Figure 1. The pollen of domesticated maize does not function on the silks of some strains of teosinte, its wild progenitor, but teosinte pollen functions on maize silks.
In collaboration with the laboratory of Jerry L. Kermicle (University of Wisconsin-Madison) we are studying factors that prevent the pollination of some strains of wild teosinte by domesticated maize. Used as ovule parent, maize pollinated with these teosintes produces a full set of kernels and highly to fully fertile F1 plants. For many strains of teosinte however the cross in the opposite direction often fails (Figure 10). The available data concerning the collections of mexicana that block pollination by maize suggested they derive from regions where teosinte grows exclusively as a weed and flowers in synchrony with the local maize. The cross-incompatibility trait of teosinte can be introduced into maize by recurrent backcrossing of plants carrying the trait to standard maize plants, producing a maize line carrying a segment of a teosinte chromosome that confers the ability of these maize plants to reject pollen from standard maize lines (Figure 2). Genetic studies have identified three different loci that contribute to genetic isolation of these teosintes from maize: Tcb1, Ga1, and Ga2.
Unilateral compatibility presents a unique opportunity to characterize barriers to crossing genetically. Crosses in the compatible direction provide material for analysis; the reciprocal cross assays for incompatibility. These circumstances prompted us to undertake study of maize teosinte compatibility with the general, long-term goals of identifying the major genes involved, incorporating them singly into maize lines and there determine their actions (Figure 2). Cross-incompatibility requires a pistil expressed factor that rejects non-cognate pollen (e.g. Tcb1-s silks rejecting tcb1 pollen) and a pollen expressed factor that allows fertilization by cognate pollen (e.g. Tcb1-s pollen on Tcb1-s silks). These two factors are tightly linked genetically and are typically inherited as a unit for all three cross-incompatibility loci described in maize and teosinte.