Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2006
Publication Date: 6/1/2006
Citation: Barkley, N.L., Roose, M.L., Krueger, R., Frederici, C.T. 2006. Assessing genetic diversity and population structure in a citrus germplasm collection utilizing simple sequence repeat markers (ssrs). Theoretical and Applied Genetics. 112:1519-1531. Interpretive Summary: The Citrus Variety Collection located at the University of California, Riverside is a 97-year-old collection that was originally created to preserve valuable citrus germplasm. Since citrus has been selected and bred for thousands of years, many of the varieties in this collection do not have complete passport records detailing parentage or geographic origin. Additionally, the level of genetic diversity in this collection has never been assayed using qualitative methods. Molecular markers, specifically simple sequence repeat markers (SSRs), were used to assay the diversity in the entire collection and various species of Citrus. The genetic relationships between accessions were also assayed. Furthermore, the population structure was evaluated and the putative parentage of nybrid accessioins was examined. This data demonstrated that there are only a few naturally occurring Citrus species and the remaining are hybrids between these naturally occurring forms.
Technical Abstract: Twenty-four simple sequence repeat (SSR) markers were used to detect molecular polymorphisms among 370 apparently sexually-derived Citrus accessions from the Citrus Variert Collection maintained at the University of California, Riverside. Genetic diversity statistics were calculated for each individual SSR marker, the entire pouplation, and for specified Citrus groups. Phylogenetic relationships among all citrus accessions and putative non-hybrid citrus accessions were determined by constructing neighbor-joining trees. There was strong support for monophyly at the species level when hybrid taxa were removed from the data set. Additionally, Citrus accessioins were assigned to populations or multiple populations of their genotype indicated admixture by a model-based clustering approach. This approach identified five pouplations in this data set. These separate analyses (distance and model based) both support the hypothesis that there are only a few naturally occurring species of Citrus and most other types of Citrus arose through various hybridization events between these naturally occurring forms.