Phenotypic abnormalities in the progeny of plants regenerated from callus

--M. D. Garcia, Maria del Carmen Molina and O. Caso

Maize plants have been regenerated from callus after 17, 32, 48 and 50 months of subcultures. This callus was obtained from one maize embryo (cv. Colorado Klein) and cultured in vitro on medium containing 2,4-D (Garcia et al., MNL 64:72-73, 1990).

Seventy percent of plants regenerated after 17 months showed normal phenotype and karyotype whilst 30% showed light phenotypic abnormalities and some chromosomic alterations.

The plants regenerated after 32 months of subcultures showed normal phenotype in 8% but the other 92% showed great somaclonal variation with different degrees of phenotypic abnormalities, including the appearance of similar traits to those produced by the genes dwarf and shrunken, which have never been observed in the original Colorado Klein population.

The meiotic study of this progeny revealed that 95% of the plants had chromosomic aberrations such as deficiencies, duplications, inversions and translocations, and 70% of the same plants had an extra chromosome.

One hundred percent of plants regenerated after 48 and 50 months of subcultures showed great phenotypic abnormalities, and the chromosomic behavior is being studied now.

Plants regenerated after 32 months of subcultures were self-pollinated or open-pollinated if pollen was sterile. The kernel color of 75% of the ears obtained mutated from yellow flint to white flint.

The analysis of the offspring of one of these ears (white flint kernel) showed great variation both for plant height and ear insertion, plants with two ears at the same node, non-branched tassels, colored anthers, albino plants and completely deformed plants.

The most dramatic and spectacular segregation was observed in the ear, because whilst the original one was white flint, the self-pollination of the offspring of it produced: 1) ears supporting yellow flint kernels, similar to the original population; 2) ears with yellow and white flint kernels; 3) ears supporting dent kernels as well as yellow and white sugary ones; 4) ears with dent and white sugary kernels in the rate 3:1; 5) ears with flint, dent and sugary kernels; 6) some ears with viviparous kernels and brittle endosperm.

It is important to point out that each plant regenerated from callus obtained from one embryo has a different phenotype and genotype, but a high number of recessive genes (not present in its parental types) appear. This means that each cell or group of cells regenerates a new maize plant with a different genetic constitution.

Lee (M.S. thesis, 1984) supposes that the high number of chromosomic breakages and the appearance of a high number of recessive genes could be due to the action of transposable elements, which would be responsible for the high genetic variation observed in those regenerated plants. This author also points out that the genome stress occurred while in vitro culture activates certain elements which behave as silent genes during the normal stage.

It could be also considered that these changes are produced by effects of the 2,4-D (a mutagenic substance), because when the culture term increases, alterations do too.

Whatever the mechanisms producing genetic variation at the callus, it is important to remark that this method could be used as a source of genetic variation, as the changes occurred have been shown to be heritable.

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