Comparative Genomics Reveals 13 Different Isoforms of Mytimycins (A-M) in Mytilus galloprovincialis

Transcriptomic and functional analysis of the antiviral response of mussels after a poly I:C stimulation

The study of mussels (Mytilus galloprovincialis) has grown in importance in recent years due to their high economic value and resistance to pathogens. Because of the biological characteristics revealed by mussel genome sequencing, this species is a valuable research model. The high genomic variability and diversity, particularly in immune genes, may be responsible for their resistance to pathogens found in seawater and continuously filtered and internalized by them. These facts, combined with the lack of proven mussel susceptibility to viruses in comparison to other bivalves such as oysters, result in a lack of studies on mussel antiviral response. We used RNA-seq to examine the genomic response of mussel hemocytes after they were exposed to poly I:C, simulating immune cell contact with viral dsRNA. Apoptosis and the molecular axis IRFs/STING-IFI44/IRGC1 were identified as the two main pathways in charge of the response but we also found a modulation of lncRNAs. Finally, in order to obtain new information about the response of mussels to putative natural challenges, we used VHSV virus (Viral Hemorrhagic Septicemia Virus) to run some functional analysis and confirm poly I:C's activity as an immunomodulator in a VHSV waterborne stimulation. Both, poly I:C as well as an injury stimulus (filtered sea water injection) accelerated the viral clearance by hemocytes and altered the expression of several immune genes, including IL-17, IRF1 and viperin.

Chromosome-Level Genome Assembly of the Blue Mussel Mytilus chilensis Reveals Molecular Signatures Facing the Marine Environment

The blue mussel Mytilus chilensis is an endemic and key socioeconomic species inhabiting the southern coast of Chile. This bivalve species supports a booming aquaculture industry, which entirely relies on artificially collected seeds from natural beds that are translocated to diverse physical-chemical ocean farming conditions. Furthermore, mussel production is threatened by a broad range of microorganisms, pollution, and environmental stressors that eventually impact its survival and growth. Herein, understanding the genomic basis of the local adaption is pivotal to developing sustainable shellfish aquaculture. We present a high-quality reference genome of M. chilensis, which is the first chromosome-level genome for a Mytilidae member in South America. The assembled genome size was 1.93 Gb, with a contig N50 of 134 Mb. Through Hi-C proximity ligation, 11,868 contigs were clustered, ordered, and assembled into 14 chromosomes in congruence with the karyological evidence. The M. chilensis genome comprises 34,530 genes and 4795 non-coding RNAs. A total of 57% of the genome contains repetitive sequences with predominancy of LTR-retrotransposons and unknown elements. Comparative genome analysis of M. chilensis and M. coruscus was conducted, revealing genic rearrangements distributed into the whole genome. Notably, transposable Steamer-like elements associated with horizontal transmissible cancer were explored in reference genomes, suggesting putative relationships at the chromosome level in Bivalvia. Genome expression analysis was also conducted, showing putative genomic differences between two ecologically different mussel populations. The evidence suggests that local genome adaptation and physiological plasticity can be analyzed to develop sustainable mussel production. The genome of M. chilensis provides pivotal molecular knowledge for the Mytilus complex.

Unveiling the Impact of Gene Presence/Absence Variation in Driving Inter-Individual Sequence Diversity within the CRP-I Gene Family in Mytilus spp

Mussels (Mytilus spp.) tolerate infections much better than other species living in the same marine coastal environment thanks to a highly efficient innate immune system, which exploits a remarkable diversification of effector molecules involved in mucosal and humoral responses. Among these, antimicrobial peptides (AMPs) are subjected to massive gene presence/absence variation (PAV), endowing each individual with a potentially unique repertoire of defense molecules. The unavailability of a chromosome-scale assembly has so far prevented a comprehensive evaluation of the genomic arrangement of AMP-encoding loci, preventing an accurate ascertainment of the orthology/paralogy relationships among sequence variants. Here, we characterized the CRP-I gene cluster in the blue mussel Mytilus edulis, which includes about 50 paralogous genes and pseudogenes, mostly packed in a small genomic region within chromosome 5. We further reported the occurrence of widespread PAV within this family in the Mytilus species complex and provided evidence that CRP-I peptides likely adopt a knottin fold. We functionally characterized the synthetic peptide sCRP-I H1, assessing the presence of biological activities consistent with other knottins, revealing that mussel CRP-I peptides are unlikely to act as antimicrobial agents or protease inhibitors, even though they may be used as defense molecules against infections from eukaryotic parasites.

The Immune System of Marine Organisms as Source for Drugs against Infectious Diseases

The high proliferation of microorganisms in aquatic environments has allowed their coevolution for billions of years with other living beings that also inhabit these niches. Among the different existing types of interaction, the eternal competition for supremacy between the susceptible species and their pathogens has selected, as part of the effector division of the immune system of the former ones, a vast and varied arsenal of efficient antimicrobial molecules, which is highly amplified by the broad biodiversity radiated, above any others, at the marine habitats. At present, the great recent scientific and technological advances already allow the massive discovery and exploitation of these defense compounds for therapeutic purposes against infectious diseases of our interest. Among them, antimicrobial peptides and antimicrobial metabolites stand out because of the wide dimensions of their structural diversities, mechanisms of action, and target pathogen ranges. This revision work contextualizes the research in this field and serves as a presentation and scope identification of the Special Issue from Marine Drugs journal “The Immune System of Marine Organisms as Source for Drugs against Infectious Diseases”.

Molecular cloning and expression analysis of mytilin-like antimicrobial peptides from Asian green mussel Perna viridis

Mytilin is one of the most important CS-αβ peptides involved in innate immune response in Mytilidae. In this study, we successfully identified four mytilin-like antimicrobial peptides (pernalins) from Asian green mussel Perna viridis by aligning the P. viridis transcriptome with 186 mytilins and myticins related sequences collected from the transcriptome data of six Mytilus species. Analysis on gene structure showed that pernalin genes had high conservation with mytilin B of Mediterranean mussel Mytilus galloprovincialis. Interestingly, all pernalin genes have a similar tissue expression feature, evidenced by the highest transcription level observed in the hemocytes and followed by the mantle. The lowest transcription level was observed in the foot and gills. qRT-PCR analysis showed that all pernalin genes were significantly down-regulated at each time points from 3 h to 48 h after Vibrio parahaemolyticus infection, suggesting their timely immune responses after bacterial infection.