Transovarial Transmission of Symbiotic Bacteria in Mastotermes darwiniensis (Isoptera: Mastotermitidae): Ultrastructural Aspects and Phylogenetic Implications

Intracellular symbiotic bacteria in specialized cells (bacteriocytes) of the fat body are found in all cockroaches and in the termite Mastotermes darwiniensis Froggatt. DNA sequence analysis of the bacteria in the two taxa shows them to be phylogenetically related; thus, it has been suggested that the bacteriocyte symbiosis was established in an ancestor common to cockroaches and termites, with a loss of the symbiosis in all other known termite lineages. The offspring of both cockroaches and Mastotermes acquire the symbiosis via transovarial transmission. However, transmission of the bacteria in Mastotermes has not been examined since the pioneering works carried out by Koch in the 1930s. Here we report on transovariole transmission in primary and neotenic reproductives, and show that the ovarioles of both harbor fat body, which contains bacteriocytes. The conspicuous number of ovarian bacteriocytes in neotenic queens corresponds to the pattern previously observed in female nymphs. Bacterial transmission to offspring during the life of the colony thus occurs through continuous provisioning of symbionts to the ovariole from bacteriocytes in ovarian fat body. This pattern parallels that of cockroaches as does the entire mechanism of transovarial transmission.