| Increased occurrence of haploids and twin seedlings
due to an alien cytoplasm H. KIHARA and K. TSUNEWAKI National Institute of Genetics, Misima, Japan In order to investigate the effects of an alien cytoplasm on the manifestation of various genotypes the authors have been placing by successive backcrosses nuclei of various wheats, including synthesized wheat and Triticale, in the cytoplasm of Aegilops caudata. During these experiments the authors have noticed that the alien cytoplasm increased the frequency of haploids arid twin seedlings and are reporting their observations. For convenience's sake wheat strains with their own cytoplasm are termed in this report "autoplasmic" and those with Aegilops cytoplasm "alloplasmic". Haploids: Records on the occurrence of haploids are at present available for two varieties, Triticum aestivum ssp. vulgare var. erythrospermum (T. vulgare erythr.) and Taylor's Triticale. The occurrence of haploids in auto- and alloplasmic lines of these varieties is summarized in Table 1. Haploid frequency of the alloplasmic line of T. vulgare erythr. is estimated to be somewhat between 0.7 and 2.7%, while that of the autoplasmic line was 0.0%; the difference is significant. The alloplasmic line of Taylor's Triticale produced haploids at a much higher rate than alloplasmic T. vulgare erythr., namely about 53 %. Apparently the nucleus of Taylor's Triticale is more influenced by caudata cytoplasm than the pure line nucleus of T. vulgare erythr. in regard to parthenogenetic development of haploid embryos. Twin seedlings: The occurrence of twin seedlings in auto - and alloplasmic lines of wheat (including Taylor's Triticale) was studied in detail in 1962. The results are summarized in Table 2. In autoplasmic wheat there is some tendency to increase the frequency of twin seedlings in hybrids between lines which produced more twins than pure lines. The average total frequency was about 0.7 per 1.000. It was increased about tenfold in the alloplasmic lines of wheat and about one-hundredfold in alloplasmic Taylor's Triticale. In this regard, too, the nucleus of the latter was more sensitive to caudata cytoplasm than those of wheats (including T. vulgare erythr.). Discussion: The observed difference in the frequency of haploids is primarily determined by that of parthenogenetic egg cells, as haploid embryos of wheat are known to be well viable, when the polar nuclei are fertilized to form the endosperm (KIHARA and KISHIMOTO 1942). The frequency of haploid parthenogenesis is symptom of abnormal embryogenesis, since in this case the egg cell develop contrary to rule into an embryo without fertilization. No detail mechanism has been studied on the occurrence of twin seedlings. However, their frequency can be considered as another indication of abnormal embryogenesis. The present results clearly indicate that the occurrence of both haploid and twin seedlings in various wheat varieties was increased by the alien cytoplasm introduced from Ae. caudata. The increase is striking in Taylor's Triticale. Thus, the alien cytoplasm was the cause of abnormal development, though the extent of the response varied with the genotype. The behavior of Taylor's Triticale is peculiar, because of the extremely high frequency of haploids and twins in the alloplasmic line. This Triticale is a fixed descedant of a hybrid between two Triticale strains, Mains 56 (2n=56, AABBDDRR) and Bledsoe 56 (2n=56, AABBDDRR), both produced and crossed by Mr. J. Taylor of USDA. The fixed line from his cross used in our experiment, however, has only 42 (21II) chromosomes. An investigation will show in the future which chromosomes have been lost in the process of fixing the employed line. |