N-methyl-N'-nitro-N-nitrosoquanidine (MNG) is one of the most powerful mutagens known for higher plants. Unlike ethylmethanesulfonate, MNG treatment of mature pollen grains does not produce a uniform, vigorous M1 generation. Throughout the M1 population are small plants, and this small plant effect does not segregate in Mendelian ratios in later generations. These small plants fall into four separate phenotypic categories. Type 1: This plant is half the normal height and normal in color. The leaves are one-half to two-thirds the normal width and fewer in number than normal plants. The nodes are not compressed. Type 2: This plant is also one-half the normal height and normal in color. The leaves are one-half to normal in width. The differences from Type 1 plants are that the number of leaves is normal and the nodes are compressed. Type 3: These plants are under 20 inches in height. They are pale green and the leaves are grainy and erect. The leaves are also less than a quarter the width of normal leaves and the tips are necrotic. Most plants have only four leaves and are female sterile. Type 4: The plants are less than one-half normal height and grow non-uniformly. The leaves are creased with fine white striping and corrugations, and are thinner than normal. Data analyzed from selfed small plants show the effect is not due to a normally segregating single dominant or recessive mutation. Differential transmission of gametes is not responsible for the aberrant ratios found in data from selfed small plants. The germination of pollen from normal and small plants was the same. The length of pollen tubes was graphed against the number of pollen tubes, and the graphs showed only one type of pollen was present. Data from outcrosses show the effect is not attributable to multiple dominant or recessive mutations. If the small plant effect was due to multiple mutations, outcrossing to inbreds would show plants of intermediate height. Several generations of outcrossing show no plants of intermediate height. Cytogenetic data prove that the small plant effect is not a result of aneuploidy. Restriction analysis of small plant, mitochondrial DNA shows no band differences from normal inbreds.
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