National Institute of Genetics, Mishima, 411 Japan
"Alien pollen primacy" (APP), a kind of selective fertilization controlled by the female organ, was detected in a mixed pollination experiment of two rice varieties, IR36 and T65 (Sato 1988). Namely, the pollen of both IR36 and T65 tended to have more chances of fertilization in alien than in their own genotype. Now, I have learned that this phenomenon was caused by differential rates of pollen-tube elongation on their own and alien stigmas. Pollen grains of IR36 and T65 were deposited on their own (selfing) and alien (outcrossing) stigmas, and the stigmas were sampled 4, 8, 16 and 30 minutes after pollination. They were stained with anilin-blue solution (1 mg/l, 30 deg.C, 2 min.), and were observed under fluorescent microscope.
The elongation of pollen tubes started within one minute after pollination, and germination rate reached a plateau of 80% or more in four minutes. No difference in germination rate was observed between self- and cross-pollination (Table 1).
Table 1.Germination rate and pollen tube length of pollen grains
of IR36 and T65 deposited on their own and alien stigmas
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Time Germination Pollen tube length (micra)
Pollen Stigma __________________________
(min.) rate (%) Mean Max.
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IR36 IR36 4 87 25 40
8 86 55 240
16 80 - 1,250
30 83 - To ovary
T65 4 91 30 55
8 67 210 220
16 83 - 1,500
30 77 - To ovary
T65 IR36 4 74 20 50
8 94 180 400
16 71 - 1,800
30 82 - To ovary
T65 4 100 40 80
8 87 110 320
16 78 - 1,500
30 84 - To ovary
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At 8 and 16 minutes after pollen deposit, it was found that pollen tubes
were much longer in cross-pollination than in selfing (Table 1). The pollen
tubes reached the ovary 30 minutes after pollination in both selfing and
outcrossing.These observations indicate that alien pollen are accepted by the stigma and style in preference to its own pollen, and APP results from differential pollen- tube elongation. In indica x japonica hybrids, segregation distortion is often observed, and is considered to be due to genes for F1 sterility and certation linked with certain marker genes. In addition, genes causing APP may be a factor in segregation distortion.
Reference
Sato, Y. I., 1988. "Alien pollen primary" detected in rice. RGN 5: 80-81.