Genetic Analysis of Citron (Citrus medica L.) Using Microsatellite Markers

Citron (Citrus medica L.) constitutes a commercially important group of Citrus in many parts of the world. It is one of the three original species from which most cultivated citrus accessions are derived. Morphologically, citron fruits are diverse ranging from small ellipsoid to gigantic ellipsoid to elaborate fingered types. We have studied the genetic variability of 48 citrons collected from various parts of the world. Thirty accessions were sampled from Yunnan province of China, part of the region where Citrus medica originated and maximum diversity of citrons exist in the wild.  Eighteen citrons were sampled from the Citrus Variety Collection in Riverside, CA, and were originally obtained from different parts of the world. Citrons are known to be mono-embryonic, and previous studies have generally found little heterozygosity. For our study, we obtained data from 23 microsatellite markers distributed throughout the genome. We recorded 97 alleles and an average of 4.21 alleles per locus. Only 7 samples were homozygous in our study. 24 samples showed 5-20% heterozygosity, 15 samples had 22-52% heterozygosity.  A distance matrix was created using a Microsatellite analyzer software (Nei’s standard genetic distance algorithm) and Neighbor joining trees were constructed. The 48 citrons clustered into 3 distinct groups with an average genetic distance ranging from 0.3-0.4 between the groups. Cluster A had citrons from Mediterranean and South American countries; cluster B had fingered and some non-fingered citrons indigenous to the Yunnan province; Cluster C had some wild citrons from China. Similar results were obtained using population STRUCTURE analysis.