Abstract:
In the Western Cape region of South Africa dormancy release and the onset of growth does
not occur normally in apple (Malus x domestica Borkh.) trees during spring due to the
mild winter conditions experienced and fluctuations in temperatures experienced during
and between winters. In this region the application of chemicals to induce the release of
dormancy forms part of standard orchard management. Increasing awareness of the
environmental impact of chemical sprays and global warming has led to the demand for
new apple cultivars better adapted to local climatic conditions. We report the construction
of framework genetic maps in two F1 crosses using the low chilling cultivar ‘Anna’ as
common male parent and the higher chill requiring cultivars ‘Golden Delicious’ and
‘Sharpe’s Early’ as female parents. The maps were constructed using 320 simple sequence
repeats (SSR), including 116 new markers developed from expressed sequence tags
(ESTs). These maps were used to identify quantitative trait loci (QTLs) for time of initial
vegetative budbreak (IVB), a dormancy related characteristic. Time of IVB was assessed 4 times over a 6-year period in ‘Golden Delicious’ x ‘Anna’ seedlings kept in seedling bags
under shade in the nursery. The trait was assessed for 3 years on adult full-sib trees derived
from a cross between ‘Sharpe’s Early’ and ‘Anna’ as well as for 3 years on replicates of
these seedlings obtained by clonal propagation onto rootstocks. A single major QTL for
time of IVB was identified on linkage group (LG) 9. This QTL remained consistent in
different genetic backgrounds and at different developmental stages. The QTL may
co-localize with a QTL for leaf break identified on LG 3 by Conner et al. (1998), a LG that
was, after the implementation of transferable microsatellite markers, shown to be
homologous to the LG now known to be LG 9 (Kenis and Keulemans, 2004). These results
contribute towards a better understanding regarding the genetic control of IVB in aplle and
will also be used to elucidate the genetic basis of other dormancy related traits such as time
of initial reproductive budbreak and number of vegetative and reproductive budbreak.
