Chemosensory-Related Genes in Marine Copepods

Abstract

Living organisms deeply rely on the acquisition of chemical signals in any aspect of their life, from searching for food, mating and defending themselves from stressors. Copepods, the most abundant and ubiquitous metazoans on Earth, possess diversified and highly specified chemoreceptive structures along their body. The detection of chemical stimuli activates specific pathways, although this process has so far been analyzed only on a relatively limited number of species. Here, in silico mining of 18 publicly available transcriptomes is performed to delve into the copepod chemosensory genes, improving current knowledge on the diversity of this multigene family and on possible physiological mechanisms involved in the detection and analysis of chemical cues. Our study identifies the presence of ionotropic receptors, chemosensory proteins and gustatory receptors in copepods belonging to the Calanoida, Cyclopoida and Harpacticoida orders. We also confirm the absence in these copepods of odorant receptors and odorant-binding proteins agreeing with their insect specificity. Copepods have evolved several mechanisms to survive in the harsh marine environment such as producing proteins to respond to external stimulii. Overall, the results of our study open new possibilities for the use of the chemosensory genes as biomarkers in chemical ecology studies on copepods and possibly also in other marine holozooplankters.

Keywords: chemosensory proteins; copepods; gene discovery; gustatory receptors; ionotropic receptors; odorant receptors; transcriptome.

Figure 1

Chemosensory-related gene diversity. Distribution of transcripts encoding for chemosensory-related genes in copepods, in the insect Drosophila melanogaster (D. melanogaster) and in the cladoceran Daphnia pulex (D. pulex). CRGs include ionotropic receptors (IR8a, IR21a, IR25a, IR76b, IR93a, IRCS2), gustatory receptors (GRs), chemosensory proteins (CSPs). For the copepods examined in this study, the diversity is shown for a single member of each family (Calanidae, Caligidae, Cyclopettidae, Harpaticidae, Pontellidae, Pseudomiatomidae, Rhincalanidae, Temoridae). On x-axis, abbreviated species names: Calanus finmarchicus (C. finmarchicus), Caligus rogercresseyi (C. rogercresseyi), Paracyclopina nana (P. nana), Tigriopus japonicus (T. japonicus), Labidocera madurae (L. madurae), Pseudodiaptomus annandalei (P. annandalei), Rhincalanus gigas (R. gigas) and Temora longicornis (T. longicornis).

Figure 2

Cladogram of ionotropic receptors (IRs) identified in this study. Colors indicate the different classes. In addition to the sequences identified in this study, the analysis includes also CRGs from D. melanogaster and D. pulex and from copepods previously identified (see manuscript for details). For the analysis, amino acid sequences were aligned using ClustalW, while FAST TREE was used to build maximum-likelihood phylogenetic tree using the protein evolution model JTT + CAT. Colors are consistent with Figure 1.

Figure 3

Relative expression of chemosensory genes in Calanus finmarchicus and C. helgolandicus. In the first panel (a–d), expression is shown for four ionotropic receptors (IR8a, IR21a, IRCS2) and for a single chemosensory protein (CSP) across six developmental stages: embryos (E), early nauplii (NII-NIII) (EN), early copepodids (CI), late copepodids (CIV), preadults (CV) and females (AF). Bar graphs indicate SD of the three replicates in each sample (2 replicates for CI and CIV). Second panel (e–h) shows expression for C. finmarchicus IR8a and CSP in females exposed for two days to the diet R. baltica (CONTROL; C) and two doses of A. fundyense (low dose [LD] and high dose [HD]). Bar graphs indicate SD of the three replicates in each sample. Relative expression of IR8a and CSP is also shown for C. helgolandicus females feeding on the flagellate P. minimum (PRO) and the oxylipin-producing S. marinoi (SKE). Bar graphs indicate SD of the three replicates in each sample.