Molecular Evidence for a Missing Wild Relative of Maize and the Introgression of Its Chloroplast Genome into Zea Perennis.

The unequivocal documentation fintrogression can be a difficult task because alternative hypotheses, uch as convergence or joint retention of the ancestral condition, are difficult todisprove. Similarly, the direction of introgression between two species can often be inferred, but more rarely proven. Molecular markers offer perhaps the best means of demonstrating introgression, because they are apt to be neutral and thus are less likely than morphological traits to converge under selection within a similar environment. Molecular markers also provide the opportunity oexamine the occurrence of introgression for the independently inherited nuclear and cytoplasmic ( hloroplast and mitochondrial) genomes. Cytoplasmic genomes present a particularly good opportunity o observe introgression, because they may contain multiple markers which, unlike nuclear markers, cannot recombine. This feature of cytoplasmic genomes allows introgression to be more readily distinguished from convergence. Nevertheless, there have been relatively few reports of introgression between plant species based on molecular markers (e.g., Rick et al., 1974; Doebley et al., 1984; Palmer et al., 1983), and there is at least one case in which molecular analyses have failed to confirm a suspected case of introgression (Rieseberg et al., 1988). In this paper, I report a case of introgression fthe chloroplast genome involving a perennial wild relative of maize, perennial teosinte (Zea perennis [Hitchc.] Reeves and Mangelsdorf). This case is of particular interest for several reasons. 1) The chloroplast genome type that has become incorporated into Z. perennis is unknown among the other species of the genus and, thus, represents a missing taxon. 2) Zea perennis is the only tetraploid in the genus, while the taxa of Zea whose chloroplast genomes most closely resemble the introgressive chloroplast genome are all diploids. This suggests that introgression may have occurred between taxa of different ploidy levels. 3) This case demonstrates how phylogenies based on chloroplast-genome studies alone could be misleading when performed in the absence of analysis of the nuclear genome.