Figs. 1-9. Agamospermy observed in C1001. Explanations in text,
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26. A new genetic material: Apomictic rice C1001
Xue-Xing GUO1 and Xi-Jin MU2
1) Chengdu Institute of Biology, Acad. Sinica, Chengdu 610041, China
2) Institute of Botany, Acad. Sinica, Beijing 100044, China
Apomixis has been found in many crops, such as sorghum, maize, pearl millet, etc. Since apomixis can be used to fix heterosis, it is important to look for apomictic materials in rice (O. sativa). In 1989 spring, the senior author discovered some of progeny plants of C1001B, which was a maintainer obtained from the F7 of Sheng-Long X 17148 (a line from Zhen-San 97B///Guang-Keng A// O. rufipogon/O. longistaminata), and C1001A, which was a male-sterile strain selected from the 5th generation of backcross of Zhen-San 97A (male-sterile) with C1001B and maintained by C1001B, produced twin- or triple-seedlings. Preliminary studies of these progenies are reported in this paper.
C1001A and C1001B have the nature of agamospermy. C1001A x C1001B (=C1001A, completely male sterile) produced some seeds during four generations observed, although seed fertility varied according to seasons (Table 1). The seed fertility in summer was obviously higher than that in spring. The higher temperature in summer during reproduction may account for the higher frequency of agamospermy. C1001B emasculated by the hot-water method (45 °C, 5 min.) also gave some seed set during three generations studied. In contrast, the maintainer Zhen-San 97B did not.
Table 1. Frequency of agamospermy in C1001 (in %)
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Strain Treatment Summer 1989 Spring 1990 Summer 1991 Spring 1991
(Shuangliu) (Lingshuf) (Shuangliu) (Lingshuf)
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C1001A (male sterile) 7.83 5.65 7.42 3.62
C1001B Emusculation - 6.43 6.47 3.88
Zhen-San Emusculation - 0 0 0
97B
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The seeds of C1001A X C1001B cross and the seed set upon self-pollination of C1001B yielded multiple seedlings at a certain frequency, but the seeds set on male-sterile C1001A and emasculated C1001B produced by apomixis as described above yielded single seedlings only (Table 2). This indicated that the multiple seedling production was related to the manner of fertilization. We observed that the frequency of multiple seedlings increased with the rise of mean temperature during flowering.
Table 2. Frequency of multiple seedlings in C1001 (in %)
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Strain Treatment Spring 1991 Winter 1989 Spring 1990 Winter 1991
(Shuangliu) (Lingshui) (Shuangliu) (Lingshui)
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C1001B Self 8.1 6.4 11.3 4.5
Emusculation 0 0 0 0
C1001A Crossed with 18.1 20.2 10.3 0.9
C1001B
Isolated 0 0 0 0
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Seed anatomy showed that some seeds of C1001B had two embryos.
One was a normal pear-shaped embryo located at the normal position, and the other
was a smaller globular one located in the internal side of the normal one (Fig.
1). The two embryos were observed in some of germinating seeds of C1001B cut longitudinally.
Each of them had its own radicle, plumular axis and plumule.
Soaked seeds of C1001B germinated in dark at 35 °C for 3-4 days produced multiple seedlings: twin- or triple-seedlings (Figs. 2-4). About 90% of the twin seedlings possessed two mesocotyles. Some of them grew out of the same position. This kind of twin-seedlings were called homoestatic ones (Fig. 2). Others came out of different positions and were called heterotopic ones (Fig. 3).
To make out the origin of the twin seedlings, plants with two leaves were cut into slices continually from the root end by the paraffin method, and stained with satanic and light green. No connections of vascular bundles were observed between the twin seedlings. This demonstrated that the twin seedlings developed from two embryos, not from lowernode tillers. In the latter case, the primary seedlings and tillers should be linked together by vascular bundles.
Embryological observation of the young ovaries of C1001 sampled one to eight days after flowering showed that most of the ovules and embryo sacs developed normally. With respect to C1001B, the nucellar cells sometimes differentiated into embryos, as did the fertilized egg and polar nuclei. The process of differentiation of the nucellar cells was as follows: A particular nucellar cell was specialized close to the inner integument in the middle-lower part of embryo sacs (Fig. 5). The specialized cell was two to three times as large as other nucellar cells, and possessed denser cytoplasm, a bigger nucleus and a bigger nucleolus. After several divisions, a mass of cells was produced, some of them having more than one nucleus. Then, there appeared a protrusion under the epidermis of embryo sac (Fig. 6). With the growth of the protrusion and the disintegration of other nucellar cells around it, the protrusion separated gradually from other nucellar tissues (Fig. 7). In this stage, there were still some cells with multiple nuclei. Then, an embryo-like structure with an epidermal tissue was formed. This structure developed further into a pseudo-embryo from which a scutellum grew (Fig.8). At last, an adventitious embryo from a nuceller cell came into being (Fig. 9).
The above observations suggest that C1001 is an apomictic strain. This character is heritable. The apomictic embryos originate from nuceller cells. Fertilized seeds had two embryos, one zygotic and the other apomictic. They germinated and produced twin seedlings. Unfertilized seeds only had the apomictic embryo and produced single seedlings.
Figs. 1-9. Agamospermy observed in C1001. Explanations in text,
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