Occurrence of Off-type Plants in japonica/indica Hybrid Rice Cultivars

Frequent occurrence of off-type plants in a given cultivar has been a serious problem in both breeder's and farmer's fields. An experiment was designed to examine the differences in rate of occurrence of off-type plants among Tongil-type cultivars (high yielding cultivars derived from indica/japonica hybridization) from which the possible cause of higher occurrence of off-type plant in a specific cultivar was deduced. Among five Tongil-type cultivars examined for morphological variant in the field, only one cultivar, Dasanbyeo, had off-type plants. When analyzed with SSR markers, off-type plants showed different band patterns from original cultivar, having several extra bands in addition to cultivar-specific band, suggesting that off-type plants were originated from Dasanbyeo, rather than originated from mixing or mishandling of seed materials with other cultivars. The possible cause of off-type occurrence seems to be natural pollination with other cuItivars adjacent to the original cultivar during seed multiplication. This was supported from the observation that self-crossed progeny of the off type plants showed a wide range of variation of agronomic traits which could not be observed when there was a smaller introduction of genes to the fixed germplasm as happened in the case of cultivar mutation. Another evidence supported this idea that Dasanbyeo showed much of difference in floral organ and behavior to other cultivar to be subjected to higher out-crossing than other cultivars examined.