Cryptic speciation in Protoceratium reticulatum (Dinophyceae): Evidence from morphological, molecular and ecophysiological data

Abstract

The cosmopolitan, potentially toxic dinoflagellate Protoceratium reticulatum possesses a fossilizable cyst stage which is an important paleoenvironmental indicator. Slight differences in the internal transcribed spacer ribosomal DNA (ITS rDNA) sequences of P. reticulatum have been reported, and both the motile stage and cyst morphology of P. reticulatum display phenotypic plasticity, but how these morpho-molecular variations are related with ecophysiological preferences is unknown. Here, 55 single cysts or cells were isolated from localities in the Northern (Arctic to subtropics) and Southern Hemispheres (Chile and New Zealand), and in total 34 strains were established. Cysts and/or cells were examined with light microscopy and/or scanning electron microscopy. Large subunit ribosomal DNA (LSU rDNA) and/or ITS rDNA sequences were obtained for all strains/isolates. All strains/isolates of P. reticulatum shared identical LSU sequences except for one strain from the Mediterranean Sea that differs in one position, however ITS rDNA sequences displayed differences at eight positions. Molecular phylogeny was inferred using maximum likelihood and Bayesian inference based on ITS rDNA sequences. The results showed that P. reticulatum comprises at least three ribotypes (designated as A, B, and C). Ribotype A included strains from the Arctic and temperate areas, ribotype B included strains from temperate regions only, and ribotype C included strains from the subtropical and temperate areas. The average ratios of process length to cyst diameter of P. reticulatum ranged from 15% in ribotype A, 22% in ribotype B and 17% in ribotype C but cyst size could overlap. Theca morphology was indistinguishable among ribotypes. The ITS-2 secondary structures of ribotype A displayed one CBC (compensatory change on two sides of a helix pairing) compared to ribotypes B and C. Growth response of one strain from each ribotype to various temperatures was examined. The strains of ribotypes A, B and C exhibited optimum growth at 15°C, 20°C and 20-25°C, respectively, thus corresponding to cold, moderate and warm ecotypes. The profiles of yessotoxins (YTXs) were examined for 25 strains using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). The parent compound yessotoxin (YTX) was produced by strains of ribotypes A and B, but not by ribotype C strains, which only produced the structural variant homoyessotoxin (homoYTX). Our results support the notion that there is significant intra-specific variability in Protoceratium reticulatum and the biogeography of the different ribotypes is consistent with specific ecological preferences.

From bi-polar to regional distribution of modern dinoflagellate cysts, an overview of their biogeography

“…The case of Operculodinium centrocarpum sensu Wall and Dale 1966 is very interesting as the relationship between process length and salinity shows strong positive correlation in the Baltic Sea and the opposite in the North Pacific (Mertens et al 2009). This phenomenon can now be explained by cryptic speciation in Protoceratium reticulatum (a name of the motile stage for Operculodinium centrocarpum sensu Wall and Dale 1966) that was proposed by Mertens et al (2012a) and supported by molecular evidence recently reported by Wang et al (2019). Similarly, in low salinity seas, high variability of process development in Spiniferites cruciformis and the cyst of Gonyaulax baltica may be weakly correlated with surface salinity (Ellegaard et al 2002;Mudie et al, 2002).…”

Paleoceanography and dinoflagellate cyst stratigraphy across the Lower–Middle Pleistocene Subseries (Calabrian–Chibanian Stage) boundary at the Chiba composite section, Japan

“…The only other relatively common species is the thermophilic Spiniferites pachydermus sensu Mertens et al (2015) (3.7-18%). Protoceratium reticulatum is a potentially yessotoxinproducing (Liu et al 2017;Wang et al 2019), bloomforming opportunistic dinoflagellate found in Japanese waters today (e.g., Koike et al 2006). It is a cosmopolitan species whose cysts, known in the literature also as Operculodinium centrocarpum sensu Wall and Dale (1966), can dominate today at high latitudes including the Okhotsk Sea (Bonnet et al 2012) and off Hachinohe, northeastern Japan, which is under the influence of the Tsugaru Warm Current (Matsuoka 1976) and Oyashio Current.…”

Environmental Factors Affecting Spatial Dinoflagellate Cyst Distribution in Surface Sediments Off Aveiro-Figueira da Foz (Atlantic Iberian Margin)

“…Protoceratium reticulatum (=Operculodinium centrocarpum) is considered one of the most cosmopolitan species (Wall et al, 1977;Dale, 1996;Marret et al, 2020;de Vernal et al, 2020). Recent work has shown the existence of at least three genotypes with different ecological preferences (Wang et al, 2019). Despite this, cysts of P. reticulatum have been successfully used as part of ecological signals related with environmental instability in different marine ecosystems.…”