THE INCIDENCE AND EFFECTS OF HYBRIDIZATION BETWEEN CULTIVATED RICE AND ITS RELATED WEED RED RICE (ORYZA SATIVA L.).

The objectives of this study were (1) to determine if hybridization occurs naturally between cultivated rice and a closely related weed, red rice (both Oryza sativa L.) and (2) to determine the incidence of hybridization and possible convergence of red rice with cultivated rice. Both morphological and electrophoretic characters were used to confirm the existence of hybrids. A total of 12,000 seeds were collected from red rice plants from fields of six different rice cultivars and sown in a common garden. Hybrids were generally taller and had longer, wider flag leaves than either red rice or cultivated rice. In addition, hybrids produced more tillers than red rice. Overall, hybrids were vegetatively robust plants, demonstrating heterosis and expressing morphological characteristics of both parents. That these plants were hybrids was confirmed by the existence of electrophoretic banding patterns common to both synthetic and natural hybrids. The incidence of hybridization was estimated as the percentage of hybrids found in red rice populations while morphological characteristics were used to test for convergence. Percentage hybridization ranged from 1% in the Lemont cultivar to 52% in the Nortai variety. The greater number of hybrids found in the Nortai variety was attributed to a later flowering time of this variety and the overlap of its flowering time with that of the F1 hybrids. F1 hybrids were observed to flower later than either red rice or the early season varieties of cultivated rice. Thus, early season varieties have a temporal separation in flowering time from the F1 hybrids that prevents back crossing. Unlike the other hybrids that were vegetatively robust, Nortai hybrids were less robust and more similar to the cultivar. These results suggest that later season cultivars may have a higher incidence of hybridization and introgression with red rice and that this may lead to morphological convergence of the weed toward the crop. Convergents were observed in the Nortai variety. Hybridization between cultivated and red rice has the potential to increase the adaptability of red rice populations by promoting genetic diversity. © 1990 The Society for the Study of Evolution.