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 major genes.

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.

Acknowledgment

This study was supported in part by Yunnan Natural Science Foundation and West Africa Rice Development Association (WARDA).

References

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