Development of genic SSR markers from transcriptome sequencing of pear buds
A total of 8375 genic simple sequence repeat (SSR) loci were discovered from a unigene set assembled from 116282 transcriptomic unigenes in this study. Dinucleotide repeat motifs were the most common with a frequency of 65.11%, followed by trinucleotide (32.81%). A total of 4100 primer pairs were designed from the SSR loci. Of these, 343 primer pairs (repeat length ≥15 bp) were synthesized with an M13 tail and tested for stable amplification and polymorphism in four Pyrus accessions. After the preliminary test, 104 polymorphic genic SSR markers were developed; dinucleotide and trinucleotide repeats represented 97.11% (101) of these. Twenty-eight polymorphic genic SSR markers were selected randomly to further validate genetic diversity among 28 Pyrus accessions. These markers displayed a high level of polymorphism. The number of alleles at these SSR loci ranged from 2 to 17, with a mean of 9.43 alleles per locus, and the polymorphism information content (PIC) values ranged from 0.26 to 0.91. The UPGMA (unweighted pair-group method with arithmetic average) cluster analysis grouped the 28 Pyrus accessions into two groups: Oriental pears and Occidental pears, which are congruent to the traditional taxonomy, demonstrating their effectiveness in analyzing Pyrus phylogenetic relationships, enriching rare Pyrus EST-SSR resources, and confirming the potential value of a pear transcriptome database for the development of new SSR markers.