Technic for Identification of Gametophytic Mutations.
In the mutation technic used in the experiment just described, gametophytic mutations are not detected if they have no visible effect upon pollen development; and if they produce defective pollen, they are not distinguishable from short deficiencies. Another difficulty is that many of the sporophytic mutations are questionable because of possible over-lapping of the normal phenotype.
Both of these difficulties may be avoidable, for limited chromosome regions, by the use of inversions to inhibit crossing-over. A trial of this method with one inversion, was made in 1940, in an experiment comparing UV and Xray treatments in a manner otherwise similar to that of the experiment just described. The method may be used more effectively with a combination of inversions in various chromosomes.
The treated parent was I wx, the untreated parent carried rearrangement-9 (McClintock 1939) with i Wx. This rearrangement eliminates crossovers in a large part of chromosome 9. The F2 seeds therefore are of three types -- one fourth I wx, homozygous for the treated normal chromosome; one fourth i Wx, homozygous for the untreated chromosome; and one half I Wx, heterozygous for thetreated and untreated chromosomes. Induced chromosome 9 alterations are linked with I wx. They are manifested in three ways:
(1) By pollen defects linked with wx. In iodine-stained pollen specimens extremely slight effects on pollen size or development may be recognized, far below the limit of detection in unlinked segregation.
(2) By modified ratios for I and Wx. Gametophyte mutations or deficiencies without visible effect on pollen development, if they prevent functioning of pollen, modify the 3:1 ratios to 2:2 and 4:0 respectively. If they permit reduced functioning, they permit the segregation of a reduced proportion of wx seeds. (A reduced proportion of wx seeds may result also from a Ga-mutation inhibiting functioning if separated from the rearrangement by crossing-over.)
(3) By seed and seedling mutations linked with I Wx. Here also the linkage permits the detection of some mutants which would be doubtful or undetectable without linkage.
The mutants are crossed with C Wx (normal chromosome) for genetic location in three-point tests. Gametophyte mutations not transmitted through pollen may be recovered from the heterozygous I Wx seeds, and when pollinated by c wx (normal) yield heterozygotes in which the location of the Ga-factor may be determined by crossing on C wx or c wx. Deficiencies and other chromosbmal alterations not lethal to the female gametophyte may be recovered similarly, for cytological examination in plants free from the rearrangement.
The spontaneous frequency of the various types of alteration is shown in the same F2 ears by segregations of the same kinds linked with i Wx instead of I wx.
The results of this experiment, as regards chromosomes other than #9, were similar to those of the previous experiment, except for differences incidental to the use of different wave lengths and dosages, which will not be discussed here.
The number of chromosome 9 alterations of each type identified is shown below:
|(1) Defective Pollen|
|(2) Low Transmission||12||4||2||0|
These constitute a representative sample of the genetic alterations induced by Xrays and UV, all located within a region well suited for critical comparison genetically and cytologically.