Endosymbiont transmission in the ovary of the brown planthopper Nilaparvata lugens studied by transmission and
scanning electron microscopy
Drews, M., Zissler, D., Sander, K.
Institut für Biologie I (Zoologie), Hauptstr.1, D-79104 Freiburg, Germany
N. lugens harbours two species of endocytobiotic microorganisms, a yeast (dimensions 10 µm by 2 µm) and a gram-negative bacterium (1.5 µm by 0.4 µm). The yeasts are localized in a central syncytium in the abdominal fat body whereas the bacteria are also found in the hemolymph spaces between the outer fat body cells. After leaving the fat body, part of the microorganisms reach the ovary via the hemolymph. While in contact with the hemolymph, the surface layer of the yeasts gains strongly in electron density. The bacteria, however, look the same in hemolymph and in host cells. In previtellogenic ovarian follicles the oocyte posteriorly forms a cone-shaped extension which is covered by a narrow tube of follicular epithelium. This tube forms the "immigration zone", the only part of the epithelium to which symbionts from the hemolymph attach. The cone soon retracts, leaving an "immigration cavity" into which the symbionts pass. From there they move into a pit (the "reception pit") that has meanwhile formed in the posterior oocyte pole; it is continuous with the immigration cavity. The follicle cells of the immigration zone project extensions into the hemolymph which contact and engulf the microorganisms; concomitantly, the electron density of the yeast surface decreases. Enclosed in vesicles, the yeast and bacterial cells pass through the epithelial cytoplasm. They are then exocytosed into the immigration cavity. The yeasts apparently pass unharmed through the follicular cells whereas many bacteria become subject to destruction in the cytoplasm. It thus seems that the yeasts, which by far outnumber the bacteria, are better adapted to endocytobiosis than the latter. Towards the end of symbiont immigration, the oocyte seals the reception pit by fusion of its rims. In the oolemma-lined cavity thus formed near the egg cell's posterior pole, quite some space is left between the individual microorganisms. In this respect, the "symbiont ball" of N. lugens differs from the quite compact mass of endocytobionts found in the eggs of many other leafhopper species.
Our investigations were kindly supported by Ciba-Geigy, Basel, through special arrangement by Dr. V. Dittrich and Dr. G. Voss.
| LOCATION |
DATE |
TIME |
| Lecture Hall I |
Thursday, April 9 |
10:40 am |