International Rice Research Institute, P. O. Box 933, Manila, Philippines
Twin seedlings are known to originate from fertilized/unfertilized two or multiple egg nuclei or from the division of the zygote. Depending on their origin, twin seedlings may be diploid/diploid, diploid/haploid or haploid/haploid. Bashaw (1980) suggested that high frequency of twin seedlings was one of the indicators of apomixis in plants. This genetic tool in which seed is produced through asexual process, can help to develop true breeding hybrids with permanently fixed heterosis. Its use in hybrid rice is truly worthwhile long-term goal. Four rice lines showing twin seedling frequency of 5.0 to 32.4% were identified from 1979 to 1985 in China (Li Yuan-Ching, Yuan Long-Ping and J.N. Rutger, in preparation). Yuan, Li, and Deng (1990) conducted embryo sac analysis of these four twin
Table 1. Frequency of twin and triplet seedlings in selected rice cultivars, IRRI (1990)
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Cultivar IRRI Origin Seedlings Twin Triple
Acc. No. observed seedlings seedlings
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Hou Men 76583 China 1428 2(0.14) 1(0.07)
Hua Luo 76602 China 605 1(0.16) -
Hong Jiao Zhi 76565 China 624 1(0.16) -
Blue Belle 45198 Guyana 539 1(0.19) -
ARC 6227 12257 India 195 1(0.51) -
ARC 7217 12329 India 452 1(0.22) -
ARC 7287 12355 India 617 1(0.16) -
Radin China 4 10 Malaysia 435 1(0.23) -
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Figures in parenthesis denote percent.
seedling lines and found adventitious embryos originating from nuceller cells
of embryo sac at frequency of 2.5 to 5.1%. These seedlings were of maternal
origin and were considered as a source of facultative apomixis.We screened 783 rice accessions from International Rice Germplasm Center at IRRI. Seeds (195-1428) of each accession were incubated in Petri dishes (15 cm diameter) at a temperature of 32 deg +/-1 deg C. The germinating seedlings were analyzed for twins and triplets after seven days of incubation. Eight cultivars (3 from China, 3 from India, one from Guyana and one from Malaysia) showed twin seedling traits with a frequency of 0.14 to 0.51% (Table 1). One rice cultivar from China also showed triplet seedlings with a frequency of 0.07%.
Eight of the nine twin seedlings showed separate mesocotyls. Three of the twin seedlings did not grow more than one week after their identification. Six twin seedlings and one triplet are being grown to 1) analyze their embryo sac development, 2) cross with a dominant marker to assess the extent of their apomixis, and 3) multiply their seeds to study their offspring for frequency of twin seedling trait.
References
Bashaw, E. C. 1980. Apomixis and its application in crop improvement. In: W. R. Fehr and H. H. Hadley (eds.), Hybridization of Crop Plants, p. 45-63. American Society of Agronomy and Crop Science Society of America,Madison, WI.
Yuan, L. P., Y. C. Li and H. Deng, 1990. Progress of studies on rice twin seedlings. Abst. 4th Annual Meeting, Rockefeller Foundation, International Programme on Rice Biotechnology, Intern. Rice Res. Inst., Philippines.