Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/1998
Publication Date: N/A
Citation: FANG, D., KRUEGER, R., ROOSE, M.L. PHYLOGENIC RELATIONSHIPS AMONG SELECTED CITRUS GERMPLASM ACCESSIONS REVEALED BY INTER-SIMPLE SEQENCE REPEAT (ISSR) MARKERS. JOURNAL OF AMERICAN SOCIETY OF HORTICULTURAL SCIENCES. 1998. Interpretive Summary: The genus Citrus has an unclear taxonomy. Relationships among putative taxa are complicated by such factors as a high frequency of bud mutation, high cross-compatibility, and a long history of cultivation. Traditionally, citrus classification relied on morphological and some biochemical criteria. More recently, isozymes and molecular genetic techniques have been used. This study compares restriction fragment length polymorphisms (RFLP) and random amplified polymorphic DNA (RAPD) for analysis of phylogenetic relationships within the genus Citrus. RFLP had previously been used for evolutionary and taxonomic studies in Citrus and other spp; RAPD have been used for mapping and cultivar identification in Citrus but not in phylogenetic analysis of Citrus. These techniques were used to analyze accessions in the USDA-ARS-NCGR/UCR Citrus Variety Collection. Distance matrices and dendrograms were generated. Results from RFLP and RAPD analysis were generally similar. Most 'species' which apparently arose via hybridization had high levels of heterozygosity and no unique fragments. A species which had apparently not arose by hybridization had unique bands, indicating that it was probably not involved in any of the hybridization events leading to the apparently hybrid spp. Pummelo (C maxima) was affiliated with the papedas. Citron (C medica) clustered with C indica when only non-hybrid taxa were examined, and among limes, lemons, and relatives when all taxa were examined. Mandarins did not show distinctive groupings among themselves or with other species. In general, the data did not support the Tanaka system of citrus classification. The data indicated that some accessions were probably mis-identified.
Technical Abstract: Relationships among 88 accessions representing 45 Citrus species, three man-made hybrids, and six related genera were examined for restriction fragment length polymorphisms (RFLP). Thirty-two Citrus and three Microcitrus accessions were also examined by random amplified polymorphic DNA (RAPD) analysis. A measure of relative heterozygosity was estimated based on the mean of the number of fragments per individual per probe- enzyme combination (PEC) divided by total number of fragments per PEC for all non-hybrid Citrus individuals. The presence in a Citrus species of a rare band found also in a related genus was taken as an indiction of possible introgression, while the presence of several fragments unique to 1 species was used to indicate non-involvement of that species in hybridization events. Most species that have been described in the literature as hybrids had high heterozygosity indices and no unique fragments. Distance matrices and dendrograms were generated using simple matching coefficient and neighbor-joining cluster analysis. RFLP and RAPD data gave approximately the same results. These data shoed C. maxima was affiliated with the papedas C. hongheensis and C. latipes. C. medica clustered with C. indica when only non-hybrid taxa were examined, or among limes, lemons, and relatives when all species were considered. Mandarins did not show strongly supported groupings among themselves, nor with other species. These data showed that several accessions were probably assigned to the wrong species.