Frequently, maize geneticists encounter poor ear setting among their crosses. Often an environmental effect (hot summers) is suggested. When poor setting is found when there is a clear expectation of good setting, then one takes notice and investigates. Such a case was found in 1975 among crosses involving controlling elements.
When a1-m(pa-pu)/a1 sh2 genotypes (Peterson, 1970, TAG) or its derivatives are crossed by a1 et/a1 males, ears with reduced seed set (RSS) are observed. Among RSS progenies, there is a consistent array of progeny types. Approximately 80 to 90 percent of the ears produce from 0 to 25 kernels per ear (class A), 6 to 23 percent produce 26 to 200 kernels per ear (class B) and 0 to 3 percent produce over 200 kernels per ear (class C). RSS occurs with specific male parents since crossing a1-m(pa-pu)/a1 sh2 by other male testers (a1 sh2/a1 sh2, a1-m(r)/a1-m-1) yields ears with normal seed set (NSS).
The reciprocal cross, a1 et/a1 et x a1-m(pa-pu)/a1 sh2, results in NSS. The resulting F1's of this NSS cross also give NSS when they are crossed by a1 et/a1 et males.
The RSS effect is heritable and is maintained through several successive crosses of a1-m(pa-pu)/a1 sh2 x a1 sh2/a1 sh2 males. No segregation of the RSS effect is observed in progenies of these crosses nor in the selfed progenies of a1-m(pa-pu)/a1 sh2.
The accumulated results of these crosses have provoked the following hypothesis for RSS behavior. It is considered that the a1-m(pa-pu)/a1 sh2 lines carry a genetic factor controlling the incompatible condition of the flower (specifically the silks). The a1 et/a1 et plants carry a genetic factor controlling the incompatible condition of the pollen and interacting with the silks. The hypothesized interactions are as follows:
RSS female x RSS male = RSS
RSS female x N male = N
N female x RSS male = N
N female x N male = N
The transmission of RSS is complex and it appears that cytoplasmic-chromosomal interaction might be involved. Studies are in progress to test various aspects of this hypothesized interaction.
Kitisri Sukhapinda and Peter A. Peterson
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