In radish, the mitochondrial gene orf138 found in Ogura cytoplasm is responsible for male sterility, whereas the nuclear restorer gene (Rf gene) inhibits the expression of orf138. To investigate the distribution of the Rf gene for Ogura male sterile cytoplasm in Japanese wild radish, 226 plants collected from 15 regions in Japan and two additional sites of Korea were used as materials. PCR analysis indicated that 42% of the wild radishes possessed orf138. On the other hand, 207 plants (91.6%) were judged to have an Rf gene by the observation of pollen fertility in wild radishes themselves and in hybrids obtained by crosses with a male sterile variety having Ogura cytoplasm. For plants having restorer function, we analyzed the nucleotide sequence of the Rf gene, orf687, identified to date. Among the 207 plants having the Rf gene, 199 amplified the orf687 fragment of the expected size by PCR. PCR products were further analyzed by mismatch-specific endonuclease digestion and PCR-RFLP analysis. As a result, plants that showed an identical RFLP pattern to the known Rf type of orf687 were restricted to 30 plants (14.5% of the 207 plants) collected mainly in populations of southern region of Japan. It was also found that most of the plants (148 plants; 71.5% of 207 plants) possessed the sequence of the orf687 corresponding to the rf genotype lacking the restorer function. We conclude that the Rf gene widely distributed in Japanese wild radish is not orf687.
The aphidophagous gall midge Aphidoletes aphidimyza (Rondani) (Diptera: Cecidomyiidae), a dominant natural enemy of aphids, has been used as a biological control agent in many countries to control aphids in greenhouses. As developmental arrest in diapause lowers the effectiveness of natural enemies, we studied the effect of photoperiod and temperature on the incidence of diapause in a Japanese strain of A. aphidimyza by examining diapause induction under different day-length conditions in the laboratory. The critical day length for diapause induction was determined to be 12.7 h at 20°C. Diapause incidence was completely prevented at 30°C even though the photoperiod used was 11L13D. We also examined diapause induction under changing temperature conditions while maintaining the critical day length (12.7L11.3D). Diapause incidence was 100% in both field and greenhouse conditions under alternating temperatures of 20/16 or 25/16°C while the critical day length of 12.7 h was maintained. The Japanese strain of A. aphidimyza was sensitive to diapause entry from the first to the third noncocooned instar larval stages. Its eggs do not seem to be sensitive to diapause induction. Our results suggest that constant short-day conditions for at least four days are needed to induce diapause in the Japanese strain of A. aphidimyza.
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