Characterization of host-microbe factors in shell diseases in Homarus americanus and Cancer borealis: bacteriome and whole transcriptome analysis

Abstract

Shell diseases affect many economically important crustacean species, they are unrelated diseases with diverse symptoms, but which generally involve shell erosion. Shell diseases have been shown to be complex, often involving polymicrobial dysbiosis and host susceptibility and resistance factors. These diseases include impoundment shell disease (ISD) which affects the American lobster (Homarus americanus) and black spot shell disease (BSD) which affects the Jonah Crab (Cancer borealis). Despite the economic losses caused by these diseases, little is known about key factors in their host-pathogen interactions, such as host immune responses or microbial community dynamics. The first objective of this thesis was to characterize the transcriptional responses of H. americanus and C. borealis to ISD and BSD in three tissues of immune interest: circulating haemocytes, hepatopancreas and shell epidermis. After RNA-Seq analyses, specific transcriptional patterns were found in each tissue and animal. Gene expression differences were most notable in the study involving H. americanus, especially in the shell epidermis where over 3600 differently expressed genes (DEGs) were found. Expression differences were also observed in the hepatopancreas (134 DEGs) and circulating haemocytes (853 DEGs) of H. americanus against ISD. Expression differences in C. borealis affected by BSD were notable in the shell epidermis (606 DEGs) but almost absent in the hepatopancreas (28 DEGs) and circulating haemocytes (seven DEGs). Some immune-related DEGs were found in both species and in different tissues, indicating relevance to shell diseases. Histopathological analyses showed some symptoms of shell diseases in the shell epidermis, especially in H. americanus, but no differences were found in the hepatopancreas of these animals. After bacteriome analyses, signs of dysbiosis were found in the bacterial communities associated with ISD in H. americanus, such as decreases in diversity. Some bacterial genera, such as Vibrio and Colwellia, were associated with ISD lesions, but were also found in healthy samples. The bacteriome analyses in the C. borealis study were performed in individuals in the early stages of BSD development in search for potential initial colonizers of the disease. No signs of dysbiosis were found, but a few bacteria were related to shell lesions, such as Aquimarina and Colwellia.