--Gurmit Singh and S.S. Gill
During germination trials in the laboratory on cv. Partap-I, while sorting out normal and abnormal seedlings, some seedlings were encountered which had two coleoptiles (Figure 1). In the literature, nothing was found on seedlings with two coleoptiles in maize. In rice, seedlings containing two coleoptiles were designated as "twin seedlings" by Professor Yuan Long, Director of the Hunan Hybrid Rice Research Centre in Changsha, China. The frequency of the twin seedlings was 0.25 percent. The experiment was repeated many times, and each time the twin seedlings were planted in a pot but none grew beyond 10-15 days. Rice embryo sac analysis indicated that most twin seedlings arose from fertilized two-egg nuclei or multiple egg nuclei and cleavage embryos. Yuan reported adventitious embryos in the embryo sac with frequencies of 2.6 to 5.1 percent, which originated from the nucellar cells.
According to Yuan these twin seedlings indicate apomixis in plants. Apomixis or asexual reproduction is a genetic tool to develop true-breeding hybrids with permanently fixed heterosis. This facultative apomixis could be used by (1) using some chemicals that can selectively kill fertilized eggs but not the adventitious embryos; or (2) transferring the long dormancy character to such twin seedlings and sowing F1 hybrid seeds before the alleviation of dormancy.
During a search for the twin seedlings trait at IRRI, seven rices (of
452 screened so far, from India, China and Guyana) exhibited this trait
with a frequency of 0.14 to 0.51 percent. One rice line from China also
possessed triple seedlings at the rate of 0.67 percent. The frequency of
twin seedlings can be increased by germinating the seeds without shell.
Nitrogen in the germination media, hydrogen peroxide treatment and germinating
the seeds at optimum temperature also enhanced this trait.
However, the significance of the twin seedlings in maize has to be realized. Work on the establishment of the twin seedlings characteristic is in progress.
1. Seedling with two coleoptiles.
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