Genetic differentiation between and within ecotypes of pike (Esox lucius) in the Baltic Sea

Abstract

1. Aquatic systems often lack physical boundaries for gene flow, but ecological and behavioural barriers can form surprisingly fine spatial scale genetic patterns that challenge traditional, large scale management. To detect fine spatial scale structures, understand sources of intraspecific diversity, and design appropriate management plans requires identification of reproductively isolated units.2. This study reports on genetic differentiation in pike (Esox lucius) within a coastal area stretching 55 km from south to north in the central Baltic Sea. Pike is here an economically and ecologically important top predator that has declined in abundance.However, population structures have mostly been studied on large spatial scales, and without considering the potential for genetic divergence between the sympatric anadromous fresh water and the resident brackish water spawning ecotypes.3. To this end, 487 individuals from the east coast of Sweden and the island of Öland, representing sympatric anadromous and resident spawning individuals, categorized to ecotype based on spawning location or otolith microchemistry, were genotyped for 10 microsatellites and used to test for divergence between ecotypes. Furthermore, divergence between regions (island/mainland), neighbouring spawning locations (n = 13) and isolation by distance within and between regions were evaluated for the anadromous ecotype.4. The results revealed strong genetic differences between regions, between spawning locations separated by as little as 5 km and the first evidence of genetic differentiation between resident and anadromous ecotypes; despite a high dispersal capacity of pike and a high connectivity within the study area. The signatures of isolation by distance indicated that connectivity among populations differed between regions, probably reflecting availability of spawning habitats. 5. To safeguard against the challenges and uncertainties associated with environmental change, adaptive conservation management should aim to promote high intrapopulation functional genetic diversity without compromising the continued integrity and coexistence of the different ecotypes and of locally adapted sub-populations.

Comparing the Performance of Microsatellites and RADseq in Population Genetic Studies: Analysis of Data for Pike (Esox lucius) and a Synthesis of Previous Studies

“…Unfortunately, declines in pike populations in the Baltic Sea have been observed during the last decades (Lehtonen et al, 2009;Ljunggren et al, 2010;Nilsson et al, 2019;Olsson, 2019), and management actions, e.g., restoration of spawning habitats, have therefore been conducted to support the populations (Larsson et al, 2015). Population assignments that could be used to aid in decisions concerning such management efforts have mainly been based on studies utilizing neutral microsatellite markers Wennerström et al, 2016;Eschbach et al, 2019;Nordahl et al, 2019).…”

“…The differences in the ability to detect population structure, both in the present and the INTRODUCTION Investigations of population genetic structure reveal patterns of biological diversity and inform about the relative importance of underlying evolutionary drivers, such as gene flow, genetic drift and selection (Loiselle et al, 1995). For example, evidence of genetic differentiation among populations is indicative of restricted gene flow, and informs about geographical and/or ecological barriers to dispersal or gene flow (Yıldırım et al, 2018a;Nordahl et al, 2019). Strong genetic differentiation may also reflect divergent selection (Charlesworth and Charlesworth, 2017).…”

“…Population genetic studies can thus provide insights into how the different processes have influenced the evolution of populations and species (Charlesworth and Charlesworth, 2017). Knowledge about population genetic structure is also essential when designing conservation efforts (e.g., translocations, supplementations and (re)-introductions) to ensure successful management (Stephenson, 1999;Hutchinson, 2008;Wright et al, 2015;Nordahl et al, 2019).…”

“…Most microsatellites behave as selectively neutral markers, but some have been found to be influenced by selection (e.g., in Cortazar-Chinarro et al, 2017), presumably because of their linkage to other loci that are under selection. Because of the high mutation rate of microsatellites, they are highly polymorphic markers that can inform about recent population divergence (Edwards and Bensch, 2009;Zink et al, 2013;Hodel et al, 2017), and have proven successful in detecting population structure even among closely related populations (e.g., Wei et al, 2013;Yıldırım et al, 2018a;Nordahl et al, 2019). Due to logistical constrains, microsatellite studies have traditionally used a relatively low number of markers (<25), which brings about a risk of underestimating genetic structure due to insufficient numbers of polymorphic loci (Hodel et al, 2017).…”

“…In view of the current development, it seems important to systematically evaluate and compare the utility and relative performance of the two different marker types when applied to the same set of sampling populations and individuals. Given that knowledge and understanding of genetic structure is required for informed decision making pertaining to protection and utilization of biodiversity (Stephenson, 1999;Hutchinson, 2008;Wright et al, 2015;Nordahl et al, 2019), it is also necessary to investigate whether any differences in the relative performance (e.g., resolution and conclusiveness of estimates of population genetic structure) of microsatellites and RADseq markers are context specific or consistent across species, populations, and environmental settings.…”