6. Paired mosaics (twin spots) have been found to involve C, CI, Pr, P, Wx and some unknown aleurone color modifiers. Wx twin spots are very faint and show only in certain material with light iodine staining. The evidence indicates that some unpaired spots start as paired mosaics but one or the other altered cell is non-viable or fails to produce tissue that reaches the surface. Unpaired c mosaic areas are usually larger and more numerous than twin spots involving the same gene in the same seeds. Many of these unpaired spots probably do not start as twin spots.
In C Wx heterozygous seeds both genes go together in about 60% of both twin spots and single spots and C alone in about 40%. A shift of Wx without C has not been observed. The dark part of a C Wx twin spot may also show a further change to colorless, normal or still darker cells. In some cases these are twin spots within twin spots. Wx may shift with C the first time and not the second, or neither or both times.
Obviously these results can not all be accounted for by mutation, non-disjunction or deletion. Some kind of interchange between homologous or non-homologous chromosomes is indicated. Proof of an exchange between the C and Pr chromosomes is at hand in white and red paired mosaics in heterozygous C Pr seeds. Such mosaics are rare. Chromosomal aberration does not seem to be adequate to account for the frequent twin spots in which the two parts are equal in size and outline and crossing-over, between homologous chromosomes as shown by Stern for Drosophila (GENETICS 21:625-730) seems probable.
Proof of somatic crossing-over in plants will have to await further evidence. It may be found in 2N tissue where dominant linked genes are contributed from each parent. The 3N endosperm mosaics are not satisfactory for this purpose.
Translocation stocks having either Su or Pr with C and Wx are desired. Seed will be appreciated if such stocks are available.
Aleurone and endosperm mosaics vary in frequency in different families from none in a thousand seeds to thousands of mosaics on a single seed. They are easily seen with a low power binocular microscope. A Bausch and Lomb BKT5 microscope with a revolving drum and .7, 1 and 2x objectives and 1Ox eyepieces has been found convenient. The light is also important. In addition to the well-known plain spots and the twin spots that are frequent in some families, large cells, giant cells, depressions and outgrowths are easily seen. The growth changes may accompany color and other known gene changes and clearly result from somatic segregation. Depressions and outgrowths are sometimes paired, alone or with color changes. Somatic segregation has an important bearing on the cancer problem and any evidence should be put on record.
D. F. Jones