Comparative assessment of genetic diversity in Albanian olive (Olea europaea L.) using SSRs from anonymous and transcribed genomic regions

A total of 560,578 sequencing reads obtained from the de novo NGS transcriptome sequencing of Olea europaea were searched for the presence of tri- and tetra-nucleotide repeat motifs, which resulted in 375 tri-nucleotide microsatellites and 252 tetra-nucleotides. Twelve expressed sequence tags (EST)-derived SSRs that gave a reproducible pattern, enabling easy scoring and allele size determination, were further tested for polymorphism on 165 cultivars O. europaea subsp. sativa and on 18 oleasters (O. europaea subsp. sylvestris). The main diversity parameters of EST-SSRs were compared to the genomic SSRs that have been most frequently used so far in olive genotyping and mapping studies. Diversity parameters were also compared for different lengths of repeat motifs (di-nucleotide, tri-nucleotide, and tetra-nucleotide microsatellites). The occurrence of null alleles was only observed in anonymous SSRs, while their frequencies were negative for all EST-SSR loci. Eight out of 12 EST-SSRs were classified as highly informative, indicating their applicability in assessing molecular diversity and genetic relatedness in olive. The 12 novel EST-SSR markers selected as a subset of all identified loci containing SSR motifs showed their suitability for evaluation of genetic diversity and population structure and for genetic mapping. They were also linked to genes and, as shown by annotation analysis, four of them have potential for interpretation of allelic variability related to disease resistance and can be evaluated as a potential diagnostic marker for disease resistance. The development of new EST-SSRs provides a valuable tool in molecular breeding programs for both cultivated and wild olives.