ABSTRACT: The transcriptome of Blochmannia floridanus, the endosymbiont of the carpenter ant Camponotus floridanus, is presented during eight developmental stages of its holometabolous host by use of a whole genome DNA-macroarray. The detected transcription patterns indicate the presence of local transcription units as well as global regulatory mechanisms. Yet, the overall regulation scale is very modest, rarely exceeding a factor of three which is in line with the low number of transcriptional regulators present in this reduced genome. A large number of genes show differential expression in different life stages and a distinct expression pattern of genes possibly involved in symbiotic function as compared to housekeeping genes is apparent. However, these transcriptional changes are small as compared to the changes in the number of bacteria during host development which is highest in pupae and in young imagines. Control of replication of the bacteria in certain life stages may therefore be the decisive parameter influencing overall gene expression of Blochmannia in the animal. The few highly expressed genes like those encoding molecular chaperones exhibit a significantly higher G+C-content than moderately expressed genes. Keywords: Host stage-dependent gene expression C. floridanus laboratory colonies were maintained under constant conditions at 25°C with a 12 h light cycle in artificial nests and fed twice a week with cockroaches (Nauphoeta cinerea), Bhatkar agar (Bhatkar and Whitcomb, 1970), and honey water (50% wt/wt) ad libitum. Developmental stages were defined as L1 (very young larvae, still clustered), L2 (older larvae, approx. 2-4 mm), P1 (pupae before metamorphosis), P2 (pupae after metamorphosis, still uncolored), P3 (pupae shortly before eclosion with dark abdomens), W1 (workers shortly after eclosion, not completely melanized, no aggressive behavior), W2 (fully melanized adult workers from the nest, age not distinguishable), and W3 (adult workers from isolated subcolonies, at least 4 months old). For RNA preparations, midguts of the respective life stage (L2 - W3) were dissected and bacteria were isolated by homogenization and centrifugation. Endosymbionts from small larvae (L1) were isolated from whole animals. RNA for each cDNA array experiment was isolated from individual bacterial preparations; from L1 to W2, each developmental stage was represented by at least three independent experiments and samples from at least three different ant colonies. Two ant colonies (C79 and C118) were represented in every stage from L1 to W2 with additional samples from six other colonies. Old W3 workers were collected from two isolated subcolonies derived from the same mother colony, each sampled in duplicate.
