TRISOMIC TRANSMISSION IN CLARKIA UNGUICULATA

VASEK, F. C. (U. California, Riverside.) Trisomic transmission in Clarkia unguiculata. Amer. Jour. Bot. 48(9): 829-833. 1961.-Seven primary trisomic plants derived from a triploid-diploid cross were self-pollinated. The 7 progenies included diploids and trisomics, the latter varying in frequency from 16 to 30%. In addition, 2 of the progenies included tetrasomic plants. Crosses were made between diploids and either trisomics or tetrasomics. The extra chromosome of 1 progeny was readily transmitted through the pollen of trisomic and tetrasomic plants. When a trisomic of the same progeny was used as a seed parent, only diploids and tetrasomics were found among the offspring, indicating a duplication of the extra chromosome. The extra chromosomes of other progenies were not transmitted through either pollen or eggs in controlled diploid-trisomic crosses but trisomics of these progenies were recovered after self-pollination. It is suggested that differential pollen-tube growth precluded transmission to diploid-trisomic hybrids and that under conditions of reduced pollen competition the extra chromosome normally would be transmitted through pollen. The extra chromosomes generally occur as univalents at metaphase and are ordinarily included in telophase nuclei. SEVEN primary trisomic plants were included in the progenies derived from crosses between triploid and diploid Clarkia unguiculata (Vasek, 1956). Inasmuch as no morphological trait was found to be correlated with the presence of any extra chromosome, the 7 plants were identified as trisomics on the basis of chromosome number. All the chromosomes (n 9) are sufficiently similar that individual chromosomes cannot be identified cytologically. Thus, it is not known whether these 7 trisomics had the same extra chromosome or whether some or all were trisomic for different chromosomes. The purpose of this study has been to determine: (1) the method of transmission of extra chromosomes; (2) differences between trisomics with regard to transmission; and (3) pairing characteristics of extra chromosomes. METHODS-The original trisomics were arbitrarily numbered from 1 to 7. These were each selfpollinated, and the resulting progenies were grown during 1959 as Trisomic Progeny 1, Trisomic Progeny 2, etc. Plants in these 7 progenies were numbered consecutively and scored for chromosome number, chromosome pairing and pollen fertility. Crosses were made between plants of these 7 progenies in numerous combinations and the resulting hybrid progenies were grown in 1960 when they, in turn, were scored for chromosome number, chromosome pairing and pollen fertility. Cytological observations were made at first meiotic metaphase and telophase of pollen mother cells. Anthers were fixed in 1:3 glacial acetic acid: absolute ethanol, stored in 70% ethanol and subjected to hydrolysis in 1 N HCI for approximately 12 min at 60 C before squashing in acetocarmine. Slides were immediately made permanent by mix1 Received for publication March 13, 1961. I am grateful to Professor Harlan Lewis for his valuable suggestions regarding the manuscript and to the National Science Foundation for financial support. ing a drop of Hoyer's medium (Beeks, 1955) with the acetocarmine just before the cover slip was added. Not only does Hoyer's medium provide a quick, easy, and effective method of preserving slides but it also clears the cytoplasm. Estimates of pollen fertility were based on the number of pollen grains that stained with cotton blue in lactophenol (Sass, 1951). Pollen grains that took up the stain were plump and full, and assumed to be viable. Grains that did not stain blue were shriveled, shrunken, devoid of cytoplasm and obviously inviable. Samples of 300 pollen grains were counted from each plant. R ESULTS-Chromosome transmission-The frequency with which the extra chromosome of trisomics is transmitted following self-pollination is shown in Table 1. Approximately 70% of the plants in these 7 progenies were diploid, 26%, were trisomic and 4% were tetrasomic. Despite the fairly low rate of transmission, trisomics were recovered in all 7 progenies. If there were no loss of chromosomes and if transmission were at random through both pollen and egg, frequencies of 50% trisomics and 25% tetrasomics would be expected. Since these frequencies were not realized, transmission is apparently not at random. The very low frequency of tetrasomics, particularly, suggests that transmission of the extra chromosome occurs mostly through the egg or the pollen, but ordinarily not through both. Crosses were made between plants that were diploid and either trisomic or tetrasomic. The frequency of each chromosomne-number type in the resulting hybrid progenies is shown in Table 2. Three progenies derived from diploids pollinated by tetrasomics of Progeny 7 yielded an average of 43%, trisomics and 28% tetrasomics, indicating clearly that not only did transmission of the extra chromosoines occur through the pollen but that both