Zea perennis
Zea perennis, the perennial teosinte, is a true grass species in the genus "Zea" and a teosinte. It is one of the two perennial species in the genus "Zea". The other perennial, "Z. diploperennis," is the sister taxon of "Z. perennis". Those two species also form a clade with "Z. luxurians". Together, the three species make up the "Luxuriantes" section in the genus "Zea". "Z. perennis" is the sole tetraploid in the genus and fertile hybrids with diploid "Zea" species are rare. Ribosomal ITS evidence suggested introgression between "Z. perennis" and "Z. mays" that must have come from  either crossing the ploidy barrier or been from the diploid ancestral pool. "Z. perennis" is generally considered to be an autotetraploid from some ancestral population of "Z. diploperennis". Due to the economic importance of maize, significant scientific interest exists in using the genes of the other "Zea" species for crop improvement. "Z. perennis" is of particular interest because of the potential for maize to become a perennial crop. However, difficulty in using genes from "Z. perennis" in "Z. mays mays" for crop improvement has occurred because the genes used often contain unwanted teosinte traits. "Z. perennis" is tropical and not  winter hardy, which has led to problems in using its genes to make a perennial form of maize. To overcome this, breeding efforts have focused on deeper rhizomes that can survive below the frost line.