Breeding Barriers in the Interspecific Cross of Oryza sativa L. and Oryza minuta Presl.

Abstract

We examined the pre- and post-fertilization barriers in the interspecific cross of O.sativa, a cultivated Thai rice species and O.minuta, a wild species native to Asia. A 9.6% seed set was obtained from 649 pollinated spikelets. However, the seeds had abnormal embryos with degenerating endosperm. The F1 hybrid embryos failed to mature on the maternal plants. The pre-fertilization barriers observed were anomalous and stunted growth of pollen tubes in the style. Some of the pollen tubes reached the micropyle and the average rate of fertilization for the interspecific cross was 61.4%. In the interspecific cross, the fastest pollen tube reached the micropyle 10 hours after pollination, while it took only 5 hours in the parents. The major post-fertilization barrier observed was embryo breadkown. The hybrid embryos ceased to develop at the pre-globular, early globular or globular stage within 6-12 days after pollination (DAP). The size of the interspecific hybrid embryo at this stage was equivalent to that of a 1-to 2-day-old self-pollinated embryo. Arrested embryo growth and subsequent abortion around 10-12 DAP was associated with the failure of endosperm development. Only a small amount of free endosperm nuclei or cellular endosperm was observed in the embryo sac. Hybird embryos were rescued by culturing 10-to 14-day-old immature embryos on Murashige & Skoog (MS) agar medium containing 3 mg/l benzylaminopurine, 1 g/l yeast extract, 15% coconut water and 3% sucrose. Active tillering was observed on media supplemented with 0.3% activated charcoal. All the 24 F1 hybrid plants thus obtained were completely sterile. Colchicine treatment to the hybrids may provide an opportunity to derive some valuable fertile allohexaloids with desirable genes from the wild parent.