| Genetic control of parthenogenesis in common wheat
YASUHIKO MUKAI Department of Biology, Osaka Kyoiku University, Ikeda, Osaka 563, Japan It has been known that the egg cell of Triticum aestivum strain Salmon with Aegilops kotschyi cytoplasm develops parthenogenetically at a high frequency due to an interaction between a 1BL-1RS translocation chromosome and an alien cytoplasm. In this study, mechanism controlling parthenogenesis of the egg cell was investigated embryologically and genetically using (kotschyi)- Salmon and its aneuploids. Seven aneuploids of (kotschyi)-Salmon were produced in the backcrossed or selfed offspring from the cross, (kotschyi)-Salmon x ditelosomics 1BS of T. aestivum cv. Chinese Spring. The short arm of 1B chromosome of Chinese Spring (1BS) carries a fertility-restoring gene, Rfv1, for the kotschyi cytoplasm . Since these lines were backcrossed six times with Salmon's pollen, they are almost pure for the Salmon genetic background. Embryological studies of (kotschyi)-Salmon revealed the following facts: (1) The egg cell starts haploid parthenogenesis before the fertilization and develops up to a globular embryo (20- 50 cells). (2) The parthenogenetic embryos never develop any more in non-pollinated ovules. (3) When polar nuclei are fertilized by pollination, most haploid embryos develop normally owing to the formation of endosperm. (4) After the egg cell started development, one of synergids sometimes develops just like an egg cell. (5) When this synergid is fertilized, a diploid embryo is formed. The embryo sac contains the haplo-diplo twin embryos. Frequencies of parthenogenetic embryo and haploid in (kotschy)-Salmon and its anueploids are shown in Table 1. From both embryological and genetical data, fate of the egg cells in four lines is summarized in Fig.1. Di- and monosomics 1BL-1RS of (kotschyi)-Salmon formed parthenogenetic embryos in the half of ovules; the other half of egg cells died. In disomic 1BL-1RS, haploids were obtained at a frequency of 90.4% of germinated seeds. The single diploids could be derived from the diploid partners of n-2n twins, because the parthenogenetic haploid embryos were weak and died occasionally. Monosomics 1BL-1RS produced two kinds of haploid embryos. The haploid embryos lacking the 1BL-1RS translocation chromosome (2n=20) were very weak and often died during embryo development. When aneuploids of (kotschyi)-Salmon having one or more 1BS arms were backcrossed with Salmon's pollen, the diploid offspring always had the 1BS arm. Conversely, the 1BS arm was never transmitted to the haploid offspring. In alloplasmic aneuploids carrying two or three 1BS arms, the egg cell lacking this arm was occasionally produced owing to desynapsis in meiosis, resulting in a parthenogenetic haploid embryo. On the bases of the results, we can draw the following conclusions: (1) When the female sporophyte has the 1BL-1RS translocation chromosome, the egg cell without the 1BS arm develops parthenogenetically and forms haploid embryos. In the egg cell, the translocation chromosome is not necessary for induction of parthenogenesis. (2) On the other hand, the 1BS- carrying egg cell never develops parthenogenetically though it has the translocation chromosome coinsidently. Then, this egg cell takes part normally in the fertilization, resulting in diploid offspring. (3) Therefore, 1RS arm of the translocation chromosome carries a gene (Ptg) inducing haploid parthenogenesis sporophytically; 1BS arm carries a gene (Spg) suppressing haploid parthenogenesis gametophytically. This study was supported in part by a Grant-in-Aid (No. 61304014) from the Ministry of Education, Science and Culture, Japan. |