Abstract
Seedlessness is a desirable characteristic for both fresh and processed citrus markets and one of the most important breeding objectives. In this paper, progress on studies for seedless breeding of citrus in Japan is reviewed. Among the several types of male sterility, anther abortion is the strictest male sterility in citrus and was shown to be controlled by both nuclear and cytoplasmic genes. Several seedless cultivars with male sterility have been developed. The mechanism and inheritance of the strictest female sterility derived from ‘Mukaku Kishu’ (Citrus kinokuni hort. ex Tanaka) were clarified and seedless cultivar and parental lines with this female sterility have been released. Some self-incompatible cultivars show seedlessness when coupled with parthenocarpy. S (self-incompatibility) genotypes of several cultivars have been estimated. Tetraploid plants, as parents of triploid offspring, were obtained from nucellar seedlings, and by ploidy mutation, colchicine treatment, and protoplast fusion. Triploid plants were produced from the combination of not only tetraploid and diploid crosses but also diploid and diploid crosses. New triploid seedless cultivars were bred by programmed cross-breeding and protoplast fusion.
