Effects of Caste on the Expression of Genes Associated with Septic Injury and Xenobiotic Exposure in the Formosan Subterranean Termite

Abstract

As social insects, termites live in densely populated colonies with specialized castes under conditions conducive to microbial growth and transmission. Furthermore, termites are exposed to xenobiotics in soil and their lignocellulose diet. Therefore, termites are valuable models for studying gene expression involved in response to septic injury, immunity and detoxification in relation to caste membership. In this study, workers and soldiers of the Formosan subterranean termite, Coptotermes formosanus, were challenged by bacterial injection or by no-choice feeding with a sublethal concentration (0.5%) of phenobarbital. Constitutive and induced expression of six putative immune response genes (two encoding for lectin-like proteins, one for a ficolin-precursor, one for the Down syndrome cell adhesion molecule, one for a chitin binding protein, and one for the gram-negative binding protein 2) and four putative detoxification genes (two encoding for cytochrome P450s, one for glutathione S-transferase, and one for the multi antimicrobial extrusion protein), were measured via quantitative real time polymerase chain reaction and compared within and among 1) colonies, 2) treatment types and 3) castes via ANOVA. Eight genes were inducible by septic injury, feeding with phenobarbital or both. Colony origin had no effect on inducibility or differential gene expression. However, treatment type showed significant effects on the expression of the eight inducible genes. Caste effects on expression levels were significant in five of the eight inducible genes with constitutive and induced expression of most target genes being higher in workers than in soldiers.

Evidence for reduced immune gene diversity and activity during the evolution of termites

“…The social insect colony is a highly organization society with specialized castes. Previous studies in termites have revealed caste specific expression patterns that reflect the specialized functions of castes within colony (Husseneder and Simms, 2014;Jones et al, 2017;Mitaka et al, 2017;Mitaka et al, 2016;Scharf et al, 2003). In this study, we show that constitutive immune gene expression is strongly caste specific in N. castaneus, reflecting a division of social roles and indicating a significant degree of caste-specific immune defense.…”

“…It possesses a sterile soldier caste, a reproductive caste and so-called false workers, which carry out shared tasks such as proctodeal trophallaxis and allogrooming (Davis et al, 2018;De Bie et al, 2006), while retaining the physiological capacity to develop into reproductive individuals under the right colony conditions (Korb and Hartfelder, 2008) . The subsocial wood feeding, C. meridianus, which represents the key lineage of Cryptocercus that is important in any comparative analysis of termite evolution because of important transitional traits such as subsociality, a wood diet with associated protist gut symbionts, and developmental similarities with termites (Inward et al, 2007a;Lo and Eggleton, 2010;Nalepa, 2015 For sequencing, equal amounts of total RNA from 8 and 4 injected individuals were pooled for termites and cockroaches for each treatment for library preparation, respectively. Each caste, species and treatment were represented by 2 libraries (N= 12, 4 and 4 total libraries for N. castaneus, C. meridianus and B. orientalis, respectively).…”

“…As with the evolution of the metazoan immune system which is thought to have emerged via the co-option of pre-existing molecular modules and functions into novel defensive pathways, it has been hypothesized that social immune systems originated via similar processes (Pull and McMahon, 2020), with a potentially crucial role for behavioural (Harpur et al, 2019) as well as immune gene adaptations (He et al, 2018;Kutsukake et al, 2019). In line with this view, many genes, including immune-related genes, have been shown to display caste-specific expression patterns (Husseneder and Simms, 2014;Jones et al, 2017;Mitaka et al, 2017;Mitaka et al, 2016;Scharf et al, 2003). In addition, enhanced antimicrobial defenses have been recorded in some social insects compared with their solitary relatives (Hoggard et al, 2011b;Stow et al, 2007;Turnbull et al, 2011;Turnbull et al, 2012).…”

“…1, Fig. 2), including two solitary cockroach species, two species of subsocial Cryptocercus wood-feeding cockroaches, which are the closest living relatives of the termites (Inward et al, 2007a), and 14 species of termites selected from a diverse range of evolutionary lineages across the clade.…”

“…Cryptocercus roaches represent a key lineage in any comparative analysis of termite evolution because they possess important transitional traits such as subsociality, a wood diet with associated protist gut symbionts, and developmental similarities with termites (Inward et al, 2007a;Lo and Eggleton, 2010;Nalepa, 2015). The termite species selected include 8 lower termites representing a range of social modes and ecologies and 6 higher termites belonging to Termitidae, a group which is thought to have undergone further transitions in symbiotic and social evolution (Bucek et al, 2019).…”