Cybridization in Citrus unshiu Marc. (Satsuma Mandarin) and C. sinensis (L.) Osbeck (Sweet Orange)

In citrus, male sterility is one of the most important characteristics because seedless fruits can be produced in a cultivar having both male sterility and parthenocarpy (Yamamoto et al. 1995). In many higher plants, male sterility is caused by nuclear-cytoplasmic interaction (Kaul 1988). In Citrus also, aborted anthers, which are the strongest male-sterihty factor (Iwamasa 1966), are caused by nuclear-cytoplasmic interaction, and only limited cultivars such as Citrus unshiu (satsuma mandarin) have sterile cytoplasm (Yamamoto et al. 1997). C unshiu, originating in Japan, is thus an important species for producing seedless cultivars. Furthermore, the species is superior in tree habit, productivity, disease resistance, and fruit quahty (early ripening, easy peeling, seedlessness) and adapts to the Japanese climate. Hence, production of C. unshiu accounts for approximately 70% of the Japanese Citrus production and is spreading all over the world. On the other hand, C. sinensis (sweet orange) is the most economically important Citrus in the world because of its excellent fruit quality. Although there are four types of C. sinensis (common orange, navel orange, blood orange, and acidless orange), all cultivars of C. sinensis have been developed from mutations.