Lack of trophic polymorphism despite substantial genetic differentiation in sympatric brown trout (Salmo trutta) populations

Abstract

Sympatric populations occur in many freshwater fish species; such populations are typically detected through morphological distinctions that are often coupled to food niche and genetic separations. In salmonids, trophic and genetically separate sympatric populations have been reported in landlocked Arctic char, whitefish and brown trout. In Arctic char and brown trout rare cases of sympatric, genetically distinct populations have been detected based on genetic data alone, with no apparent morphological differences, that is "cryptic" structuring. It remains unknown whether such cryptic, sympatric structuring can be coupled to food niche separation. Here, we perform an extensive screening for trophic divergence of two genetically divergent, seemingly cryptic, sympatric brown trout populations documented to remain in stable sympatry over several decades in two interconnected, tiny mountain lakes in a nature reserve in central Sweden. We investigate body shape, body length, gill raker metrics, breeding status and diet (stomach content analysis and stable isotopes) in these populations. We find small significant differences for body shape, body size and breeding status, and no evidence of food niche separation between these two populations. In contrast, fish in the two lakes differed in body shape, diet, and nitrogen and carbon isotope signatures despite no genetic difference between lakes.These genetically divergent populations apparently coexist using the same food resources and showing the same adaptive plasticity to the local food niches of the two separate lakes. Such observations have not been reported previously but may be more common than recognised as genetic screenings are necessary to detect the structures. K E Y W O R D Sbody shape, geometric morphometrics, gill rakers, population genetic structure, stable isotopes, stomach content 1

Complex genetic diversity patterns of cryptic, sympatric brown trout (Salmo trutta) populations in tiny mountain lakes

“…However, such lakes are often substantially larger and deeper than those in the present study, potentially permitting ecological divergence between the populations. In our case, we find no indications of ecological niche separation between populations A and B (Andersson et al 2016). To our knowledge, there is only one additional case where cryptic, sympatric populations of brown trout have been detected within a very restricted geographical area, i.e.…”

“…The population structures occur in stable sympatry at about the same frequency within both lakes and show no obvious morphological dichotomies (Palmé et al 2013). An extensive screening for phenotypic differences of the two clusters detected only minor tendencies for morphological divergence, and no indication of trophic niche separation (Andersson et al 2016), in spite of a high genetic differentiation (F ST ≈ 0.1; Palmé et al 2013). Differences did occur between lakes, however, indicating the same pattern of plasticity with respect to diet and morphology within both genetic clusters (Andersson et al 2016).…”

“…An extensive screening for phenotypic differences of the two clusters detected only minor tendencies for morphological divergence, and no indication of trophic niche separation (Andersson et al 2016), in spite of a high genetic differentiation (F ST ≈ 0.1; Palmé et al 2013). Differences did occur between lakes, however, indicating the same pattern of plasticity with respect to diet and morphology within both genetic clusters (Andersson et al 2016). This lack of trophic niche separation is in agreement with suggested food web limitations of nutrient-poor subarctic lake ecosystems (Karlsson et al 2009).…”

“…that of Lakes Bunnersjöarna in central Sweden (Allendorf et al 1976;Ryman et al 1979). Neither of the populations within Lakes Trollsvattnen and within Lakes Bunnersjöarna can be discriminated by the naked eye (Ryman et al 1979;Andersson et al 2016), a feature that separates these two cases from other reports on sympatric brown trout populations.…”

“…Identifying and understanding patterns of population genetic structure is an important task in evolutionary and conservation biology, because such intraspecific, genetic biodiversity represents results of evolutionary processes that provide the basic units for future adaptation processes (Allendorf et al 2013). Research over the past decade has reported that genetic biodiversity can affect species productivity, diversity and viability (Reusch et al 2005;Lindley et al 2009), resilience to environmental stressors (Frankham 2005;Hajjar et al 2008;Hellmair and Kinziger 2014) and adaptation to changing environmental features including climate change (McGinnity et al 2009;Barshis et al 2013).…”