Comparative Genomic Insights into Endofungal Lifestyles of Two Bacterial Endosymbionts, Mycoavidus cysteinexigens and Burkholderia rhizoxinica

Endohyphal bacteria (EHB), dwelling within fungal hyphac, markedly affect the growth and metabolic potential of their hosts. To date, two EHB belonging to the family Burkholderiaceae have been isolated and characterized as new taxa. Burkholderia rhizoxinica (HKI 454(T)) and Mycoavidus cvsteinexigens (B1-EBT), in Japan. Metagenome sequencing was recently reported for ilziortierella elongata AG77 together with its endosymbiont 3d. cvsteinexigens (Mc-AG77) from a soil/litter sample in the USA. In the present study, we elucidated the complete genome sequence of B1-EBT and compared it with those of Mc-AG77 and HKI 454(T). The genomes of B1-EBT and Mc-AG77 contained a higher level of prophage sequences and were markedly smaller than that of HKI 454(T). Although the B1-EBT and Mc-AG77 genomes lacked the chitinolytic enzyme genes responsible for invasion into fungal cells, they contained several predicted toxin-antitoxin systems including an insecticidal toxin complex and PIN domain imposing an addiction-like mechanism essential for endohyphal growth control during host colonization. Despite the different host fungi, the alignment of amino acid sequences showed that the HKI 454(T) genome consisted of 1,265 (32.6%) and 1,221 (31.5%) orthologous coding sequences (CDSs) with those of BI-EBT and Mc-AG77, respectively. This comparative study of three phylogenetically associated endosymbionts has provided insights into their origin and evolution, and suggests the later bacterial invasion and adaptation of B1-EBT to its host metabolism.