While working to develop a nuclear male-sterility system for hybrid seed corn production (Phillips, MNL 49:118 and 52:67; West, M.S., Univ. Minnesota 1984), three nonallelic, recessive, nuclear male-sterile: female-fertile mutants were recovered. Seed samples have been deposited with the Maize Genetics Stock Center. These mutants have received the designations ms22-A632A, ms23-A619B, and ms24-LT.
These new male-sterile mutants were compared cytologically with the known male-steriles as described by Beadle (Genetics 17:413, 1932) and Albertsen and Phillips (Can. J. Genet. Cytol. 23:195, 1981). Microspore mother cells (MMC's) of ms22-A632A degenerate prior to pachynema. MMC cell walls appear extremely fragile before degeneration and may be the site of the mutant's primary effect. Allelism tests were conducted with ms8, ms9, and ms17 because these male steriles had been described previously as exhibiting early MMC breakdown. There were no sterile plants in any of the F1 progenies of the crosses with ms8 (72 plants), ms9 (62 plants), or ms17 (17 plants).
Degeneration of ms23-A619B is also during meiosis I, although intact prophase II configurations have been found. We sometimes have observed that the tapetal cells lose staining intensity before the MMC's exhibit cellular disorganization. Whether this is a cause or effect has not yet been determined. As with ms22-A632A, MMC cell walls appear fragile. The nuclear events of meiosis continue, however, until meiosis II. Because this pattern of microsporogenesis breakdown is similar to that described for ms8, we made the appropriate testcross, but found no sterile plants among 38 F1 progeny. We did find, however, ms23-A619B to be allelic with a previously undescribed male sterile (ms*-Bear7) from Earl Patterson (Univ. Illinois) by testcrossing a homozygous recessive ms*-Bear7 plant with a known ms23-A 619B heterozygote. The F1 progeny segregated 8 fertiles:11 steriles (X2 = 0.47; .90*P*.75), indicating the same locus but from independent mutations. We previously had testcrossed ms*-Bear7 plants with ms1 (20 plants), ms2 (18 plants), ms5 (13 plants), ms7 (21 plants), ms8 (21 plants), ms9 (21 plants), ms10 (17 plants), ms11 (20 plants), ms12 (17 plants), ms13 (19 plants), ms17 (22 plants), ms20 (21 plants) and po (39 plants) and observed that all progeny were fertile. Relationship of ms22-A632A with ms23-A619B was tested by observing F1 and F2 progenies. Forty-three F1 plants were completely fertile. The selfed progeny segregated 414 fertile plants:315 sterile plants, a good 9:7 fit (X2 = 0.02; .90*P*.75) for two different loci.
Homozygous recessive mutant ms24-LT extrudes anthers and exhibits occasional engorged pollen grains among degenerated pollen grains when the mature anther locule is viewed under low power. No effective pollinations were achieved, however, when homozygous recessive ms24-LT plants were used as males. I2-KI staining also revealed irregular starch accumulation in pollen ranging from undetectable to fully engorged. This suggests that ms24-LT may be involved in the inhibition of pollen starch synthesis or deposition. Germinability of these engorged pollen grains has not been examined in vitro. Migration of vegetative and generative nuclei also appears irregular. The generative nucleus eventually degenerates. This male sterile appears to degenerate later than ms5, ms11, or ms14, which had been previously described as being late-degenerating mutants. We found it to be nonallelic to ms14 (25 F1 plants examined). It has not yet been testcrossed with ms5 and ms11.
New male-sterile mutants ms22-A632A and ms24-LT constitute the earliest and latest, respectively, degenerating male-sterile mutants thus far described. Male-sterile mutant ms23-A619B is cytologically, but not genetically, similar to ms8. An independent mutation for the same locus has occurred in another previously unknown male sterile, ms*-Bear7. Mutant ms24-LT, because it appears to proceed nearly to the completion of pollen development, might be induced to complete microsporogenesis and succeed to fertility by an appropriate chemical agent or environmental condition, as has been suggested previously for ms14 (Albertsen, Ph.D., Univ. Minnesota, 1980).
David P. West and Marc C. Albertsen
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