA scores to investigate whether patterns of morphological divergence are greater than random genetic drift.

Results/Conclusions

Baltic coast and coastal lake populations show little morphological and genetic variation within and between groups suggesting that these populations experience similar ecological conditions and that time since isolation of coastal lakes has been insufficient to demonstrate divergent morphology in coastal lake populations. Inland populations, on the other hand, showed much greater morphological and genetic variation characteristic of long periods of isolation. Inland populations from lakes without predators generally have larger body size, and smaller spine length relative to body size, suggesting systematic reduction in spine length. In contrast, inland populations with predators exhibit a wider range of spine lengths relative to body size suggesting that this trait is responding to local predation pressure differently among these populations. Taken together the results suggest that predation plays a role in shaping morphological variation among isolated inland populations.