Although the occurrence of sterility in hybrids between Oryza sativa
and O. glaberrima has long been recognized (Chu et al. 1969;
Morinaga et al. 1957; Morishima et al. 1962,1963; Tao et
al. 1997), the genetic mechanism is unclear. To study this phenomenon,
we pollinated two O. glaberrima accessions, IRGC101767 and IRGC102203,
with japonica cultivars, most of which derive from Yunnan Province.
All F1 hybrids were highly sterile, with about 50% of pollen
grains showing typical abortion and 50% showing spherical abortion (Table
1). After pollination of the hybrids with their japonica parent
as recurrent parent, some of the crosses gave rise to a number of semi-sterile
plants (Table 2).
As the population sizes were too small to allow any genetic conclusions
to be drawn, a larger population (275 BC1F1 plants)
was developed by pollination of IRGC102203/IRAT216 by IRAT216. The average
fertility of pollen grains was 19%, significantly higher than that obtained
from the 37 individuals in Table 2. This result indicates that it is necessary
to increase the population size to obtain more fertile individuals from
interspecific BC1F1 between O. sativa and
O. glaberrima. Pollen fertility for all plants was below 57.5%,
except for one plant of normal fertility (Figure 1). When 50 randomly
selected BC1F1 plants were pollinated by IRAT216,
a BC2F1 population of 203 plants was obtained. The
average pollen fertility increased to 33.7%. When 50 BC2F1
individuals with semisterility or high sterility were pollinated by IRAT216,
the pollen fertility was 36.5% for 419 BC3F1 individuals.
From the distributions for BC1F1 to BC3F1
shown in Figure 1, it is apparent that semi-sterile individuals became
more and more frequent as the number of backcrosses increased. The distribution
was discontinuous, with a fertile peak at 97.5%, and a trimodal distribution
below 57.5% where
peaks occurred at 42.5-47.5%, 22.5-27.5%, and 12.5-17.5%. This indicated
that the inheritance of fertility in interspecific hybrids involved several
When one BC1F1 individual with pollen fertility
of 32.1% was bagged for selfing, 76 plants of the BC1F2
population showed an average pollen fertility of 89.2%, with few individuals
below 57.5%. This result indicated that both male and female gametes were
involved in sterility, and the sterility was mainly gametophytic.
This study was supported in part by Yunnan Natural Science Foundation
and West Africa Rice Development Association (WARDA).
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