Experimental immune challenge drastically reduces gut microbiome diversity and triggers fertility- and longevity-dependent changes in gene expression in the ant Temnothorax rugatulus

Senescence occurs in all organisms and is a consequence of an accumulation of molecular damage due incomplete somatic repair. An up-regulation of repair mechanisms or a reduced genesis of those damages can facilitate long lifespan, as evidenced by social insect queens that can live for decades. Also in many ant workers longevity is a phenotypic plastic trait and queen removal can induce fertility and an extension in lifespan. Here we investigate the molecular mechanisms underlying this lifespan extension, including the role of immunity and the gut microbiome. The immune system is pivotal for maintaining gut microbiome stability and we thus also assessed whether a strong immune response can shape microbiome makeup. We induced fertility and challenged the immune system of Temnothorax rugatulus ant workers and studied responses in fat body gene expression and gut microbiome composition. Fertile workers upregulate molecular repair mechanisms, potentially linked to their extended longevity. The immune challenge altered the expression of several thousand of genes in the fat body, especially immune genes and this transcriptomic response depended on worker fertility. For example, only fertile, immune-challenged workers upregulated genes of the alpha-ketoglutarate pathway. This versatile compound that enhances macrophage activity can also increase longevity by down-regulating the TOR pathway and oxidant production. Our study therefore sheds light onto the molecular basis of the positive association between fertility and longevity in social insects. Using 16S high-throughput sequencing we observed a dramatic loss in bacterial diversity in ant guts 24h after the immune-challenge. Total bacterial abundance did not decrease, suggesting that some bacteria proliferated. The expression of immune genes in the fat body was linked to gut microbiome composition, revealing candidate genes for the regulation of the microbiome by its insect host. Our study is one of the first to demonstrate a negative effect of immune flare-ups on gut microbiome stability in insects, suggesting a new physiological cost of immunity.