Aim

The species of the genus Chionodraco (Notothenioidei) are the most abundant icefish on the continental shelf of the Weddell Sea. While previous studies indicated that only Chionodraco hamatus and Chionodraco myersi inhabit the Weddell Sea, the third Chionodraco species, Chionodraco rastrospinosus, was recently sampled in the area. Since C. rastrospinosus is supposed to be found only at the Antarctic Peninsula and Scotia Arc, this study aimed at confirming the species classification of C. rastrospinosus by molecular methods and identifying its putative source population. Given the documented evidence of introgression among the three species, we tested whether the newly found C. rastrospinosus shared any genetic variability with the other Chionodraco species. To explain the pattern of distribution of the Chionodraco species, we aimed at estimating the hydrodynamic connectivity between the Antarctic Peninsula and the Weddell Sea.

Location

Antarctic Peninsula, southern Scotia Arc and the south-eastern Weddell Sea

Methods

We genotyped 19 microsatellites and sequenced the mitochondrial D-loop for 560 Chionodraco individuals. We simulated the dispersal of more than 3 million drifters (Lagrangian model).

Results

The molecular analyses support the presence of C. rastrospinosus in the Weddell Sea and its homogeneity with C. rastrospinosus from the Antarctic Peninsula. Bayesian clustering identifies three putative hybrids among C. rastrospinosus and the other congenerics. Lagrangian simulations do not support connectivity driven by the oceanographic features of the Antarctic Peninsula and Weddell Sea via passive larval dispersal only.

Main conclusions

This study documents, for the first time, the presence of C. rastrospinosus in the Weddell Sea unveiling more biodiversity than previously known in this region. The sympatry of the three Chionodraco species explains the occurrence of occasional, ongoing events of hybridization in the genus. Alternative possible hypotheses need to be tested in future studies about the mechanisms maintaining the interspecific connectivity in Chionodraco spp.

Panel of 19 microsatellites loci for 564 individuals belonging to the species Chionodraco hamatus, Chionodraco myersi and Chionodraco rastrospinosus. All loci were previously characterized for the three Chionodraco species by Agostini et al. (2013). Fragment lengths were estimated by Genoscreen (www.genoscreen.fr) on a ABI PRISM 3700 DNA Analyzer (Applied Biosystems). Scoring and binning were performed by two operators, independently, with Peak Scanner ver. 1.0 (Applied Biosystem) and with the Excel macro Flexibin (Amos et al., 2007) respectively. The input files were generated with CREATE ver. 1.38 (Coombs, Letcher, & Nislow, 2008).

Agostini, C., Papetti, C., Patarnello, T., Mark, F. C., Zane, L., & Marino, I. A. M. (2013). Putative selected markers in the Chionodraco genus detected by interspecific outlier tests. Polar Biology, 36(10), 1509–1518. https://doi.org/10.1007/s00300-013-1370-0

Amos, W., Hoffman, J. I., Frodsham, A., Zhang, L., Best, S., & Hill, A. V. S. (2007). Automated binning of microsatellite alleles: Problems and solutions. Molecular Ecology Notes, 7(1), 10–14. https://doi.org/10.1111/j.1471-8286.2006.01560.x

Coombs, J. A., Letcher, B. H., & Nislow, K. H. (2008). Create: A software to create input files from diploid genotypic data for 52 genetic software programs. Molecular Ecology Resources, 8(3), 578–580. https://doi.org/10.1111/j.1471-8286.2007.02036.x

There are four different dataset in Genepop format: one that includes all species ("all-chionodraco-species") and one for each species separately ("hamatus", "myersi", "rastrospinosus").
Each dataset, according to the Genepop format, is made by two files: the proper microsatelite dataset (e.g. "name-file-GENEPOP.txt") and a file showing the name of the samples which the dataset is divided in (e.g. "name-file-POPULATION-NAMES.txt").

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