Identification of transcripts relating to host-parasite interactions and feeding-associated activities through de novo transcriptome sequencing of branchiuran fish lice, Argulus foliaceus (Linnaeus, 1758) and Argulus coregoni Thorell, 1865
Authors: AmbuAli Aisha , Taggart John B. , Bekaert Michael , Wehner Stefanie , Monaghan Sean J. , Bron James E. 
Published: March 2021

Abstract
BackgroundThe genus Argulus, whose members are widely known as “fish lice”, comprises a group of obligate ectoparasites that cause argulosis. This parasitic disease affects both wild and cultured fish populations and can lead to significant mortalities. Our previous work confirmed the presence of immunomodulatory or other active products in the secretions of Argulus foliaceus (Linnaeus, 1758) using proteomic analysis conducted on secretory / excretory products (SEPs). Some of the compounds identified have also been described for other host-associated Crustacea, particularly the parasitic copepod Lepeophtheirus salmonis (Krøyer, 1837). The identification and characterisation of such components can contribute to the understanding of the host-parasite relationship for argulids and may also assist in the development of novel control methods, however, due to limitations in existing genomic resources, only 27 proteins could be confirmed in SEPs. In this study, high-throughput sequencing, supported by the use of a range of bioinformatics tools and methodologies were employed on host-associated stages of A. foliaceus and A. coregoni Thorell, 1865 to explore elements of the transcriptome associated with host-parasite interactions and feeding-associated activities.ResultsThe transcriptome data produced in the current study comprises the largest current genomic data set for the genus Argulus, with 84,256,934 reads, assembled into 66,940 contigs for Argulus foliaceus, and 10,840,092 reads, assembled to give 40,954 contigs, representing the first transcript sequences for Argulus coregoni. A pre-existing transcriptome dataset for A. siamensis was also analysed. An overview of the functional roles of the transcripts and their respective proteins for the three Argulus species was performed using GO analysis. A phylogenetic tree was also constructed, using the top 100 shared genes for the four transcriptome datasets available for Argulus. From this analysis, A. coregoni was revealed to be more closely related to A. foliaceus than to A. siamensis. The application of OrthoVenn software to the Argulus spp. transcriptome datasets identified 6,674 shared gene clusters. The transcriptomic analysis in this study, supported by earlier proteome work, has contributed to identifying, for the first time, a range of proteins / genes in Argulus, and more widely for the Branchiura, that have been previously characterised as being important immune mediators for other aquatic and terrestrial blood-feeding arthropods. Data are available via EBI with the project number PRJEB34947.Conclusions In addition to confirming the presence of a range of key immune mediators in Argulus spp. for the first time, this study has provided important genomic tools for researchers seeking to better understand the biology and host interactions of Argulus spp. The created resources unlock the possibility of characterising protein targets relating to host-parasite interactions and feeding-associated activities and can thereby assist the future development of vaccines, veterinary drugs, functional feeds and other tools that might contribute to improved integrated pest management of these globally important parasites.