Haploid and aneuploid corn cultures

Haploid tissue cultures were initiated from mature embryos of Stock 6 (Coe, 1959, Am. Nat. 93:381) and young haploid yellow green seedlings produced from a cross with yg2 in the female. Cultures were induced on a slightly modified MS medium. Stock 6 cultures were very slow growing and were abandoned after one year; yg2 haploid cultures continue to thrive after more than 18 months. Ploidy analysis using chromocentre and chromosome counts indicated a persistent, high proportion of haploid cells in both cultures after 10 months. The yg2 cultures at present include sectors which are purely haploid as well as diploid and mixed sectors. No intact aneuploid cells were found.

Monosomic corn cultures were also established and their growth and ploidy stability studied. Progeny of a cross between heterozygous r-X1 stock and Mangelsdorf's Multiple Tester were screened for monosomics using recessive seedling markers and chromosome counting; each line was karyotyped. Monosomics (including double monosomics) involving all 10 chromosomes were identified as well as mono-telosomics, mono-trisomics, primary trisomics and double trisomics. Cultures were initiated from roots and stem tissue of seedlings at 2-3 leaf stage. Despite heavy infection losses cultures monosomic for chromosomes 2, 6, 7, 9 and 10 were established. After four months a very small proportion of cells were tetraploid; some purely monosomic and some mixed sectors were found. No significant differences in growth rate were observed between different monosomic cultures. In general the ploidy of the monosomics was more stable than the haploid cultures previously described.

The majority of cell lines in cereals and in dicots are highly polyploid and/or aneuploid, although it is not clear whether aneuploidy as such causes cell line induction by some gene imbalance or whether there is only an indirect relationship. To study this question further in corn, where cell lines rarely occur, we prepared cultures from seedlings which were primary trisomics for all chromosomes but 2 and 8. During two years of subculture none of the trisomic lines gave rise to cell lines. However, cultures trisomic for chromosomes 6 and 10 consistently grew much faster than the others. Extra ribosomal DNA sequences in trisomic 6 and perhaps in 10 (Phillips et al., 1974, Genetics 77:285) might be responsible for this increased vigour, but the differences might equally be the result of undefined differences in genetic background. After ten months most of the cultures remained purely trisomic. Trisomics 7 and 9 were abandoned because of their poor growth. Telocentric chromosomes were found in cells of cultures originally trisomic for chromosomes 1, 3 and 4 but it is not yet clear which chromosomes are involved in this change. Cells with 22 chromosomes were eventually observed in cultures trisomic for 5.

We have recently selected immature embryos which are haploid (using Stock 6), monosomic for chromosome 1 (after r-x1 x Adh-) and mono-telosomic for the long arm of chromosome 1 (using A/B interchanges). In all cases the embryos were selected using the recessive Adh1- mutant (Schwartz and Osterman, 1976, Genetics 83:63; Cheng and Freeling, 1976, MGNL 50:11) in conjunction with allyl alcohol (details in a paper submitted to Genetics). Tissue cultures were induced on the scutella of the immature embryos and both monosomic and mono-telosomic plants have been regenerated from the cultures. The embryos, cultures and plants will serve as a source of protoplasts for mutant selection experiments.

H. S. Dhaliwal and P. J. King


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