DNA Sequence Support for Reclassification of the Endemic Southern Caspian Sea Deepwater Goby as Neogobius bathybius (Formerly Ponticola; Perciformes: Gobiidae) and Recent Population Expansion of a Continuous Population
The taxonomic status and phylogenetic relationships of the Deepwater Goby, Ponticola bathybius, an endemic species in the southern Caspian Sea, which were previously unknown, are examined here. Deepwater Goby samples were collected from three southern Caspian Sea localities (southwest, mid-south, and southeast), and phylogenetic analyses were conducted using mitochondrial cytochrome b (cyt b) and S7 nuclear gene sequences with maximum likelihood and Bayesian approaches. Additionally, the Deepwater Goby's population genetic structure and demographic history were evaluated for cyt b. Results were highly congruent, providing strong support for inclusion of the Deepwater Goby in the genus Neogobius (tribe: Neogobiini), from its previous placement in the genus Ponticola (tribe: Ponticolini). AMOVA analyses showed significant differences between western and eastern sampling localities (P < 0.05) of Deepwater Goby, with moderate FST (0.095) divergence. The haplotype network appeared star-like in formation, with no apparent genetic structure, indicating high connectivity and gene flow, and supporting a single continuous population. Since the Deepwater Goby exhibits low mobility after settlement, except for some migration between deeper and shallower waters, gene flow primarily occurs through spread of pre-settlement post-larvae via regional water currents. Mismatch distribution and neutrality tests both indicated that the Deepwater Goby likely underwent a recent demographic expansion in the southern Caspian Sea, estimated at ∼247 kya during the Pleistocene epoch, coinciding with similar expansions of other Caspian Sea taxa.
Sampling locations in the southern Caspian Sea. Geographic coordinates of site 1: 37.500778, 49.397639; site 2: 37.448472, 49.783528; site 3: 36.757361, 51.015028; site 4: 36.729669, 51.108277; site 5: 36.713333, 51.199389; site 6: 36.877472, 53.466917; site 7: 36.890111, 53.578139; site 8: 36.908806, 53.723722; site 9: 36.911417, 53.810778.
Maximum likelihood (ML) phylogeny of the subfamily Benthophilinae and outgroups based on the mtDNA cyt b gene. Numbers at the nodes represent statistical support for ML and BI analyses, respectively. Bootstrap support values <63 and Bayesian posterior probabilities <65 are not shown, * = 100%. See Data Accessibility for tree file.
Maximum likelihood (ML) phylogeny of the subfamily Benthophilinae and outgroups based on the nuclear DNA intron S7. Numbers at the nodes represent statistical support for ML and BI analyses, respectively. Bootstrap support values <63 and Bayesian posterior probabilities <65 are not shown, * = 100%. See Data Accessibility for tree file.
Time-calibrated phylogeny for representative Gobiidae (Ponto-Caspian gobies of the Gobius lineage), based on cyt b gene sequences. Bars indicate 95% highest posterior density (HPD) distributions for the time estimates. See Data Accessibility for tree file.
Maximum likelihood tree of unique haplotype sequences of Neogobius (formerly Ponticola) bathybius. SW: South-West; SE: South-East; and MS: Mid-South regions in the Caspian Sea. Numbers at the nodes represent statistical support for maximum likelihood and Bayesian analyses, respectively. Bootstrap support values <55 and Bayesian posterior probabilities <80 are not shown, *=100%. See Data Accessibility for tree file.
Haplotype network reconstructed from cytochrome b sequences. Each circle represents a single haplotype; circle size is proportional to the frequency of that haplotype in the sample; the branch length is relative to the mutational steps between haplotypes. See Data Accessibility for tree file.
Contributor Notes
Associate Editor: M. T. Craig.