ENDONUCLEAR BACTERIAL SYMBIONTS IN 2 TERMITE SPECIES - AN ULTRASTRUCTURAL-STUDY

Intracellular symbionts (endosymbiotic bacteria), either isolated or i n clusters, were found within the cytoplasm and the nucleus of female germ cells and of trophocytes in nymphs and reproductives of two termi te species, Kalotermes flavicollis (Isoptera: Kalotermitidae) and Reti culitermes lucifugus (Isoptera: Rhinotermitidae), collected in the wil d. This finding represents the first record of endonuclear symbiotic b acteria in a metazoan species. The bacteria reach the host ovarian cel ls by crossing the tunica propria. The ultrastructural morphology of t he bacteria closely resembles that of cytoplasmic symbionts found in o ther insect orders (such as Dictyoptera and Homoptera) and their prese nce does not appear to be correlated with bacteriocytes (or mycetocyte s) as in the more primitive termite Mastotermes darwiniensis (Isoptera : Mastotermitidae). The endonuclear symbionts, as well as the cytoplas mic ones, are enveloped by a plasma membrane, by a cell wall typical o f Gram-negative bacteria and by a perisymbiotic membrane, the last two separated by an outer periplasmic space. In some of the endonuclear s ymbionts, however, the perisymbiotic membrane is lacking. Different ki nds of inclusions, apparently unrelated to mesosomes, are visible with in the endonuclear symbionts. A wide array of vesicles is found in the outer periplasmic space and inside the host chromatin, in close proxi mity to bacteria. Some vesicles apparently originate from the outer la yer of bacterial cell wall and are freed in the host chromatin by a fu rther process of budding and vesiculation of the perisymbiotic membran e. These observations support the hypothesis of product exchanges betw een endosymbionts and the host cell. The presence of endonuclear symbi onts (also seen in the act of dividing within the nucleus) does not ap parently affect the meiotic processes or cause host cell degenerations , although a less dense chromatin, aggregates of granular material, th in cylindroid bodies and bundles of microfibrils are indeed observed w ithin the nucleoplasm. A continuity between the perisymbiotic membrane and the RER is often detected in the cytoplasm and some endosymbionts appear to be closely associated with mitochondria.