A selfish gene governing pollen-pistil compatibility confers reproductive isolation between maize relatives

Abstract

Some populations of maize's closest relatives, the annual teosintes of Mexico, are unreceptive to maize pollen. When present in the pistil (silk and ovary) a number of maize genes discriminate against or exclude pollen not carrying the same allele. An analogous gene Tcb1-s was found in some teosinte populations but not in sympatric or parapatric maize. It was polymorphic among populations of teosinte growing wild, but regularly present in populations growing in intimate association with maize as a weed. Introduction of Tcb1-s into maize substantially to fully restored compatibility with Tcb1-s carrying teosintes. Although Tcb1-s pollen can fertilize tcb1 tcb1 maize, it is at a competitive disadvantage relative to tcb1 pollen. Hence, the influence of Tcb1-s on crossability is bidirectional. In the absence of maize, Tcb1-s can increase in teosinte populations without improving their fitness. In the presence of maize, Tcb1-s appears to have been co-opted to provide reproductive isolation for adaptation to a cultivated habitat.

Figure 1.

Seed set on emasculated plants of 13 annual teosinte populations pollinated with maize. Results expressed relative to when teosinte pollinates itself (± SE). Open bars, tcb1 and Tcb1-m teosinte populations; shaded bars, teosintes containing Tcb1-s. (A) Ga1-m tcb1 maize pollinator. (B) Increase in seed set due to introgressing Tcb1-s into the maize pollinator, that is, the difference between Ga1-m tcb1 and Ga1-m Tcb1-s as pollinators. *0.05 > P > 0.01, **P < 0.01, based on one-tailed t-tests.