Genetic Diversity and Population Structure Analysis of Citrus Germplasm with Single Nucleotide Polymorphism Markers

Citrus (Citrus sp.) germplasm collections are a valuable resource for citrus genetic breeding studies, and further utilization of the resource requires knowledge of their genotypic and phylogenetic relationships. Diverse citrus accessions, including citron (Citrus medica), mandarin (Citrus reticulata), pummelo (Citrus maxima), papeda (Papeda sp.), trifoliate orange (Poncirus trifoliata), kumquat (Fortunella sp.), and related species, have been housed at the Florida Citrus Arboretum, Winter Haven, FL, but the accessions in the collection have not been genotyped. In this study, a collection of 80 citrus accessions were genotyped using 1536 sweet orange-derived single nucleotide polymorphism (SNP) markers, to determine their SNP fingerprints and to assess genetic diversity, population structure, and phylogenetic relationships, and thereby to test the efficiency of using the single genotype-derived SNP chip with relatively low cost for these analyses. Phylogenetic relationships among the 80 accessions were determined by multivariate analysis. A model-based clustering program detected five basic groups and revealed that C. maxima introgressions varied among mandarin cultivars and segregated in mandarin F-1 progeny. In addition, reciprocal differences in C. maxima contributions were observed among citranges (Citrus sinensis x P. trifoliata vs. P. trifoliata x C. sinensis) and may be caused by the influence of cytoplasmic DNA and its effect on selection of cultivars. Inferred admixture structures of many secondary citrus species and important cultivars were confirmed or revealed, including 'Bergamot' sour orange (Citrus aurantium), 'Kinkoji' (C. reticulata x Citrus paradisi), 'llyuganatsu' orange (Citrus tamurana), and palestine sweet lime (Citrus aurantifolia). The relatively inexpensive SNP array used in this study generated informative genotyping data and led to good consensus and correlations with previously published observations based on whole genome sequencing (WGS) data. The genotyping data and the phylogenetic results may facilitate further exploitation of interesting genotypes in the collection and additional understanding of phylogenetic relationships in citrus.