Multiple gene analyses of caligid copepods indicate that the reduction of a thoracic appendage in Pseudocaligus represents convergent evolution

摘要

Background: The Caligidae is a family of parasitic copepods containing over 30 recognised genera. They areudcommercially important parasites as they cause disease in numerous finfish aquaculture facilities globally.udMorphological features are used to distinguish between the genera and Pseudocaligus has traditionally beenuddifferentiated from Caligus solely by the presence of a much reduced form of the fourth thoracic leg. Currentlyudthere are numerous DNA sequences available for Caligus spp. but only the type species, Pseudocaligus brevipedis,udhas molecular data available, so systematic studies using molecular phylogenetic analyses have been limited.udMethods: Three gene regions, SSU rDNA, 16S and CO1, for Pseudocaligus fugu from puffer fish from Japan andudPseudocaligus uniartus from rabbit fish from Indonesia are sequenced and molecular phylogenetic analysesudperformed in order to infer phylogenetic relationships between Pseudocaligus and other caligid copepods.udResults: The analysis revealed that there was no discrete grouping of Pseudocaligus spp. and that they had audpolyphyletic distribution within Caligus taxa. Pseudocaligus fugu grouped with Caligus elongatus and contained audunique synapomorphy in the SSU rDNA region only seen in members of that clade. Pseudocaligus uniartusudformed a well-supported group, in the SSU rDNA analyses, with a Caligus sp. that also infects rabbit fish, but wasudunresolved in the other analyses. Pseudocaligus brevipedis consistently and robustly grouped with Caligus curtusudand C. centrodonti in all analyses. The majority of Lepeophtheirus spp. form a monophyletic sister group to theudCaligus clade; however, L. natalensis is unresolved in all analyses and does not form part of the mainudLepeophtheirus clade.udConclusions: These findings do not support the morphological-based distinction between Pseudocaligus and Caligus,udsuggesting that the reduced fourth leg is a feature that has evolved on multiple occasions throughout caligid evolution.udCongruent molecular phylogenetic data support groupings based on the presence of morphological features, such asudlunules, geography and host fish type rather than appendage morphology. Therefore, we support the synonymy ofudPseudocaligus with Caligus.