GENETIC DIVERSITY IN THE ORANGE SUBFAMILY AURANTIOIDEAE .2. GENETIC-RELATIONSHIPS AMONG GENERA AND SPECIES

Genetic relationships were studied by means of ten isoenzymatic system s, at the genus and species level, using two distances and four method s of aggregation in a germplasm collection of 198 cultivars and access ions of 54 species belonging to Citrus and 13 related genera. The most consistent results were obtained by the chord distance and the neighb or-joining clustering method. Citrus species were distributed in two m ain groups: the orange-mandarin group and the lime-lemon-citron-pummel o group. The species C. halimii and C. tachibana are not included in t hese groups. Mandarin species fall into three main subgroups: one incl udes C. sinensis; the second, C. aurantium, the third, small-fruit spe cies. The citron, the pummelo and the ancient lemon subgroups form a c luster to which the species belonging to subgenus Papeda and the culti vated limes, lemons and bergamots are related. Microcitrus spp, to whi ch Severinia buxifolia and Atalantia ceylanica seem to be related, clu ster with the lime-lemon-citron-pummelo group while Fortunella is clos e to the orange-mandarin group. Poncirus trifoliata, the most importan t species for citrus rootstock improvement is located far from Citrus but connected to it through Fortunella spp. A broad distribution of sp ecies has been found that should be taken into account to sample new g enotypes in the search of desired characters in order to fully and eff iciently use genetic resources for citrus improvement.