Abstract
AbstractIn the North Atlantic and the Arctic Ocean, four species of the copepod genus Calanus dominate the zooplankton biomass. Because of their morphological resemblance, knowledge of their respective distribution range has long been biased by misidentification, until the recent use of molecular tools uncovered numerous areas of sympatry. As hybridization between Calanus finmarchicus and Calanus glacialis has been claimed in the East‐Canadian Arctic based on microsatellites, we investigated further the potential for interbreeding in newly uncovered areas of sympatry. Calanus species and stage composition were analyzed during winter in two Norwegian subarctic fjords, using molecular markers developed specifically for species identification and hybrid detection between C. finmarchicus and C. glacialis. Overall, C. glacialis were the most abundant throughout the winter, followed by C. finmarchicus and Calanus hyperboreus with only a few records of Calanus helgolandicus. The presence of C. glacialis, C. hyperboreus, and C. finmarchicus' nauplii was recorded, indicating that these species reproduce locally. In January and February, the simultaneous occurrence of males and females of both C. finmarchicus and C. glacialis suggested a potential for interspecies mating. However, genetic admixture tests performed on all 1126 individuals revealed no signal of hybridization, implying a strong reproductive isolation mechanism. We conclude that no evidence supports a potential for hybridization between C. finmarchicus and C. glacialis.
Highlights
The most cost-effective and easiest way to reliably identify the Calanus species involves the use of a combination of six molecular markers type insertion/deletion (InDel) (Smolina et al 2014) following the optimized protocol described in Choquet et al (2017)
Atlantic and the Arctic Ocean (Nielsen et al 2014; Smolina et al 2014; Choquet et al 2017). Their performance was tested over thousands of individuals across the North Atlantic and the Arctic Ocean, and their validity was confirmed by comparison with traditional 16S mitochondrial DNA sequencing for species identification (Nielsen et al 2014; Smolina et al 2014; Choquet et al 2017)
C. finmarchicus, C. glacialis, C. hyperboreus, and C. helgolandicus were all genetically identified during the winter 2016–2017, confirming records from previous studies (Lindeque et al 2004; Choquet et al 2017)
Summary
In their study, Smolina et al (2014) selected InDel markers derived from partial sequences of genome and transcriptome from both C. finmarchicus and C. glacialis These markers are nuclear, they are inherited by both parents, and they were chosen to be codominant (both alleles are expressed when cooccurring in an individual). As a result, these InDel markers are the most reliable tools currently available for detecting putative hybrids between C. finmarchicus and C. glacialis (Nielsen et al 2014; Smolina et al 2014). There are, documented examples of hybridization occurring in freshwater zooplankton (e.g., Daphnia: Wolf 1987)
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