Two cases of simultaneous bz-rcy to bz-n(rcy) and Sh to sh have been observed. Both cases arose from the cycling state bz-rcy812215 (see following note) out of a population of 1733 bz-rcy, Cy gametes.
bz-n(rcy)-sh-835211w: In the first case three bronze, non-spotted shrunken kernels (835211w-1, w-2, w-3) were isolated as an ear sector from Cross 1: C Sh bz-rcy; Cy x C sh bz; no Cy and as such most likely arose as a single Sh to sh event. The heritability of the shrunken phenotype was confirmed by crossing these exceptional kernels by bz-m4 (Cross 2). The absence of round kernels in the resulting ears confirms that the Sh to sh event caused a heritable change.
The absence of spots on the original bronze shrunken isolates was unexpected since Cross 1 contained many Cy elements. This is suggestive that coincident with the Sh to sh event a bz-rcy to bz-n(rcy) (non-responsive to Cy) event occurred. In order to test this possibility bronze shrunken kernels from Cross 2 were crossed by bz-rcy. Colored spots on the bronze background of round kernels confirmed that Cy was present in the original shrunken isolates 835211w-1 and 835211w-3 and that bz-rcy changed to bz-n(rcy).
bz-n(rcy)-sh-835211x-1: In the second case a single bronze shrunken, non-spotted kernel was found on an ear resulting from Cross 1. This new sh (bz-n(rcy)-sh-835211x-1) was handled in the same fashion as described above. The results were the same; bz-n(rcy)-sh-835211x-1 is heritable and coincident with the Sh to sh event and bz-rcy to bz-n(rcy).
The likely sequence of events that caused these two cases of simultaneous loss is as follows:
1. Trans-active signals from an independently segregating Cy element induced the rcy receptor element at the bz-rcy allele to transpose.
2. Excision was imprecise, resulting in the production of a bz allele that no longer responds to Cy. Simultaneously with this loss of receptivity, two map units distal to the Bz locus, Sh changed to sh.
Patric S. Schnable and Peter A. Peterson
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