A Comparison of Differentially Expressed Genes Associated with Chill Accumulation during Endodormancy in Malus x Domestica borkh. Varieties ‘Anna’ and ‘Lady Williams’

Chill requirement in fruit trees is a great concern for apple producers in the temperate sub tropical countries. Most apple producing countries receive more than 1500 Richardson Units (RU) annually, in comparison to apple producing regions in South Africa, which receives between 400-1100 RU. As a result chilling requirement of the trees, during endodormancy, is not fulfilled, causing prolonged dormancy symptoms, like reduced branching and extended fruiting period. The two apple varieties, ‘Anna’ and ‘Lady Williams’ were used to compare the differentially expressed genes during chill accumulation. ‘Anna’ is a low chill apple variety that requires less than 300 hours of chill. ‘Lady Williams’ is a very high chill variety that requires more than 1500 hours of chill. QTL analysis was performed for a Lady Williams’ x ‘Anna’ mapping population using the apple 8K Infinium SNP-array. One major QTL on linkage group 9 and several minor QTLs on linkage groups, 2, 8 and 15 were detected. RNA was extracted from Lady Williams and Anna at 100h time intervals and analysed by RNA-Seq. The QTLs were investigated by mapping RNA-seq reads against the loci and surrounding regions to identifying possible genes within it. A number of transcripts were highlighted as being differentially expressed between time intervals. The differentially expressed genes from Anna were compared to those from Lady Williams. Differences between the patterns of expression were identified for further analysis.