Identification of QTLs linked to fruit quality traits in apricot (Prunus armeniaca L.) and biological validation through gene expression analysis using qPCR

Nine important fruit quality traits—including fruit weight, stone weight, fruit diameter, skin ground colour, flesh colour, blush colour, firmness, soluble solids content and acidity content—were studied for two consecutive years in two F 1 apricot progeny derived from the crosses ‘Bergeron’ × ‘Currot’ (B×C) and ‘Goldrich’ × ‘Currot’ (G×C). Results showed great segregation variability between populations, which was expected because of the polygenic nature and quantitative inheritance of all the studied traits. In addition, some correlations were observed among the fruit quality traits studied. QTL (quantitative trait loci) analysis was carried out using the phenotypic data and genetic linkages maps of ‘B×C’ and ‘G×C’ obtained with SSR and SNP markers. The most significant QTLs were localised in LG4 for soluble solids content and in LG3 for skin and flesh colour. In LG4, we can highlight the presence of candidate genes involved in D-glucose and D-mannose binding, while in LG3, we identified MYB genes previously linked to skin colour by other authors. In order to clearly identify the candidate genes responsible for the analysed traits, we converted the QTLs into expression QTLs and analysed the abundance of transcripts in the segregating genotypes ‘GC 2–11’ and ‘GC 3–7’ from the G×C population. Using qPCR, we analysed the gene expression of nine candidate genes associated with the QTLs identified, including transcription factors (MYB 10), carotenoid biosynthesis genes (LOX 2, CCD1 and CCD4), anthocyanin biosynthesis genes (ANS, UFGT and F3’5’H), organic acid biosynthesis genes (NAD ME) and ripening date genes (NAC). Results showed variable expression patterns throughout fruit development and between contrasted genotypes, with a correlation between validated genes and linked QTLs. The MYB10 gene was the best candidate gene for skin colour. In addition, we found that monitoring NAC expression is a good RNA marker for evaluating ripening progression.