Apical Groove Type and Molecular Phylogeny Suggests Reclassification of Cochlodinium geminatum as Polykrikos geminatum

Dajun Qiu,Huan Zhang,Sheng Liu,Liangmin Huang,Senjie Lin,Ahmed Moustafa

doi:10.1371/journal.pone.0071346

Dajun Qiu, Huan Zhang + Show 4 more

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https://doi.org/10.1371/journal.pone.0071346

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Abstract

Traditionally Cocholodinium and Gymnodinium sensu lato clade are distinguished based on the cingulum turn number, which has been increasingly recognized to be inadequate for Gymnodiniales genus classification. This has been improved by the combination of the apical groove characteristics and molecular phylogeny, which has led to the erection of several new genera (Takayama, Akashiwo, Karenia, and Karlodinium). Taking the apical groove characteristics and molecular phylogeny combined approach, we reexamined the historically taxonomically uncertain species Cochlodinium geminatum that formed massive blooms in Pearl River Estuary, China, in recent years. Samples were collected from a bloom in 2011 for morphological, characteristic pigment, and molecular analyses. We found that the cingulum in this species wraps around the cell body about 1.2 turns on average but can appear under the light microscopy to be >1.5 turns after the cells have been preserved. The shape of its apical groove, however, was stably an open-ended anticlockwise loop of kidney bean shape, similar to that of Polykrikos. Furthermore, the molecular phylogenetic analysis using 18S rRNA-ITS-28S rRNA gene cistron we obtained in this study also consistently placed this species closest to Polykrikos within the Gymnodinium sensu stricto clade and set it far separated from the clade of Cochlodinium. These results suggest that this species should be transferred to Polykrikos as Polykrikos geminatum. Our results reiterate the need to use the combination of apical groove morphology and molecular phylogeny for the classification of species within the genus of Cochlodinium and other Gymnodiniales lineages.

Highlights

Gymnodiniales is a major order of dinoflagellates, which are ecologically important because many of its species, such as Cochlodinium polykrikoides, Karenia mikimotoi, cause harmful algal blooms
Our results clearly indicated that C. geminatum should be placed in the genus Polykrikos, reiterating that the traditional cingulum turn number is unreliable and should be replaced by apical groove morphotyping combined with molecular phylotyping for the classification of Cochlodinium and related species
Morphological characters Cells of C. geminatum collected during the bloom occurred in pairs (Figure 1a, b; Figure 2e-g; Figure 3a), each were ellipsoidal with size of 20.0–29.9 mm in length and 20.2–30.6 mm in width, giving averaged size of 26.262.3 mm (n = 29) in length and mm (n = 37) in width (Figures 2, 4)

Summary

Introduction

Gymnodiniales is a major order of dinoflagellates, which are ecologically important because many of its species, such as Cochlodinium polykrikoides, Karenia mikimotoi, cause harmful algal blooms While their correct classification is crucial for understanding their varying ecological characteristics, the traditional genus delimitation criteria in the unarmoured order of dinoflagellates have proven to be problematic [1], [2]. The combination of apical groove morphological and molecular phylogenetic studies has led to the uncovering of many relationships that were previously unknown [1,2,3,4,5,6,7,8,9,10,11] Some genera, such as the genus of Cochlodinium, are still defined based on the old genus discriminating criteria [12], [13]. It is necessary to re-examine species in Cochlodinium based on the combination of apical groove morphology and molecular phylogeny

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