Breeding programs and the development of regenerable callus cultures

--David R. Duncan, David P. Deutscher and Jack M. Widholm

Several reports indicate that the ability of a maize genotype to produce regenerable callus cultures from immature embryos is genetically controlled (Tomes and Smith, Theor. Appl. Genet. 70:505-509, 1985; Williams et al., In Vitro 25:95-100; and others). These experiments used F1 hybrids, made from inbreds known to culture well and inbreds known to culture poorly, that were then backcrossed onto the parental materials. Also, there seems to be a general trend in maize tissue culture indicating that the more agronomically elite an inbred is, the more difficult it is to culture. Thus, A188 produces good cultures easily and B73 generally produces poor cultures or on rare occasions produces a good culture. Considering the genetic control of maize culturability, we thought that perhaps breeders might be selecting against culturability while selecting for useful agronomic traits. Such a scenario could explain the "trend" of agronomically elite lines producing poor cultures.

To test this hypothesis, we looked at culturability versus agronomic traits of Illinois Foundation Seed (IFS) inbreds developed from the same breeding population and in the final stages of the IFS breeding program. This experiment compared callus initiation of these inbreds with the agronomic production of these inbreds in test crosses. Two media and three field locations in the midwest were used. The donor plants used in callus production were not grown in the three production fields.

Among the 21 lines tested, from 80% to less than 1% of the embryos cultured formed regenerable callus (Fig. 1). When this percent of callus induced was compared to yield, ear height, root lodging and stalk lodging no correlation could be detected (Fig. 2).

These results indicate that using this breeding stock and these agronomic traits, culturability was not selected against in the IFS breeding program. These results also suggest that the loss of culturability with agronomic improvement of maize may best be explained by the incorporation into breeding programs of material initially devoid of gene(s) for culturability.

Figure 1. The distribution of Illinois Foundation Seed genotypes based on their production of regenerable callus from immature embryos. The y axis represents the summation of percent callus initiation from the different media.

Figure 2. The relationship between percent callus initiation and yield performance (ranked highest to lowest) of 21 Illinois Foundation Seed genotypes. The x axis represents the summation of percent callus initiation from two different media.

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