--Luther E. Talbert and John F. Doebley

The genus most closely related to Zea (x=10) is Tripsacum (x=18). At least thirteen species are recognized in Tripsacum. Although hybrids between Zea and Tripsacum species have been produced experimentally, good evidence for natural hybridization is wanting. The cytology of Tripsacum andersonii provides some evidence for hybridization. This species contains 2n=64 chromosomes, and is hypothesized to contain three genomes from Tripsacum and one genome from Zea.

Two lines of evidence have suggested that Mu transposable elements may be useful as molecular markers to monitor hybridization between Zea and Tripsacum. First, Mu elements are dispersed throughout the Zea genome, and approximately 40 elements may be visualized using a probe specific for the Mu termini (e.g. Chandler et al., Genetics 114:1007-1021, 1986; Talbert et al., J. Mol. Evol., in press). Second, our results suggest that Mu elements may have become a component of the Zea genome after the divergence of the genera Zea and Tripsacum (Talbert and Chandler, Mol. Biol. Evol. 5:519-529, 1988). Thus, the presence of Mu elements in the genome of a Tripsacum species would be strong evidence for hybridization with Zea.

Figure 1. Southern blot with Tripsacum and Zea DNAs restricted with EcoRI/HindIII and hybridized to a probe for the Mu termini. Lane a: Tripsacum dactyloides, b: Tripsacum andersonii, c: Tripsacum pilosum, d: Tripsacum peruvianum, e: Zea luxurians, f: Zea mays ssp. mays.

A Southern blot with DNA from several Zea and Tripsacum species hybridized to a probe for the Mu termini is shown in Figure 1. Hybridizing sequences are seen in the Zeas and Tripsacum andersonii. No hybridizing sequences are seen in the other Tripsacums. These data strongly support the hypothesis that Tripsacum andersonii is a hybrid with a Zea. Apparent similarity of banding patterns for the Mu termini in T. andersonii (lane b) and Z. luxurians (lane e) provides preliminary support for our morphological evidence that Zea luxurians was the Zea species involved. Studies are currently underway to confirm this by determining the numbers and types of Mu elements present in the respective genomes. Additionally, analysis of maternally inherited chloroplast DNA sequences may enable us to determine the maternal parent of this Tripsacum-Zea hybrid.

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