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Tree Genetics and Molecular Breeding 2014, Vol.4, No.2, 1
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165-bp genotype of NH026a tended to be distributed
on the higher browning and lower acidity side,
whereas progeny with the 151-bp genotype generally
exhibited lower browning and higher acidity.
Furthermore, fruit juice browning and fruit acidity
showed a significant negative correlation (
r
= −0.69),
with fruit of higher acidity progeny tending to have
lower fruit juice browning, and vice versa (Figure 4).
1.3 Physical mapping of the QTLs on chromosome 16
Analysis of the SSR markers CH02a03 and CH05c06
identified six recombinants in the Fj × Mp population.
Two classes of recombinants were not encountered:
those bearing fruit with lower acidity (<0.64 g/100 mL
and higher browning (>3.6), as well as those showing
higher acidity/lower browning. Screening of contig
sequences within the estimated QTL region of 1.4 Mb
(CH02a03–CH05c06) identified 18 SSR markers
(Table 1). Among the 18 markers, amplification of 15,
LG16-769020-1, -769020-2, -837763, -1003247-1,
-1159074, -1199876, -1352468, -1437845, -LAR-SSR,
-1526288, -1568112, -1661111, -1728791, -1788357
and -1860066, yielded polymorphic fragments that
mapped to LG 16 of ‘Fuji’. The 15 sets of SSR
markers were then analyzed in the recombinants, and
the region of recombination was successfully
identified in each recombinant (Figure 5a). From these
data, graphical genotypes of the QTL
region were
generated. LG16-1199876, NH026a, LG16-1352468,
-1437845, -LAR-SSR, -1526288, and -1568112 showed
co-segregation with the QTL in the recombinants and the
mapping population, suggesting that these markers
would be useful for marker-assisted breeding for fruit
juice browning and fruit acidity. The QTL regions for
both fruit juice browning and fruit acidity were delimited
between LG16-1159074 and -1661111 based on key
recombinants of Fj × Mp R-6 and Fj × Mp G-38 (Figure
5a). ABLAST search against the apple genome sequence
(Genome Database for Rosaceae (GDR) BLAST server
(http://www.rosaceae.org)
identified
two
contig
sequences containing the two markers—MDC010932.632
for LG16-1159074 and MDC006784.248 for
LG16-1661111—that are physically separated by
approximately 514 kb (Figure 5b).
1.4 Candidate genes for QTLs
To elucidate the genetic mechanisms underlying fruit
juice browning and fruit acidity QTLs, the estimated
514 kb region identified in the apple genome was
examined by
in silico
analysis for genes possibly
related to fruit juice browning and fruit acidity. A total
of 108 genes were predicted in the estimated QTL
region, including ALMT (MDP0000252114) and
leucoanthocyanidin reductase (LAR) (MDP0000171928).
2 Discussion
In this study, we analyzed the genetic basis of fruit
juice browning and fruit acidity using an Fj×Mp
population. The parents of the studied population had
contrasting phenotypes: ‘Fuji’, the leading cultivar in
Japanese and global apple production, is characterized
by low acidity (0.36 g/100 mL ± 0.02) and a high
degree of fruit juice browning (5.00±0.00), whereas
‘Maypole’, which bears small, astringent, sour fruit,
has high acidity (1.60 g/100 mL ± 0.04) and a
relatively low degree of fruit juice browning
(2.28±0.05). In the Fj × Mp population, both fruit
juice browning and fruit acidity varied greatly among
progeny (Figure 1a; Figure 1b).
The bimodal distribution observed for fruit juice
browning suggests the presence of a major QTL
controlling this trait in apple (Figure 1a). In contrast,
fruit acidity showed a normal distribution with a peak
around 0.7 g/100 mL (Figure 1b), a value at the higher
end of the acidity range among modern apple cultivars
(Iwanami et al., 2012). Mid-parent values of fruit
acidity were close to family means of the Fj × Mp
population. In crosses in which the acidity of one
parent is in a higher category, the distribution of
acidity has been reported to shift towards a higher
category (Iwanami et al., 2012). The tendency towards
high acidity in the Fj × Mp population can thus likely
be attributed to the pollen parent ‘Maypole’, which
bears fruit of extremely high acidity.
A major QTL for fruit juice browning was detected on
LG 16 of ‘Fuji’. In the mapping population used in
this study, minor QTLs for fruit juice browning were
also detected on LG 10 of ‘Maypole’ and LG 17 of
‘Fuji’. Previous studies have identified several QTLs
for flesh browning on LGs 3, 10, 15, 16, and 17
(Kenis et al., 2008; Mellidou et al., 2012; Sun et al.,
2014). Although these previous studies were all
focused on fruit
flesh
browning rather than fruit
juice