Grain traits such as length and weight are not only related to production,
but also have direct bearing on the marketability in rice. So far, quantitative
trait loci (QTL) for grain traits in rice have been identified in many
QTL studies by using all kinds of cross combinations in the sativa
gene pool and AA genome species (Tan et al. 2000). However, genetic
studies on the grain characters have not been reported in the interspecific
progeny developed using O. minuta (BBCC) due to difficulty in developing
progeny (Brar and Khush 1997). The aim of this study is to identify and
characterize QTLs underlying grain characters in a backcross progeny derived
from a cross between the O. sativa subsp. japonica cv. Hwaseongbyeo
and O. minuta (2n = 48, BBCC, Acc. No. 101141).
An introgression line, WH79006 was produced from a single plant from BC5F3
families from a cross between Hwaseongbyeo used as a recurrent parent
and O. minuta as a donor parent, which was subsequently self-pollinated
for three generations (Kang et al. 2002). WH79006 resembled the
O. sativa parent, Hwaseongbyeo, however it differed from Hwaseongbyeo
in several traits including 1,000 seed weight and days to heading (Kang
et al. 2002). These differences might be due to O. minuta
introgressions. To detect the polymorphism between Hwaseongbyeo and O.
minuta, 511 microsatellite markers of known chromosomal position were
used for the parental survey. Of the 511 markers, 231 (45.2%) showed polymorphism.
Of these 231 polymorphic markers, 45 SSR markers detected 28 independent
O. minuta introgressions in WH79006 located on all chromosomes
except for chromosome 2 (Jin et al. 2004). The high number of introgressions
in WH79006 might be due to selection for O. minuta phenotype in
the generation advancement (Kang et al. 2002). To evaluate the
effects of these segments on the grain traits, 75 F2:3 lines
derived from the cross Hwaseongbyeo/WH79006 were developed and evaluated
for grain traits. Bulked 300 brown rice of fully filled grains of each
of the F2:3 families were evaluated for seed length (sl),
seed width (sw), seed thickness (st), 1,000 seed weight
(tsw) and seed length/width ratio (lwr). The frequency distribution
of phenotypes for four traits in the 75 F2:3 lines are shown
in Figure 1. All traits showed approximately normal distribution suggesting
Forty-five SSR markers that detected O. minuta-specific introgressions
in WH79006, were used for evaluation of the genotype of the F2
population (Fig. 2). One-way ANOVA was carried out to detect association
of introgression with the grain characters. QTL was declared if the phenotype
was associated with a marker locus at P < 0.001 or with two
adjacent marker loci at P < 0.05. A total of four QTLs for 4
traits were identified (Fig. 2, Table 1). The number of QTLs identified
in this study is probably an underestimation because some of the chromosomal
regions harboring introgressions might have been undetected with the SSR
markers employed and the population size (75 lines) was small to detect
QTLs of minor effects. The seed length QTL, sl11, was detected
on chromosome 11 and the WH79006 allele increased seed length. This QTL,
sl11 affecting seed length appears to share a similar region as
the grain length QTLs (Tan
et al. 2000, Yoshida et al. 2002), and their relationships
remain to be clarified. The other three QTLs associated with sw,
tsw, and lwr, were detected on chromosomes 7, 7, and 10,
respectively. The two QTLs, sw7 and tsw7 shared the similar
region, and at these loci the WH79006 alleles increased weight and width.
Because seed width and 1,000 seed weight are
related, it is quite possible that a single gene has a pleiotropic effect
on two traits. Fine mapping of this cluster would be needed to differentiate
between tight linkage and pleiotropy of the genes involved. These two
QTLs have not been detected in the previous QTL studies between Oryza
cultivars, indicating potentially novel alleles from O. minuta.
This study was supported by a grant from the Bio Green 21 project of the
Rural Development Administration, Republic of Korea.
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