In crosses such as a2-m55064 Bt/a2 bt x a2 bt, an excess of colorless-round kemels is found. This indicates either one of two sources of these kernels. Either this a2-m is controlled by several segregating En (3 or 4) or this a2-m is autonomously mutating and gives rise to a high frequency of colorless derivatives. Typical data are given in Table 1. A means of distinguishing between these two options is to test the colorless-round segregants (a2 Bt/a2 bt) that appear in greater than expected frequency. In tests of five colorless-round progeny of crosses similar to 86 4450-21 x 5134 (Table 1), none contained an active En as determined from a test with the En tester a2-m(r). In another test of a sib family that arose from A2 Bt/a2-m55064 Bt X a2 bt, none (8/8) of the colored-round progeny from this cross showed the presence of En.
Another feature of this allele is the presence also of a "fine" factor. This "fine" factor suppresses the early mutability of the a2-m allele in giving a "fine" pattern (late and many). Further, this same suppressive feature of the "fine" factor manifests itself in a lower incidence of colored and colorless derivatives among testcross progenies of plants arising from "fine" kernels. This is evident in both Tables 1 and 2.
It is evident that the fine factor segregates independently and affects the mutability of the autonomously mutable a2-m. Note also in Table 1 (the 4450-23 progeny) that fine types arise out of progeny of the coarse testcross.
Table 1. From a testcross of fine types a2-m Bt/a2 bt (fine) X a2 bt/a2 bt) both fine and coarse types appeared in the progeny. Both these sib types, coarse (864450) and fine (864452) were testcrossed and the progeny types are given (the colorless brittle progeny are not given).
Table 2. Testcross of fine types - a2-m Bt/a2 bt (fine) x a2 Bt/a2 bt.
Peter A. Peterson
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