A new maize ring chromosome - ring10:A1179
Ring chromosomes, although rare, have been described occasionally involving maize chromosomes. In this report, I announce the identification of a chromosome 10 ring.
Origin. ring10:A1179 originated in a Mutator stock of maize and was recovered while I was screening kernel populations for Mu-induced mutations of the R locus. The initial selection appeared as a coarsely sectored kernel quite dissimilar from a typical finely spotted, Mu-induced mutation of a maize anthocyanin pathway gene. In subsequent crosses I found that the transmission of the "mutation" was not consistent with the segregation of one or two genes: progeny ears from test crosses of A1179 with r-g consistently produced 0-10% sectored progeny kernels with most ears having no sectored kernels. Although loss of Mutator activity could explain the colorless kernels, a ring chromosome was the preferred explanation because of the atypical Mu spotting pattern and the absence of Mutator symptoms in all plants.
Cytological evidence. In order to make cytological verification of a ring chromosome, I harvested several immature tassels from A1179 plants derived from sectored kernels and visited Miss Ellen Dempsey and Dr. Marcus Rhoades in Bloomington, IN. Because the suspected ring chromosome was transmitted only occasionally, many preparations were examined. Miss Dempsey's gifted hands and eyes found that a ring chromosome, best indicated in diakinesis, was unmistakable in several chromosome spreads deriving from one plant. From rough estimates, ring10:A1179 appeared to involve approximately one half of chromosome 10.
Genetic evidence. Overlap of ring10:A1179 with 10L markers besides R has not been especially enlightening. The genotype golden1/golden1; ring10:A1179 produces sectored g1, G1 plants indicating overlap of ring10:A1179 with g1 (not surprising as g1 is proximal to R). A similar experiment using w2 (distal to R) has been inconclusive.
Transmission. Transmission of ring10:A1179 was recently tested using the stock r-g/r-g; ring10:A1179; W22 (six generations backcrossed to W22). In reciprocal crosses with r-g tester plants, ring10:A1179 is transmitted in different frequencies dependent on the mode of transmission. Pollen transmission produces the highest frequency of pigmented kernels or embryos, approximately 13%. Maternal transmission produces approximately 6% pigmented kernels. Plant by plant frequency data follow: (plant number - male transmission frequency/female transmission frequency): (1 - 0.11/0.04), (2 - 0.10/0.04), (5 - 0.12/0.07), (6 - 0.18/0.07), (9 - 0.12/0.05), (12 - 0.17/0.03), (13 - 0.17/0.07), (14 - 0.09/0.08), (16 - 0.18/0.04), (17 - 0.12/0.05), (18 - 0.13/0.08). Kernels with pigmented embryos and colorless endosperm were only produced from pollen-transmitted ring 10 (4% of the pigmented kernels), possibly by heterofertilization. These were verified as noncontaminant by progeny testing and were included in the above data set. There were no plants grown from sectored kernels which did not produce sectored progeny kernels. This contrasts with plants grown in the first three generations in which most of the plants produced only colorless kernels. Possible explanations include the ring stabilizing spontaneously, in the W22 background, or the increase in number of ring chromosomes per cell.
ring10:A1179 may have originated via abortive transposition of a Mu element on chromosome 10 or by crossing over between two similar Mu elements. The origin of chromosomal abnormalities as a result of transposable element action is well documented, especially in Drosophila.
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