The following controlling element (CE) stocks have been propagated and crossed onto cms-S male-sterile testers to determine their fertility restorer (Rf) status: bz2-m (Ds-Ac); Adh1-FM335 (Ds-Ac); a1-m4 (Ds-Ac); a1-m1 (I-En); a1-m1 and wx-m8 (Spm-S); bz1-m (Ds-Ac); (P-u R-sc) (Ds2-Mp); and Mutator (Mu). In progeny of these crosses we have identified rf rf (non-restoring) plants in five of the eight CE pedigrees tested. The CE strains were also crossed as male parents onto cms-S carrying one of the spontaneously-arising restorers, Rf7 through RfX. These restorers have been located at a minimum of seven different sites in chromosomes 1, 2, 3 and 8 in maize. For the restorer tagging experiments we have emphasized those CE-Rf combinations in which one or more of the CE (autonomous or nonautonomous) tagging elements is linked with the Rf target. We are now in a position to search on a large scale among plants carrying a defined Rf element and a functional CE system for tassel phenotypes that indicate the insertion of CE elements into Rf genes.

We have shown that RfI, RfII, RfIV and RfVI are themselves transposable and we hope to characterize them at the molecular level through an alternate route by identifying their insertions into maize genes such as Adh1, Sh1, Bz1, and A1, for which gene clones are already available. We plan for extensive plantings, mainly in isolation plots, to identify the rare events sought.

Crosses have also been made to develop stocks that will afford a search for CE and/or Rf into various maize genes for disease resistance, and into maize genes whose mutant forms are associated with profound developmental changes. Protocols for the use of CE and Rf strains in tagging such genes are being developed.

John R. Laughnan, Susan Gabay-Laughnan and Prasanna Athma
 
 

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