Genetic diversity and phylogenetic relationships of citron (Citrus medica L.) and its relatives in southwest China

Understanding the genetic diversity and phylogenetic relationship of citron (Citrus medica L.) is of great importance for making conservation and utilization strategies. Southeast Asia and its vicinity are usually regarded as one of the centers of origin for Citrus. The principal aims of this work were to evaluate the genetic diversity of 56 accessions of citron and its relatives mainly from southwest China, to investigate the genetic structures of citron populations, and to construct a phylogenetic tree to understand the evolution of some hybrids among citrons and other citrus types. SSR analysis detected a total of 387 alleles ranging from 2 to 12 alleles per locus, and nearly all accessions identified could be unequivocally distinguished. The observed and expected heterozygosities averaged 0.36 and 0.49, respectively. From a Bayesian cluster analysis, citrons were characterized by two distinguished genetic structures, which corresponded to the geographical distribution in southwest China. Citron and fingered citron might derive from a common ancestor. Based on the chloroplast sequences, the phylogenetic trees were constructed with congruent topologies and similar levels of statistical support for relationships among citron and its relatives using both maximum parsimony and Bayesian inference methods. Citron formed a monophyletic clade, which was completely different from mandarin (C. reticulata) and pummelo (C. grandis). The genetic contributors of some hybrid species, such as C. limonia, C. aurantifolia, and C. limon, were also discussed, and citron appeared to be a primary contributor to the speciation of these secondary citrus species.