Haploids Obtained from Diploid×Triploid Crosses of Citrus

Abstract

Four monoembryonic diploid (2n=18) citrus cultivars and hybrids were crossed with a triploid (2n=27) hybrid. Seeds were collected from each fruit of all crosses at maturity, and fully developed seeds were germinated. The seedlings had several ploidy levels. While most of them were diploid, some were haploid, aneuploid, triploid or tetraploid. The haploid plants were observed in two cross combinations : two seedlings out of 116 in Clementine and one out of 54 in 'Lee'. There was no difference in size between haploid and diploid embryos. Haploid embryos germinated normally and their primary roots elongated vigorously. Haploid plants transplanted to soil, however, grew very weakly. Restriction endonuclease analyses of nuclear ribosomal DNA and of chloroplast DNA were performed to determine the origin of the haploids. These three haploid plants had rDNA and cpDNA fragments specific to the seed parent, but no fragments specific to the pollen parent. This finding indicates that these haploid plants are of maternal origin.