Reproductive isolation is critical for speciation. This can be achieved by geographic isolation or by having plants that grow together to prevent fertilization by pollen from foreign populations of plants. Some plants prevent fertilization by foreign pollen by using a species-specific pollinator, such as a single insect species, to deliver only the correct pollen to the female parts of the flower, called the pistil. However, in wind-pollinated species in which many types of pollen can land on the pistil, reproductive isolation is achieved in other ways, including the physiological rejection of foreign pollen grains by the pistil. This type of reproductive isolation is poorly understood because of the simple fact that plants of different types cannot be crossed together. An example of this second strategy occurs between some strains of teosinte, the wild progenitor of maize, and cultivated maize. It is postulated that selection for teosintes that can reject maize pollen has occurred during maize cultivation. Because hybrids can be made in controlled conditions by pollinating maize plants with teosinte, this situation provides a model system for the study of physiological, reproductive isolation in a wind-pollinated crop plant. By crossing maize females by teosinte pollen this trait has been conferred to some maize lines, and in many cases the trait is controlled by a single locus, Teosinte crossing barrier1 (Tcb1). The Tcb1 gene has been located to a small region of chromosome 4. In this project, the Tcb1 gene is to be cloned and its DNA sequence determined. With the DNA sequence of the gene known, the manner in which the protein encoded by Tcb1 creates a barrier to maize pollen can be studied, and studies of the role of Tcb1 like genes in reproductive isolation and speciation of wind-pollinated plant families will be possible. The broader impacts of this project include a greater understanding of the molecular basis for speciation, which has implications far beyond this study system. This project also provides training opportunities for high school students, undergraduate students and a postdoctoral researchers.
Selection of appropriate mates occurs in plants as well as in animals. In wind-pollinated species, which lack mate selection based on pollinator identity (such as having an insect pollinator that is unique to a particular flower), the interaction between the male pollen grain and the female pistil is the point of selection for an appropriate mate by the female. Rejection of a class of pollen from within the same species can also lead to reproductive isolation between two related populations growing adjacent to one another. Reproductive isolation is the key step in the initiation of new species. Geographic isolation is, perhaps, the most obvious, but other mechanisms of how a crossing barrier arises in sexually reproducing organisms are largely unknown. In domesticated maize and its wild ancestor teosinte there are three genetic complexes that are present in some populations and absent in others that confer cross-incompatibility between females carrying the genetic complex and males lacking it. These genetic systems provide partial reproductive isolation between populations. In this study, it was shown that rejection of unwanted pollen by these genetic systems may actually involve three distinct cellular mechanisms. A candidate for one of the genes was also identified and is in the process of being verified. The identity of this gene provides insight into how the male and female cells interact and the molecular events that cause rejection of unwanted pollen blocking its ability to fertilize the female. This study also determined that the two functions in one of these cross-incompatibility systems – the female function providing the barrier responsible for rejecting pollen lacking the complex and the male function in the complex that overcomes this barrier – are encoded by separable but tightly associated genes. A better understanding of the molecular events that characterize rejection of unwanted pollen in maize may lead to a more general understanding of reproductive isolation and speciation in plants. This knowledge will increase our ability to monitor and control gene flow between different domesticated plant populations and between domesticated and wild plant populations. Using combinations of cross-incompatibility systems and an understanding of the basis of pollen rejection vs. acceptance will enable control of release of transgenes to undesired populations and maintain integrity of distinct breeding populations that have been bred for different uses.
