GENETICAL AND PHYSIOLOGICAL ANALYSIS OF PSEUDO- SELF-COMPATIBILITY IN PETUNIA HYBRIDA

Transcription

1 JAPAN. J. GENETICS Vol. 48, No. 1: (1973) GENETICAL AND PHYSIOLOGICAL ANALYSIS OF PSEUDO- SELF-COMPATIBILITY IN PETUNIA HYBRIDA HIDEJIRO TAKAHASHI Laboratory of Applied Botany, Faculty of Agriculture, Kyoto University, Kyoto 606 Received October 11, 1972 It is known that most of self-incompatible species include a few self-compatible plants, and that there are two kinds of self-compatibility, i.e., true self-compatibility and pseudo-self-compatibility (East and Yarnell 1929). Pseudo-compatibility is caused by many factors, and Denward (1963) classified them as follows: (1) environment, especially in species with a `weak' incompatibility system, (2) modifier genes randomly segregating, (3) variation among S alleles in ability to allow the pollen tubes grow in incompatible pistils, (4) mutations, spontaneous or induced, (5) polyploidy, and (6) interspecific hybridization and disturbance of the polygenic basis of the incompatibility reaction. There are many reports about pseudo-compatibility, but the definition of pseudo-compatibility is obscure and distinct difference between the true self-compatibility and pseudo-self-compatibility is unknown. In the present experiments, the genes participating in true- and pseudo-compatibility were analyzed using self-compatible and pseudo-self-compatible Petunia hybrida as material, and the time needed for pollen-tube growth from stigma to ovary in the plants showing various grades of compatibility was also examined. MATERIALS AND METHODS From the plants of cultivated petunia (Petunia hybrida), partially and completely self-compatible plants were chosen, and the clones derived from them were named as clone 3 and clone 4, respectively. Experiment 1 Clone 3 and clone 4 were selfed by bud pollination. The obtained seeds were sown and the plants were grown up. From the progenies of each clone ten plants were picked out, reciprocal sib-crosses were made among the ten plants and seeds obtained were counted (X) in order to examine the grade of compatibility. On the other hand, each progeny was crossed with the pollen of clone 11J (S2S2), and the number of seeds obtained was counted (Y). The grade of compatibility (fertility) was, then, shown by seed set (%) which was defined by X JY X 100. The origin of clone 11J was already given by the author (Takahashi 1972).

2 28 H. TAKAHASHI Experiment 2 From the progenies of clone 3 (Experiment 1), 3 heterozygous plants showing various grades of self-compatibility were chosen. They were named as clone 33E (70-100%), clone 33G (10-40%) and clone 33H (0-10%), respectively. These 3 plants were self-pollinated and obtained progenies were examined for the grade of their self-compatibility. Experiment 3 Clone 33E and clone 4 were crossed as male with clone 11J which was self-incompatible. The obtained F1's were self-pollinated or crossed with the pollen of clone 11J to examine the grade of compatibility. Furthermore, among the F1 plants obtained after the crossing clone 33E and clone 11J, the most self-compatible plant was chosen, and its clone was named as clone 13JE. Clone 13JE was crossed as male with clone 11J. The obtained progenies were examined for their grade of compatibility in the same manner as mentioned above. Experiment 4 Clone 33E, 33G, 33H and 4 were self-pollinated and kept at 20 C or 30 C. After a certain hours, these plants were removed their styles and stigmas. As the control these plants were crossed with the pollen of clone 11J. The seeds obtained were counted and percentage of seed set was calculated. RESULTS AND DISCUSSION The results of Experiment 1 are given in Tables 1 and 2. Table 1 shows that the reciprccal crosses among the progenies of clone 3 give various grades of compatibility. These results mean that clone 3 is heterozygous for S alleles and is pseudo-selfcompatible. These alleles were named as S6 and S6. Table 2 shows that the reciprocal crosses among the progenies of clone 4 give either complete compatibility or incompatibility. This fact means that clone 4 is truely self-compatible having S alleles in heterozygous condition. One of the alleles has the same character as the self-compatible gene, S1, dicovered by East (1929) in Nicotiana langsdorfii. This allele pair was named as S7S f. The results of Experiment 2 are shown in Table 3. Progenies produced by selfing of pseudo-self-compatible plants showed various grades of self-compatibility. But the progenies having similar grade of self-compatibility to that of their parents is most in number. From these facts it is suggested that pseudo-compatibility is controlled by polygenic system. The results of Experiment 3 are shown in Table 4. Although all F1's obtained after the crossing clone 11J and clone 33E have S2 gene inherited from self-incompatible clone 11J, their progenies showed a little higher grade of compatibility with the pollen

3 PSEI IDO-SELF-COMPATIBILITY 29 Table 1, Compatibilities among the progenies (A-J) of a partial self-compatible plant (clone 3) Table 2. Compatibilities among the progenies (A-J) of a self-compatible plant (clone 4) of clone 11J than with the pollen of themselves. This fact means that the genes participating in pseudo-self-compatibility differ from S alleles, and are transmitted separetely from the S alleles. Meanwhile, F1's showed a little lower grade of selfcompatibility than the progenies obtained by selfing of clone 33E (Table 3). Furthermore, the progenies obtained by crossing clone 11J and clone 13JE showed much lower self-compatibility. These facts also support the view that pseudo-compatibility is probably caused by polygenic system, and such polygenes are diluted through the crossing with self-incompatible plant. In Petunia, Mather (1943) already suggested that incompatibility reaction was under polygenic control.

4 30 H. TAKAHASHI Table 3. Frequency of the progenies showing which were obtained from the selfing various grades of self-compatibility of pseudo-compatible plants Table 4. Frequency of the progenies showing various grades of were obtained from the crossing of self-incompatible plants compatibility which and self-compatible The F1's obtained after crossing clone 11J and clone 4 gave only two kinds of progenies, self-incompatible and self-compatible, and all F1's were incompatible when crossed with the pollen of clone 11J. So the cause of true self-compatibility is limited to S gene. The results of Experiment 4 performed at 30 C are shown in Fig. 1 and those carried out at 20 C are given in Fig. 2. Though the rate of pollen-tube growth was

5 PSEUDO-SELF-COMPATIBILITY 31 Fig. 1. Fertility of pseudo-self-compatible (A) and self-compatible (B) plants whose pistils and stigmas were removed at various times after the pollination at 30 C. X : clone 33E, A : clone 33G, / : clone 33H, : selfing of clone 4, 0 : crossing of clone 4, : selfing, : crossing Fig. 2. Fertility of pseudo-self-compatible (A) and self-compatible (B) plants whose styles and stigmas were removed at various times after the pollination at 20 C. X : clone 33E, A : clone 33G, / : clone 33H, : selfing of clone 4, Q : crossing of clone 4, : selfing, : crossing quite different between 30 C and 20 C, similar results were obtained at both temperatures, that is, in the selfing of pseudo-self-compatible plants, pollen-tube growth was much slower than that of crossing, and the lower the grades of their self-compatibility, the slower was their pollen-tube growth. On the other hand, in the self-compatible plant, the growth of own pollen was a little faster than that of the crosspollinated pollen. These facts mean that in pseudo-compatible plant S allele itself acts normally, but the suppression of pollen-tube growth is insufficient, so the pollen tubes grow through the style and some of them reach to ovary.

6 32 H. TAKAHASHI From the results of Experiments 1-4, it is supposed that the style of self-incompatible plant may contain a substance which suppresses the pollen-tube growth in self-pollination. The specificity of this substance depends on S allele and whose activity is controlled by polygene. This substance will be called S substance hereafter. One of the factors controlling pseudo-compatibility may be lowering the activity of the S substance. As shown in Table 1, crosses using the pollen of plant B gave lower fertility than others. This lowered fertility may depend on low vitality of plant B's pollen, because plant B is partially male sterile. Figures 1 and 2 suggest that even in the same plant each pollen has different vitality, because not only in selfing but also in crossing there was about ten hours' difference in the time needed for pollen tube to elongate from stigma to ovary. In a previous report the author showed that pollen vitality played a great role in pseudo-compatibility (Takahashi 1972). So the vitality of pollen may be another factor of controlling pseudo-compatibility. Many investigators claimed that pseudo-compatibility was caused by changes in S gene itself. For example, some workers believed that pseudo-compatibility is caused by mutation of S allele in (1) female side, (2) male side, (3) both male and female sides and (4) by revertiblemutati on (Lewis 1949; Lewis 1951; Pandey 1956; Brewbaker and Natarajan 1960; Denward 1963; Pandey 1970). But if S gene itself were changed to compatible allele, such changed S allele should act as S f in clone 4, and the plant having it should be truely self-compatible. In Trifolium re~ens, Atwood (1942) also stated that pseudo-compatibility was a quantitative character, probably determined by several additive genes, and little, if any, relationship existed between the S factors and the amount of pseudo-self-compatibility. The author supposes that pseudo-compatibility is brought about by both lowered activity of S substance which suppresses pollen-tube growth and by increased vitality of pollen. SUMMARY The genes participating in true or pseudo-self-compatibility of Petunia hybrida were analyzed. It was found that pseudo-compatibility is probably caused by polygenic system rather than S alleles, and that true self-compatibility is due to the change in S gene to S f. Furthermore, the time needed for pollen-tube growth from stigma to ovary was examined in the plants showing various grades of compatibility. The result was that the lower the grade of compatibility, the slower was the growth rate. It is supposed that in the style of self-incompatible plant exists certain substance named as S substance which suppresses pollen-tube growth of own pollen grains. The specificity of this S substance depends on S allele and its activity is probably controlled by polygene. Pseudo-compatibility may be brought about by lowered activity of the S substance and by increased vitality of pollen.

7 PSEUDO-SELF-COMPATIBILITY 33 ACKNOWLEDGMENTS The author wishes to express his gratitudes to Mr. Tei Teramura, Dr. Atushi Takimoto and Dr. Kotaro Watanabe for their kind advices. LITERATURE CITED Atwood, S., 1942 Genetics of pseudo-self-compatibility and its relation to cross-incompatibility in Trifolium yepens. J. Agr. Res. 64: Brewbaker, J. L., and A. T. Natarajan, 1960 Centric fragments and pollen-part mutation of incompatibility alleles in Petunia. Genetics 45: Denward, T., 1963 The function of incompatibility alleles in red clover (Trifolium pratense L.). II. Results of crosses within inbred families. III. Changes in the S-specificity. Hereditas 49: , East, E. M., 1929 Self-sterility. Bibliog. Genet. 5: East, E. M., and S. H. Yarnell, 1929 Studies on self-sterility. VIII. Self-sterility allelomorphs. Genetics 14: Lewis, D., 1949 Structure of the incompatibility gene. II. Induced mutation rate. Heredity 3: Lewis, D., 1951 Structure of the incompatibility gene. III. Types of spontaneous and induced mutation. Heredity 12: Mather, K., 1943 Specific differences in Petunia. I. Incompatibility. J. Genet. 45: Pandey, K. K., 1956 Mutations of self-incompatibility alleles in Trifolium pratense and T. yepens. Genetics 41: Pandey, K. K., 1970 Elements of the S-gene complex. VI. Mutations of the self-incompatibility gene, pseudo-compatibility and origin of new self-incompatible alleles. Genetica 41: Takahashi, H., 1972 Unilateral pseudo-compatibility between the inbred progenies and their parents in self-incompatible Petunia hybrida. Japan. J. Genetics 47: