Identification of differentially expressed genes in a spontaneous mutant of 'Nanguoli' pear (Pyrus ussuriensis Maxim) with large fruit

The selection of mutants is one of the most important steps in horticultural breeding. Fruit size is an important breeding objective in pear (Pyrus spp.) because of its economic importance. The pear cultivar 'Da Nanguoli' (Pyrus ussuriensis Maxim) produces larger fruit than 'Nanguoli', from which 'Da Nanguoli' was a bud mutation. The molecular basis for this bud mutation remains unknown. In the present study, ploidy analysis showed that 'Da Nanguoli' did not exhibit any alteration in the overall ploidy level. Seventy-six transcript-derived fragments (TDFs) of genes that were differentially expressed between the two cultivars were sequenced using cDNA-AFLP technology. BLAST-X analysis showed that 26(34.2%) of the TDFs had high sequence homology with known proteins in the non-redundant National Center for Biotechnology Information (NCBI) protein sequence database (i.e., E-values < -3.00). In total, 17 TDFs were chosen and their patterns of expression revealed using cDNA-AFLP and confirmed using quantitative real-time polymerase chain reaction (qRT-PCR). In addition, 98 TDFs, representing candidate genes, were studied in more detail to determine their functions in order to dissect the complex molecular mechanisms involved in the fruit-size mutation.