Comparative genomics of endosymbionts across endemic Hawaiian leafhopper (Hemiptera: Cicadellidae: Nesophrosyne) species

Nutritional endosymbionts are a vital part of a sap-feeding insect's survival. Endemic Hawaiian leafhoppers (Hemiptera: Cicadellidae: Nesophrosyne) are obligate phloem and xylem feeders, a diet that cannot nutritionally sustain animal life. Nesophrosyne relies on the bacteria, Candidatus Sulcia muelleri (hereafter known as Sulcia) and Candidatus Nasuia deltocephalinicola (hereafter known as Nasuia), that have been co-evolving for over 300 million years. This association is vital to the host by assisting in the production of 10 essential amino acids that are limited in their depauperate diet. These symbionts are intracellular and vertically inherited; therefore, they are prone to strong genetic drift with subsequent loss of essential free-living genes. Genome reduction varies widely throughout lineages causing variation among symbiont genomes, even if they are the same bacterial species. As genomic capabilities degenerate in beneficial symbionts, hosts must compensate using their own mechanisms. However, it remains unclear how this symbiosis has evolved in closely related species that have undergone an adaptive radiation. To answer this question, we investigated the genome evolution of Sulcia (190kb) and Nasuia (112kb) across 20 leafhopper species that span five Hawaiian Islands and eight host plant groups. Sulcia and Nasuia genomes exhibit gene loss across varying degrees in leafhopper species of the same genus. Additionally, Sulcia genomes are experiencing lower rates of molecular evolution compared to the co-symbiont, Nasuia. Collectively, these results indicate an ongoing symbiotic genome reduction among Hawaiian leafhopper lineages and distinct patterns of evolutionary rate in each bacterial partner.