Insights into deep‐sea adaptations and host–symbiont interactions: A comparative transcriptome study on Bathymodiolus mussels and their coastal relatives

Abstract

Mussels (Bivalve: Mytilidae) have adapted to various habitats, from fresh water to the deep sea. To understand their adaptive characteristics in different habitats, particularly in the bathymodiolin mussels in deep-sea chemosynthetic ecosystems, we conducted a comparative transcriptomic analysis between deep-sea bathymodiolin mussels and their shallow-water relatives. A number of gene families related to stress responses were shared across all mussels, without specific or significantly expanded families in deep-sea species, indicating that all mussels are capable of adapting to diverse harsh environments, but that different members of the same gene family may be preferentially utilized by different species. One of the most extraordinary trait of bathymodiolin mussels is their endosymbiosis. Lineage-specific and positively selected TLRs and highly expressed C1QDC proteins were identified in the gills of the bathymodiolins, suggesting their possible functions in symbiont recognition. However, pattern recognition receptors of the bathymodiolins were globally reduced, facilitating the invasion and maintenance of the symbionts obtained by either endocytosis or phagocytosis. Additionally, various transporters were positively selected or more highly expressed in the deep-sea mussels, indicating a means by which necessary materials could be provided for the symbionts. Key genes supporting lysosomal activity were also positively selected or more highly expressed in the deep-sea mussels, suggesting that nutrition fixed by the symbionts can be absorbed in a "farming" way wherein the symbionts are digested by lysosomes. Regulation of key physiological processes including lysosome activity, apoptosis and immune reactions is needed to maintain a stable host-symbiont relationship, but the mechanisms are still unclear.

Rapid climate-driven evolution of the invasive species Mytilus galloprovincialis over the past century

“…HSPA12A was suggested to possess cytoprotective functions in response to various stressors, especially pathogens (Libro and Vollmer 2016), toxic metals (Nicosia et al 2015), and heat stress (Wang et al 2014). RNA-directed DNA polymerase from mobile element jockey-like has RNA-directed DNA polymerase activity, and has been proven to play a key role in response to heat stress in the marine snail Nucella lapillus ) and environmental adaptation in deep-sea mussel Bathymodiolus platifrons (Zheng et al 2017). Both upregulation of HSPA12A (López-Galindo et al 2019) and RNA-directed DNA polymerase from mobile element jockey-like (Lancaster et al 2016) gene were detected in response to thermal stress.…”

Comparative transcriptomic analysis of deep- and shallow-water barnacle species (Cirripedia, Poecilasmatidae) provides insights into deep-sea adaptation of sessile crustaceans

“…Conditions on the deep-sea floor are poorly known but generally are considered too harsh for the survival of most organisms, e.g., high hydrostatic pressure, darkness, hypoxia, low temperature, and limited food availability [1][2][3][4][5]. However, a macrofauna consisting of a growing range of newly discovered animals adapted to deep-sea habitats has been reported, including crustaceans [6][7][8], polychaetes [9,10], fishes [11,12], and mollusks [13,14]. Various mechanisms have adapted them for survival in deep-sea environments: e.g., squat lobsters and mussels have developed chemoautotrophic systems of symbiotic bacteria for inhabiting hydrothermal vents and cold seeps in the seafloor [15][16][17]; and snailfish have evolved special morphological and physiological characters to survive and thrive in the hadal zone [12].…”

microRNAs Facilitate Comprehensive Responses of Bathymodiolinae Mussel Against Symbiotic and Nonsymbiotic Bacteria Stimulation

“…However, these studies failed to show the host response globally without state-of-the-art molecular tools. The successful application of next generation sequencing in deep sea mussels now provides a better solution [26,32]. In the present study, expressional alternations of Gigantidas genes during either symbiotic MOB or nonsymbiotic bacterial challenges were surveyed globally.…”