Abstract
Natural hybridization plays a key role in the process of speciation. However, anthropogenic (human induced) hybridization of historically isolated taxa raises conservation issues. Due to weak barriers to gene flow and the presence of endangered taxa, the whitefish species complex is an excellent study system to investigate the consequences of hybridization in conservation. We focused on three naturally reproductively isolated whitefish taxa in Germany: the endangered, anadromous North Sea houting (NSH) and Baltic houting (BH), which were reintroduced after local extinction, and the commercially stocked European whitefish (EW). To evaluate the genetic integrity of each taxon, source and reintroduced populations of NSH and BH, and EW populations were characterized based on two mitochondrial and 17 microsatellite loci. Additionally, we investigated gill raker counts as an adaptive phenotypic trait. Even though clear genetic and phenotypic differentiation confirmed the houtings as separate evolutionarily significant units, admixture analyses revealed an extensive hybrid zone. Hybridizations were introgressive, positively correlated with genetic diversity, and were reflected in the gill raker counts. The BH distribution range showed higher heterogeneity and stronger admixture than the NSH range. Erroneous stocking with non-native genotypes best explained these patterns, which pose challenges for the conservation of the endangered NSH and BH.
Highlights
The present age has been coined ‘the sixth extinction’ (Leakey and Lewin 1996), as current extinction rates driven by anthropogenic impacts rival those of the five previous mass extinction events on earth
The distinctness of the three sources NSH_VID, BH_PEE, and European whitefish (EW) was sustained by multiple lines of evidence: (i) phenotypic differences in gill raker counts (GRCs), which is often used as a taxonomic trait, (ii) strong and significant genetic differentiation between
Even though the genetic integrity of the source populations of North Sea houting (NSH), Baltic houting (BH), and EW was maintained, our study documented the widespread presence of three-way hybridizations
Summary
The present age has been coined ‘the sixth extinction’ (Leakey and Lewin 1996), as current extinction rates driven by anthropogenic impacts rival those of the five previous mass extinction events on earth. Cryptic drivers contributing to biodiversity loss are introgressive hybridization and reverse speciation (Seehausen 2006) associated with habitat modifications and species translocations by humans (Allendorf et al 2001). The role of hybridization in conservation is a dilemma: Whereas natural hybridization plays an important evolutionary role in the process of speciation and the Hybridization in endangered whitefish maintenance of biodiversity (Dowling and Secor 1997; Nolte and Tautz 2009), hybridization induced by human activities can be considered as harmful as it threatens the integrity of ancestral species and has contributed directly or indirectly to species extinctions (Rhymer and Simberloff 1996). Allendorf et al (2001) framed the potential outcomes of such anthropogenic hybridizations into three clearly determined categories: (i) hybridization without introgression (sterile F1s only), (ii) widespread introgression, and (iii) complete admixture, which can result in reverse speciation (Seehausen 2006). Each case poses different conservation challenges, ranging from wasted reproductive effort due to sterile F1s under (i) to the loss of parental lines under (iii) (Allendorf et al 2001)
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