The original TB-9Lc stocks were time consuming to work with because no endosperm markers were uncovered by this translocation, making selection of known hyperploids for use in mapping experiments difficult. In the absence of homozygous TB-9Lc lines, each plant in a segregating TB-9Lc population had to be examined for pollen sterility, and those plants which showed slight pollen abortion (approximately ten percent in a year with good expression) had to be test crossed to a 9L seedling marker (such as v1 or Bf1), and the progeny seedling tested in order to determine with certainty which plants carried the translocation.
An innovation was made when the Wc1 (dominant pale yellow endosperm) mutation was transferred to the translocated 9L arm, allowing the uncovering of the recessive wc1 allele (J. B. Beckett, Locating recessive genes to chromosome arm with B-A translocations, in The Maize Handbook, ed. by Freeling and Walbot, Springer-Verlag, 1993) in hypoploid endosperms. In this method, plants homozygous for Wc1 and segregating for TB-9Lc are crossed onto wc1 Y1 (yellow endosperm) standards. If the male parent in a cross carries TB-9Lc, the resulting ears segregate for large pale yellow (balanced and hyperploid endosperms) and smaller yellow kernels (hypoploid endosperms). Barring heterofertilization (a relatively rare event), the yellow kernels will have hyperploid embryos and thus carry the translocation. Crosses in which the male parent does not carry TB-9Lc will produce ears with only pale yellow kernels.
In order to improve the selection for homozygous Wc1 plants carrying TB-9Lc, we have taken advantage of the close linkage of the wx1 locus on the short arm of chromosome 9 to the TB-9Lc breakpoint (see 1993 linkage map). We have crossed a homozygous wx1 Wc1 line to a TB-9Lc Wc1 line for two generations, generating homozygous Wc1 ears segregating for wx1 kernels. On such ears, the majority of Wx1 kernels should carry TB-9Lc since TB-9Lc is in close coupling with the Wx1 allele. Of 44 plants grown from Wx1 kernels so far, 33 (75%) have carried TB-9Lc. After further testing and increase, our Wx1 9-B Wc1 B-9 (TB-9Lc) and wx1 Wc1 lines will be made available for distribution from the Maize Genetics Cooperation Stock Center.
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