Studies with mutable waxy.
An allele at the waxy locus (wxm), which mutates with a high frequency to Wx in both endosperm and germinal tissue, is under investigation. This allele is intermediate between Wx and wxs; Wxwxm plants segregate approximately 3 Wx:1 wxm; and wxmwxs plants approximately 3 wxm:1 wxs. (Ratios deviate from 3:1 in some cases due to germinal mutations.)
Typically, a wxmwxs plant when selfed gives three classes of kernels: About 1/4 waxy, less than 3/4 mosaic (waxy with various sized spots of normal starch), and a variable number (often 5-20 per cent) of kernels with normal starch endosperm.
The most readily observable mutation both somatically and germinally is from wxm to Wx. Mutation rate comparisons made between different stocks by counting the numbers of Wx kernels produced in crosses wxmwxm backcrossed or selfed, indicate differences of the following order of magnitude:
|S-47-2 × wxs||199||41||17.0|
The mutable allele probably also mutates to wxs. Four ears from a cross wxswxs × wxmwxm threw 5.3 per cent wxs seed. A mosaic kernel when grown and selfed gave the phenotypic ratio 29 Wx:212 wxm; 19 wxs - the 29 Wx and 19 wxs kernels arising by mutation. These seeds are being grown now to establish their genotype.
In a few stocks, kernels have been found consisting entirely of normal starch except for many small scattered waxy spots. Since in these cases the rest of the ear bore all normal starch kernels (Wx by mutation), these spotted kernels may represent reverse somatic mutations of a somewhat unstable Wx′ allele back to wx.
A study of the distribution of Wx and wx pollen grains in alcohol preserved tassels from wxmwxm plants (Wx grains stain blue and waxy stain red with weak IKI) indicates that mutations may occur so early in tassel development as to affect an entire branching or even a few neighboring branches. On the other hand, some branches carry anthers segregating in varying ratios, indicating later mutations. Mapping of ears from crosses wxmwxm × wxswxs has not revealed any sectored pattern as yet.