Loss of Fertility in X - Irradiated Acetabularia

In X-irradiated Acetabularia cells, cysts fail to germinate. However, meiosis of the primary nucleus and mitosis of secondary nuclei take place. The observed loss of fertility could result from a 'late' death effect or from deleterious interaction between nuclei of opposite sign inside multinucleate cysts. Algae are known to be more resistant to ionizing radiation than higher plants or animals (Chipman, 1972). The unicellular marine alga Acetabularia shows a high tolerance to both Xand gamma rays (Bacq et al., 1955; Hammerling, 1956; Six, 1958; Bonotto et al., 1970). However, early investigations on A. Wettsteinii (= A. parvula) and A, mediterranea (= A. acetabulum)' revealed that X-rays reduce fertility already at doses which have little effect on cell growth and morphogenesis (Hammerling, 1956; Six, 1958). Since the process leading to loss of fertility is not yet understood, our investigations on the effect of X-rays focused on the reproductive phase of A. acetabulurn (= A. mediterranea). Cells at Stage 4 (Bonotto and Kirchmann, 1970) were irradiated with single X-ray doses of 50 and 100 Kr and their morphological differentiation was continuously monitored. Table 1 lists representative data on the percentage of cells which had developed a cap (reproductive organ comparable to a sporangium) and cysts (gametangia) in the cap rays. The formation of cysts in 58 % and 11 % of cells, which had received a dose of 50 and 100 Kr respectively, indicates that meiosis of the primary nucleus in the rhizoid and subsequent mitotic divisions occurred, as well as the migration of the haploid secondary nuclei into the cap ray cytoplasm (for a recent review on meiosis and mitosis in Acetabularia, see Koop, 1979). Upon microscopical inspection of cyst-bearing caps, morphological alterations became obvious (Fig. 1). In Under the International Code of Botanical Nomenclature the correct name of Acetabularia rnediterranea Lamouroux is now Acetabularia acetabulurn (L.) Silva (Silva, 1952) O Inter-Research/Printed in F. R. Germany contrast to the rather uniform size and ovoid shape of cysts in control cells (Fig. 1 a) , cysts in irradiated cells are highly variable in size and morphology (Fig. 1 b). Moreover, photographic recording of cyst development revealed the formation of multinucleate cytoplasm portions rather than uninucleate ones as in control cells (Liittke and Bonotto, in preparation). Since the number of secondary nuclei visible in completed cysts exceeds that initially observed during constriction of the cytoplasm, further mitotic divisions must have taken place. For the normal sequence of meiosis and subsequent mitoses before and after cyst formation, despite the morphological alterations, no prediction on gamete formation and germination could be made. Hence cysts from single caps were collected in separate test vials and their germination capacity checked after a preceeding dark period (Koop, 1975). The experimental protocol is outlined in Table 2 together with the developmental stage of the cells. In three experiments, carried out with cells of different stock cultures, germination occurred only once (1 out of 22 caps; this corresponds to 5 %). A second attempt was made with single giant cysts isolated, subjected to dark treatment, and then returned to light in order to induce germination. They equally failed to germinate. In either case most of the cysts degenerated and turned pale after a few days in the light. Table 1. Acetabularia acetabulurn. Cap and cyst formation in X-irradiated individuals Dose Noofcaps ' % No of caps %