Duke University

A cross between Tripsacum dactyloides and Zea diploperennis
--Mary Eubanks

Tripsacum dactyloides and Zea diploperennis have been crossed using standard pollination technique and viable, fertile hybrids obtained. The cross has been effected both ways. With diploperennis as female parent, the plant is referred to as Sun Dance, and with Tripsacum as female parent, the plant is called Tripsacorn. Zea diploperennis seed was provided by Professor Hugh H. Iltis, HH of the University of Wisconsin at Madison. It was collected from populations growing in their natural habitat in Jalisco, Mexico.

In growth habit and overall general appearance, the F1 plants are more like Z. diploperennis than Tripsacum. Other morphological features are intermediate between the two parents. The hybrid is perennial by means of basal offshoots and underground rhizomes. Plants die back in autumn when temperatures reach ~25 F and produce new growth after the ground thaws in spring. Grown in outdoor nurseries in Mississippi and North Carolina, it has come back every year surviving temperatures as low as 0 F. Z. diploperennis, which is also perennial in its native habitat and greenhouse grown, has not survived winters when temperatures have dropped below 25 F.

The Tripsacum-diploperennis hybrid produces numerous culms (15-50 when field grown) with some better developed than others. The inflorescences are variable. Some are entirely staminate and appear as tassels at the summit of culms. Some, which are bisexual with staminate flowers subtended by pistillate flowers, more closely resemble the Tripsacum inflorescence. Some pistillate ears are a single rowed spike of caryopses completely enclosed in hard shell-like fruitcases, whereas others are highly unusual and have four rows of paired kernels partially enclosed by glumes. These latter ears have distinct interspacing between rachis segments and resemble the oldest archaeological maize specimens from Tehuacán, Mexico.

A unique feature of the Tripsacum-diploperennis hybrid is it can be rooted in water or soil from stem cuttings. Furthermore, excised nodal tissue placed on tobacco initiation medium produces callus. Green growth of early shoot formation and roots have developed from the calli.

The chromosome number from root tip counts and study of pollen mother cells at diakinesis ranges from 2n=16 to 2n=24. This indicates chromosome loss, a phenomenon frequently observed in interspecific hybrids, has occurred. There is a range in degree of chromosome pairing as indicated by varying numbers of bivalents and univalents at metaphase and by incomplete synapsis observed at pachytene. Trivalents, as well as chains of 3 and 4, have also been observed at diakinesis and metaphase. Preliminary cytological evidence for Tripsacum introgression is further seen in the architecture of the chromosome carrying the nucleolar organizer region. Although not yet verified by comparison of karyotypes from actual plants involved in the cross, it looks like Tripsacum chromosome 16. If it is not Tr16, it is Z. diploperennis chromosome 6 with a broken terminal end.

The hybrid has a high degree of pollen fertility as measured by scoring 100 pollen grains stained with cotton blue. Tripsacorn is 94.1% fertile and Sun Dance is 92.3% fertile.

There is segregation in F2 progeny with some plants producing 4 to 8-rowed ears and others producing single-rowed spikes. The chromosome number in the F2s is 2n=20 and pollen fertility is 93.4%.

The hybrid is cross fertile with maize. The cross has been effected with hybrid and inbred corn lines, as well as with indigenous Mexican and Andean races. Pollen fertility in these crosses is dependent on which maize lines is used in the cross. For example Zapalote Chico x Tripsacorn is 68.5% fertile and W64A x Tripsacorn is 84.6% fertile.

The unexpected fertility of the Tripsacum-diploperennis hybrid and cross-fertility with maize is potentially of great value because it is a natural genetic bridge for transferring Tripsacum genes into maize. It provides a mechanism for importing Tripsacum genes into maize in one generation by natural breeding techniques. When Tripsacum is the female parent in the hybrid, it provides unique opportunity for transferring Tripsacum cytoplasmic genes into maize. Benefits to maize observed in field tests, greenhouse plants and laboratory bioassays include heat and drought tolerance, cold tolerance, hybrid vigor, increased standability, resistance to corn rootworm and corn leaf aphid, enhancement of maize tissue culture capability. 

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

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