Prevalent ciliate symbiosis on copepods: high genetic diversity and wide distribution detected using small subunit ribosomal RNA gene.

Zhiling Guo,Senjie Lin,Tao Li,Simin Hu,Guangxing Liu,Yousong Huang,Sheng Liu,Huan Zhang,Alina Margarita Szmant

doi:10.1371/journal.pone.0044847

Abstract

Toward understanding the genetic diversity and distribution of copepod-associated symbiotic ciliates and the evolutionary relationships with their hosts in the marine environment, we developed a small subunit ribosomal RNA gene (18S rDNA)-based molecular method and investigated the genetic diversity and genotype distribution of the symbiotic ciliates on copepods. Of the 10 copepod species representing six families collected from six locations of Pacific and Atlantic Oceans, 9 were found to harbor ciliate symbionts. Phylogenetic analysis of the 391 ciliate 18S rDNA sequences obtained revealed seven groups (ribogroups), six (containing 99% of all the sequences) belonging to subclass Apostomatida, the other clustered with peritrich ciliate Vorticella gracilis. Among the Apostomatida groups, Group III were essentially identical to Vampyrophrya pelagica, and the other five groups represented the undocumented ciliates that were close to Vampyrophrya/Gymnodinioides/Hyalophysa. Group VI ciliates were found in all copepod species but one (Calanus sinicus), and were most abundant among all ciliate sequences obtained, indicating that they are the dominant symbiotic ciliates universally associated with copepods. In contrast, some ciliate sequences were found only in some of the copepods examined, suggesting the host selectivity and geographic differentiation of ciliates, which requires further verification by more extensive sampling. Our results reveal the wide occurrence and high genetic diversity of symbiotic ciliates on marine copepods and highlight the need to systematically investigate the host- and geography-based genetic differentiation and ecological roles of these ciliates globally.

Highlights

Copepods are the most numerous metazoans distributed globally in the aquatic ecosystem [1]
Some live individuals of these copepods were used in gut clearance treatment, and three species (Temora turbinata, Acartia erythraea and A. pacifica, named as starved copepods) survived the treatment, which were subjected to DNA extraction, PCR and cloning
Most of the previous studies on marine copepod symbionts were carried out using morphological observations by silver-impregnation technique or scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which are time-consuming and require specific equipment, and were limited to the equipped laboratories [2,22,25]

Summary

Introduction

Copepods are the most numerous metazoans distributed globally in the aquatic ecosystem [1] They are conventionally considered as the key trophic linkage between phytoplankton and higher trophic levels in aquatic food chains [2,3]. Their chitinous skeleton can serve as a suitable living environment for the settlement and growth of various symbiotic microorganisms [4,5]. The life styles of symbiotic apostome ciliates include ectocommensalism and invasive parasitism [4,10] They depend on copepod hosts to provide diverse and dynamic substrate and food sources to complete their life cycle [5]. Peritrich and Suctorian ciliates have been reported to live as epibionts on crustacean, including pelagic copepods [5,7,14], and they exhibit higher host-specificity on larger-sized hosts [5]

Methods

Results

Conclusion