In a previous newsletter note, Chris Cullis and I reported that most repeated DNA sequences in maize vary considerably in copy number between inbred lines. Testing 10 cloned sequences that differ in function and molecular arrangement and have reiteration frequencies of 100 to over 100,000 copies/genome, we found that nine showed significant quantitative diversity among inbred lines.
These differences appeared to be stable quantitative polymorphisms because the copy numbers measured are characteristic of all individual plants within a line and are the same in DNA extracted from different plant tissues. Therefore, we anticipated that when a cross was made between inbreds that differed in copy number for a particular sequence, the F1 plants would all be identical to each other and have copy numbers that were equal to the mean of the parental lines.
We found, however, that while each of the F1s had the same copy number for a particular sequence, that number was not necessarily the parental mean. In a manner that is specific to the genotypes involved in the cross, the copy number of a repeated sequence can be dramatically different from Mendelian expectation. Progeny from 25 crosses including reciprocals have been analyzed. A sample of the data is given below. Although the phenomenon is not yet fully described, the following appear to be true:
1. The change is not random. A particular genomic change is found each time a cross is made and in all progeny of that cross.
2. Reciprocal crosses result in the same genomic changes.
3. Neither the occurrence of F1 change nor the magnitude of the change is related to the copy numbers or difference in copy numbers of the parental lines.
4. The direction of copy number change is consistent for each sequence. In the data we have collected so far, rDNA and SS sequences either show the parental mean or a decrease in number, whereas a sequence associated with chromosome knobs, pZmK10, and one that might have a role in DNA replication pZmA101, have only shown changes that are increases over expected values.
Repeated sequence copy number changes in F1s expressed as observed number/parental mean:
Experiments are in progress to explore the timing, genetics and developmental ramifications of this rapid genomic change. Measurements are being made on DNA samples from early embryonic tissue of both F1 and inbred plants to determine when genomic changes take place. F2, F3 and backcross progeny are being examined to determine if there is continued change, stabilization or segregation for repeated sequence copy number. I am also looking at a new set of genotypes to ask whether change or stability of the F1 genome is correlated with hybrid vigor.
Other related work in progress is a study of the genomic stability of maize cells in culture. I have been following 3 embryo cultures started from a selfed ear of A188 over 6 months in culture. By two months, quantitative differences in repeated DNA sequences were apparent between each culture and the inbred plant genome. The changes that have accumulated are similar in magnitude and direction to those observed in the outcross progeny. I am continuing to follow these cultures to ask how differentiation and regeneration may affect or be affected by genomic fluctuations.
I welcome any questions or comments about genomic and phenotypic stability in maize. After March 1984 I can be reached at the Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon.
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