Integrative taxonomy of the Pseudo-nitzschia (Bacillariophyceae) populations in the NW Adriatic Sea, with a focus on a novel cryptic species in the P. delicatissima species complex

ABSTRACT

Species of the genus Pseudo-nitzschia are common inhabitants of phytoplankton communities of the northern Adriatic Sea, reaching high abundances. During 2018–2019, a total of 138 cultured Pseudo-nitzschia strains were isolated from northwestern Adriatic Sea and were characterized by using both morphological (TEM) and molecular (LSU, ITS, ITS2 secondary structure) approaches. Observation of frustule ultrastructure and phylogenetic analysis resolved a total of six taxa, i.e. P. fraudulenta, P. pungens, P. calliantha, P. mannii, P. delicatissima and P. cf. arenysensis. Results of the integrative taxonomic approach clearly revealed that P. cf. arenysensis is a cryptic species within P. delicatissima species complex. Pseudo-nitzschia cf. arenysensis is closely related to P. arenysensis, which forms a subclade and a sister group in LSU and ITS phylogenies, respectively. When comparing ITS2 secondary structure of P. cf. arenysensis and P. arenysensis, 1 CBC, 1 HCBC and 2 SNPs were recognized, suggesting reproductive isolation. Domoic acid production was not detected in any of the studied strains.

KEYWORDS:

• Cryptic species
• Domoic acid
• Phylogeny
• Secondary structure
• Ultrastructure

ACKNOWLEDGEMENTS

We are grateful to LTER-ITALY (Italian Long-Term Ecological Research Network) and to ECOSS (Ecological Observing System in the Adriatic Sea) project. Thanks to the crew of M/N Actea for their support during sampling activities. Special thanks to Luigi Gobbi for assistance with the EM.

SUPPLEMENTARY MATERIAL

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Additional information

Funding

This research was partially supported by the Italian Ministry of Health (Ricerca Finalizzata 2016) under grant number GR-2016-02363211. The cruises carried out with the M/N Actea were entirely funded by the Department of Life and Environmental Sciences (Università Politecnica delle Marche).