Improvement of anther culture response of apomictic maize-Tripsacum hybrids
--Li, YG, Bai, DP, Kindiger, B

The gene(s) controlling apomictic reproduction in Tripsacum have been assigned to the long arm of chromosome 16. In addition, the long arm of Tripsacum chromosome 16 (Tr16L) has been translocated to the long arm of maize chromosome 6 (Mz6L) generating a Mz6L-Tr16L translocation stock (Kindiger et al., Genome 39:1133-1141, 1997). The stock carries 29 intact maize chromosomes, the Mz6L-Tr16L translocation, and 8 additional Tripsacum chromosomes. The form of apomictic development in Tripsacum dactyloides is characterized as being diplosporous pseudogamy of the Antennaria type which results in the complete omission of normal meiosis. This omission of meiosis precludes the reduction of the genome in the megaspore mother cells and therefore produces unreduced 2n egg cells. The genomic constitution of apomictic hybrids can be altered either by a rare event of 2n + n mating or the generation of rarely occurring polyhaploids (Kindiger et al., Genetica 92:197-201, 1994, Kindiger et al., Crop Sci. 36:1108-1113, 1996). An alternative approach can involve microspore culture. The microspore mother cells undergo normal meiosis during microsporogenesis and the resultant microspores are viable until the first meiotic division (Kindiger, Genome 36:989-997, 1996). Therefore microspore and anther culture methods could provide an alternative approach for reducing or eliminating the unnecessary Tripsacum chromosome in the Mz6L-Tr16L translocation stock.

A total of 24,000 anthers from 80 individual apomictic plants carrying Mz6L-Tr16L translocation were grown in the green house and were cultured on both solid and liquid YP basal medium containing 0.1 mg/l 2, 4, 5-triiodobenzoic acid (TIBA) in a growth chamber at 28 C in the summer of 1996. All tassels were pretreated for 12 to 14 days at 8 C. Enlargement in a few anthers was observed after two months of culture. The poor response indicate a lack of genetic components in the Mz6L-Tr16L stock for response to anther culture. The translocation stock was, therefore, crossed to the DH139/29 maize line to improve the culture responses of this stock. The DH139/29 maize line was bred for high response to microspore culture and is protected under U.S. Patent (#5306864). The cross was again achieved through 2n + n mating and the resultant hybrids carry 39 intact maize chromosomes, the Mz6L-Tr16L translocation, and 8 additional Tripsacum chromosomes. A total of 45 hybrid seed were harvested. Two thirds of the hybrid seed were planted in the greenhouse and one third were planted in field nursery during the spring 1997. Forty-two plants were successfully established. About 16,000 anthers from the 42 plants growing in both greenhouse and field were cultured as described above in 1996 and tassels were again pretreated for 12 to 14 days at 8 C. Callus was obtained after two months from two of the anthers cultured. However, these calli grew poorly and failed to regenerate when transferred to hormone free YP regeneration medium. This preliminary study indicates that the addition of 10 more maize chromosomes from the DH139/29 anther culture responsive line to the Mz6L-Tr6L translocation stock can make improvement in callus formation from anthers of the apomictic translocation stock.

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