[Advances in insect obligate endosymbionts and their genomes--a review]

In nature, many insects, especially sap-feeding insects, harbor nutritional bacterial symbionts, which are called obligate endosymbionts. These bacteria co-evolved with their hosts for millions of years. Obligate endosymbionts are commonly found in specialized organs, named bacteriomes or mycetomes that consist of a number of insect's cells (bacteriocytes or mycetocytes). Obligate endosymbionts strictly maternally inherited, providing essential amino acids to the hosts, and relating to survival, reproduction and evolution of the insects. Because of enriched nutritional environment, compared to those free-living bacteria, the genomes of obligate endosymbionts have different characteristics, such as genome size, GC content, and gene deletion. Although the genomes of many insect endosymbionts have been carefully analysis, the gene functions of endosymbionts and the interactions between endosymbionts/hosts and endosymbionts remain unknown. Thus, to provide an insight into the co-evolution of endosymbionts and their hosts, further studies of endosymbionts at genetic level are required.