A Lotus japonicus mutant, Ljsym75, which forms ineffective symbiotic nodules and defines a new locus involved in the process of nitrogen fixation, was characterized in detail in order to identify the stage of developmental arrest of the nodules. No nitrogen-fixing activity was detectable in Ljsym75 nodules at any stage during plant development, and plant growth was markedly retarded. Ljsym75 plants formed twice as many nodules as the wild-type Gifu, and this phenotype was not influenced by the application of low concentrations of nitrate. Although the ineffective nodules formed on Ljsym75 were anatomically similar to effective Gifu nodules, Ljsym75 nodules senesced prematurely. Microscopic examination revealed that bacteria endocytosed into Ljsym75 nodules failed to differentiate into bacteroids. Moreover, the bacteria contained no nitrogenase proteins, whereas leghemoglobin was detected in the cytosol of the nodules. These results indicate that Ljsym75 is required for bacterial differentiation into nitrogen-fixing bacteroids in nodules, and thus the Ljsym75 gene was renamed sen1 (for stationary endosymbiont nodule). Linkage analysis using DNA markers showed that Sen1 is located on chromosome 4.

The cytocidal effect of HM-1 produced by Hansenula mrakii on yeast Saccharomyces cerevisiae cells was studied. The HM-1 strongly inhibited the growth of S. cerevisiae cells at a low concentration (IC50: 2.1 x 10(-8) M) by reducing the number of viable cells. The killer action of HM-1 was most efficient when cells were actively proliferating. Cells in a resting state were resistant, but they became HM-1-sensitive after about 90 min of culturing at 30 degrees C, concomitantly with the increment of budding index. In association with the reduction of viable cell number, ultraviolet light-absorbing cellular components were discharged from sensitive cells. HM-1 molecules appear to bind to susceptible cells rather loosely since cells incubated with HM-1 were able to proliferate after having been washed. By phase-contrast light microscopy and scanning electron microscopy, discharge of cell material was observed at the budding portions of HM-1-treated cells. Addition of sorbitol to make the culture medium isotonic partially reduced the cell death induced by HM-1. These results suggest that HM-1 acts on the budding region of proliferating yeast cells, resulting in pore formation, leakage of cell material and eventual cell death.

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Phase relations in the system Y z O 3 -Y Z o 3~~~~ were studied at >1400°C. Yttrium oxide with the rare-earth C-type structure transformed into a hexagonal structure at 2360" and melted at 2400°C. The eutectic between Y203 and 3Yz0,.W0, was 2334°C at =90 mol% Y,O. 3Y,Q. WO, with a rhombohedral structure changed into a cubic phase at 1765" and melted at 2360"; 5Y20,. 2W03 with a tetragonal structure changed into the cubic phase at 1740°C. This cubic phase was stable at >1740" and had a solid-solution range of 69 to 75 mol% Y203 at 1800°C. 7Y,O,. 4 w 0 3 melted incongruently at 2107°C. Y203.W03 with a monoclinic structure (a' phase) transformed into an orthorhombic structure ( E phase) at 1470", then into a tetragonal structure (/3 phase) at 1600" to 1700°C. This compound melted incongruently at 1706°C. A phase diagram is given.

Morphological studies on gametes and sperm entry into the egg of Rhodeus ocellatus were conducted using transmission and scanning electron microscopy. The spermatozoon has has a round head, a mitochondrion, and a flagellum but no acrosomal structure. The egg is enclosed in a chorion that has a micropyle at the animal pole. After a spermatozoon attaches to the well-developed microvilli of the egg surface facing the micropyle, it is covered by the microvilli and incorporated into the egg cytoplasm with membrane fusion occurring between gametes. The egg cytoplasm just beneath the micropyle swelled a little while incorporating the spermatozoon and promptly plugged the micropyle. The nuclear envelope of the spermatozoon disappeared first at the apical region of the head and then progressed posteriorly through a process of vesiculation. The nucleus of the sperm head at once initiated decondensation upon incorporation through the effects of the egg cytoplasm. The mechanism of block to polyspermy in Rhodeus eggs is discussed.