Recent research has shown that low genetic variation in individuals can increase susceptibility to infection and group living may exacerbate pathogen transmission. In the eusocial diploid termites, cycles of outbreeding and inbreeding characterizing basal species can reduce genetic variation within nestmates during the life of a colony, but the relationship of genetic heterogeneity to disease resistance is poorly understood. Here we show that, one generation of inbreeding differentially affects the survivorship of isolated and grouped termites (Zootermopsis angusticollis) depending on the nature of immune challenge and treatment. Inbred and outbred isolated and grouped termites inoculated with a bacterial pathogen, exposed to a low dose of fungal pathogen or challenged with an implanted nylon monofilament had similar levels of immune defence. However, inbred grouped termites exposed to a relatively high concentration of fungal conidia had significantly greater mortality than outbred grouped termites. Inbred termites also had significantly higher cuticular microbial loads, presumably due to less effective grooming by nestmates. Genetic analyses showed that inbreeding significantly reduced heterozygosity and allelic diversity. Decreased heterozygosity thus appeared to increase disease susceptibility by affecting social behaviour or some other group-level process influencing infection control rather than affecting individual immune physiology.

No abstract

Termites form one-piece nests in wood that can vary in their moisture content and degree of decomposition, and thus microbial richness. To estimate the microbial load of nests and the potential risk they pose for colony members, we quantified the number of microbes in the nest and on the cuticle of the dampwood termite, Zootermopsis angusticollis, and three drywood termites, Incisitermes minor, I. schwarzi, and Cryptotermes cavifrons. The number of colony forming units (CFUs) cultured from nest material samples and washes of the cuticle of larvae and nymphs were determined. CFUs recorded from nest material was low (fewer than 60 CFUs/g) in the drywood termites and comparatively high in the dampwood species, as more than 800 bacterial and fungal CFUs/g were cultured from the nest material of Z. angusticollis. Similarly, cuticular microbial loads were negligible in the drywood termites sampled, ranging from 0.5 to fewer than 16 CFUs/cm2, whereas approximately 200 CFUs/cm2 were cultured from Z. angusticollis. The nesting and feeding habits of these basal species likely influence colony microbial load and the degree of pathogen exposure, which in turn could favor adaptations to resist disease that vary with termite nesting biology.CFUcolony forming unit

Pathogens have likely infl uenced life-history evolution in social insects because their nesting ecology and sociality can exacerbate the risk of disease transmission and place demands on the immune system that ultimately can impact colony survival and growth. The costs of the maintenance and induction of immune function may be particularly signifi cant in termites, which have a nitrogen-poor diet. We examined the effect of fungal exposure on survival and reproduction during colony foundation in the dampwood termite Zootermopsis angusticollis by experimentally pairing male and female primary reproductives and exposing them to single ('acute') and multiple ('serial') dosages of conidia of the fungus Metarhizium anisopliae and recording their survival and fi tness over a 560 day period. The number of eggs laid 70 days post-pairing was signifi cantly reduced relative to controls in the serial-exposure but not the acute-exposure treatment. Reproduction thus appeared to be more resilient to a single pathogen exposure than to serial challenge to the immune system. The impact of fungal exposure was transient: all surviving colonies had similar reproductive output after 300 days post-pairing. Our results suggest that disease can have signifi cant survival and fi tness costs during the critical phase of colony foundation but that infection at this time may not necessarily impact long-term colony growth.

To determine the impact of inbreeding and outbreeding on disease resistance and survival during colony foundation, nestmate (NM) and non-nestmate (NON) primary reproductives of the dampwood termite Zootermopsis angusticollis were exposed to a single or double dose of conidia of the entomopathogenic fungus Metarhizium anisopliae. Male and female primary reproductive pairs originating from the same parent colony had higher survivorship than NON pairs in control and conidia-exposure treatments. The survival advantage of NM primary reproductives increased with the intensity of pathogen challenge and was significantly greater in the single- and double-dose treatments than in the controls. Although NM pairs had significantly lower mortality than NON pairs, the survivorship of colonies stabilized as they matured and inbred and outbred colonies did not differ in offspring production. These results demonstrate that colony foundation by NON male and female reproductives may have a disease-related survival cost during this critical phase of their life cycle. There may also be a cost associated with lower offspring heterozygosity, but in the first generation this does not appear to significantly impact colony growth.