Genetic determinism of flowering regularity over years in an apple multi-family population

Abstract

Irregular flowering over years is commonly observed in fruit trees and is assumed to be, at least partly, under genetic control. This study aimed at predicting genotype flowering behaviours and at detecting QTL associated to regularity, in a multi-family apple tree population. Successions of vegetative and floral annual shoots (AS) were observed along axes in trees belonging to five apple related full-sib families, observed at two experimental sites. Sequences were analysed using Markovian and linear mixed models including year and site effects. Indices of flowering regularity, periodicity and synchronicity were estimated, at tree and axis scales. First indices were derived from the Biennial Bearing Index (BBI). A second index was the auto-regressive correlation coefficient between flowering in consecutive years. A third index quantified the synchronicity of meristems within the trees through the measure of differences in the predictability of flowering over years, from a probabilistic viewpoint. These three types of indices were used to predict tree behaviour and to detect QTL with a Bayesian pedigree-based analysis, using an integrated genetic map containing 6,849 SNPs. The combination of the three indices efficiently predicted and classified the genotype behaviours despite few miss-classifications. Four common QTLs for BBIs and auto-regressive coefficient were highlighted (on LG4, 5, 8 and 10) and one for synchronicity (on LG9) in the integrated multi-family map, thus revealing the complex genetic architecture of the considered traits. This study proposes a posteriori sampling of axes within trees as a relevant and time-saving method to estimate tree flowering behaviours. Coupled to appropriate statistical indices, it is efficient to evaluate the tree breeding values for flowering regularity. In apple tree, biennial bearing appeared to result from high AS synchronism in flowering, i.e. with all axes alternatively flowering or not in a given year, whereas regularity resulted from either asynchronous alternating or regular flowering of AS.