A Numerical Taxonomic Study of Affinity Relationships in Cultivated Citrus and Its Close Relatives

The affinity relationships of 43 biotypes of cultivated Citrus-close relatives, interspecific and intergeneric hybrids, and clones of unknown origin-were studied to develop information for use in citrus breeding experiments. Intraspecific affinity was very high (87 to 98) in four of six cultivated Citrus species (C. aurantium, C. grandis, C. limon, and C. paradisi) but lower in C. reticulata (45 to 58) and in C. sinensis (37). Among Citrus species and relatives, the affinity pattern showed two main groups in Citrus and a third group consisting of Eremocitrus glauca and Microcitrus species. The larger Citrus group included five species (C. aurantium, C. grandis, C. paradisi, C. reticu- lata, and C. sinensis) and a smaller group of three (C. aurantifolia, C. limon, and C. medica). The charcteristics of C. grandis were domi- nant in interspecific hybrids. In hybrids of cultivated Citrus species and wild relatives, the characteristics of the latter were strongly domi- nant with one exception. The affinities of clones of unknown origin indicated probable hybrid origins of diverse genetic backgrounds. Comparisons of the data on the study clones were made with the major authoritative speculations on their derivations. Citrus grandis, C. medica, and C. reticulata are proposed as true biological species. Citrus aurantifolia, C. aurantium, C. limon, C. paradisi, and C. sinensis are proposed as unique, apomictically perpetuated biotypes of probable hybrid origin. A nonmetric multidimensional scaling solution is pre- sented to support, in part, the proposed systematic relationships. In- adequate sampling of the variation present in populations and a lack of appreciation of the effects of facultative apomixis on population samples and its relationship to genetic heterozygosity in Citrus have been the major obstacles in past efforts to produce an objective Citrus taxonomy.