New sources of cytoplasmic diversity in the Italian population of Olea europaea L. as revealed by RFLP analysis of mitochondrial DNA: characterization of the cox3 locus and possible relationship with cytoplasmic male sterility

Abstract

Restriction fragment length polymorphisms (RFLPs) of mitochondrial DNA (mtDNA) were investigated to detect intraspecific variation among the Italian cultivated and wild Olea europaea L. populations. RFLPs were visualized by Southern hybridization of EcoRI and HindIII restricted total cellular DNA using two mitochondrial genes ( cox3 and atpA) as probes. The mtDNA RFLP data gathered in our analysis allow the identification of three distinct olive tree mitotypes; each characterized by specific hybridization patterns. The result on genetic variability is also confirmed by the unweighted pair group method with arithmetic averages dendrogram (UPGMA) that combines the Italian thirty-seven different sources of Olea europaea L. into three major clusters of cytoplasmic relatedness. Therefore, at least as many maternal lineages are involved in the evolution and origin of the Italian olive tree mitochondrial genomes, with the cultivar Cerasòla showing the greatest diversity with the cox3 probe and two restriction enzymes. We, thus, report the identification of three novel diagnostic probe/enzyme combinations that can be used as markers of cytoplasmic diversity in the Euromediterranean O. europaea L. population. Further characterization of the polymorphic cox3 locus in the mtDNA of the cultivar Cerasòla by sequencing and expression analysis at the level of transcription and RNA editing pattern suggests a correlation between a duplication event of the cox3 locus and the male sterile phenotype of this cultivar. The mtDNA RFLPs provide, thus, a sensitive and reliable method to assess intraspecific cytoplasmic diversity within the economically important olive tree cultivars in Italy. Our results also provide new insights into the maternal lineages involved in the evolution of the Euromediterranean olive trees.