Reproductive isolation is one of the main obstacles to the practical
use of super-heterosis between indica and japonica hybrid
rice. Discovery of wide-compatibility genes(WCG) shed a light on intersubspecific
hybrid rice (Ikehashi et al. 1987; Wan et al. 1996). However,
those intersubspecific hybrids, completely fertile under normal condition
with the aid of WCGs, are still sensitive to relatively low temperature.
This kind of sterility , named low temperaturesensitive sterility (LTSS),
was found mainly due to the low-temperature-sensitive pollen sterility
(LTSPS, Li et al.1996, 1997), and remained one of the major problems
in intersubspecific hybrid rice breeding (Ikeda 1994, Li et al.
1996). In this study, we tried to illuminate the
genetic basis underlying LTSPS by quantitative trait loci (QTLs) analysis.
157 F2 plants derived from 3037 (indica) and 02428 (japonica,
a wide-compatibility varietiy with S5n) (Zou
et al. 1989) were planted together with the F1 hybrid
and the parents in the experimental farm of the Agricultural Academy Jiangsu
Province, Nanjing, China, 2002. Field management was basically the same
as usual except the delay of seeding time. The June 20 planting provided
a relatively low temperature stress (21~23C) for spikelet development
ing booting stage, especially for pollen development. Anthers were collected
from spikelets just before flowering, and pollen grains from crushed anthers
were suspended in potassium iodide solution (KI-I2). All round
and strongly stained pollen were scored as normal fertile and irregular-shaped
yellowish or unstained pollen grains were scored as sterile (Chaudary
et al. 1981). Frequency distribution of pollen fertility for F2
population, along with F1 and parents are shown in Fig. 1.
The linkage map was constructed with 108 SSR markers (developed by McCouch
et al. 2002) spanning 1857.8 cM along the rice genome with average
interval of 17.20 cM and used for QTL mapping with MapMaker/QTL 1.1b(Lincoln
et al. 1993). LOD score of 2.0 was used as criteria to indicate
the putative QTLs.
Two putative loci, namely qLTSPS2 and qLTSPS5, were detected
on chromosomes 2 and 5 by interval mapping, which explained 15.6% and
11.9% of phenotypic variation respectively, with additive effects of 0.021
and 0.045, dominant effects of -0.246 and -0.215, and degrees of dominance
of 11.7 and 4.8, respectively (Table 1, Fig. 2). The additive effects
of the two QTLs have been contributed by the japonica cv 02428.
The gene action is overdominance, which coincided with the phenotypic
appearance shown in Table 2, where the average pollen fertility of heterozygotes
from F2 population on either locus was lower than that of homozygotes.
The entire genome was searched at a 0.001 probability level for digenic
interaction for the trait with two-way ANOVA using all possible two-locus
combination of marker genotypes. No epistatic loci were detected, which
is similar to the low temperature-sensitive sterility (Li et al.
The two QTLs for LTSPS described here were detected with relative low
LOD scores as threshold level and only account for a total of 27.5% of
the phenotypic variation, suggesting that there should be many other minor
QTLs for LTSPS, some of which would be detected by a LOD score lower than
the threshold level used in this study. Further analysis of the data present
here is now underway.
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