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Tree Genetics and Molecular Breeding 2014, Vol.4, No.2, 1
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dominant high acidity or low pH allele and
ma
is the
low acidity or high pH allele (Maliepaard et al., 1998).
The primary role of the
Ma
locus in determining fruit
acidity has also been demonstrated by QTL mapping,
as a major QTL has been consistently detected on LG
16 (Kenis et al., 2008; Liebhard et al., 2003; Xu et al.,
2011). Recently, Xu et al. (2011) localized the
Ma
locus to a region of 150 kb on chromosome 16 in the
apple genome sequence. In addition, Bai et al. (2012)
have identified an aluminum-activated malate
transporter (ALMT) gene as a strong candidate for
Ma
.
Fruit juice browning is closely related to the
processability of apple fruit into substances such as
juice, sauce, and cut fruit. Because increased
browning greatly reduces the marketability of these
processed products (Murata et al., 1995), cultivars
with a low degree of fruit juice browning are generally
desired. Genetic analyses focusing on flesh browning
in apple have been performed by several research
groups (Kenis et al., 2008; Mellidou et al., 2012; Sun
et al., 2014), resulting in the identification of several
QTLs. In addition, Gualdo et al. (2013) have reported
QTLs for fruit flesh browning and their association
with two candidate genes encoding phenylalanine
ammonia lyase (PAL) and polyphenol oxidase (PPO).
These studies were all focused on fruit
flesh
browning
rather than fruit
juice
browning, however, and it is
unclear whether or not the two types of browning are
regulated by the same genetic mechanism.
In this study, we analyzed a ‘Fuji’ × ‘Maypole’ F
1
population to analyze the genetic basis of fruit juice
browning and fruit acidity. We identified major QTLs
for both traits on LG 16 in ‘Fuji’. In addition, the QTL
was physically determined based on genotyping of
recombinant progeny, and several candidates located
within the QTL region were examined for their
associations with fruit juice browning and fruit acidity.
1. Results
1.1
Genetic analysis of fruit juice browning and fruit
acidity
For each genotype, average phenotypic values of 3
years from 2011 to 2013 were used in genetic analysis,
because fruiting in the ‘Fuji’ × ‘Maypole’ (referred to
as Fj×Mp) population was not stable among years and
a limited number of individuals produced fruits in
each year (e.g., in 2011 fruits of 45 individuals were
harvested). Although the number and genotype of
fruiting progeny varied among years presumably due
to biennial bearing, phenotypic values of fruit juice
browning and fruit acidity in the same individuals
were almost constant and showed significant
correlation (
r
=0.68~0.70 in fruit juice browning,
r=
0.80~0.84 in fruit acidity). The degree of fruit juice
browning in the Fj×Mp population varied between
1.00 and 5.00, with a mean of 3.48±0.17. The
frequency distribution for this trait was bimodal: we
observed one peak in the higher range-high and
medium browning—representing most of the
population, and another in the lower browning range
(Figure 1A). The boundary between low and
high/medium browning seemed to be around 3.00. On
the other hand, fruit acidity ranged from 0.30 to 1.60,
with a mean of 0.84±0.03 g/100 mL, and showed a
continuous distribution with a single peak around 0.70
g/100 mL (Figure 1B).
Figure 1 Frequency distoributions of fruit juice browning (A)
and fruit acidity (B) in F
1
population of ‘Fuji’בMaypole’
Note: The number of progeny at each value represents the mean
for each progeny of up to 3 successive years from 2011 to 2013