Cytogenetic stability of aneuploid maize tissue cultures

Monosomic maize tissue cultures might be used to select recessive mutations of cellular traits. This strategy would avoid some of the problems encountered with haploid cultures such as lack of vigor, sterility of regenerated plants, and uncontrolled diploidization. Monosomic and other aneuploid plants were selected among progeny of W22 R/r-x1 crossed with genetic stocks containing recessive markers. The r-x1 allele induces aneuploidy at a frequency of about 15%. Immature tassels of selected plants were used to initiate totipotent tissue cultures. Plants were regenerated from the cultures over a period of 3 to 17 months after culture initiation. Meiotic karyotypes of microsporocytes and pollen sterility were analyzed in regenerated plants. At least 40% of the 161 plants regenerated from aneuploid cultures had altered karyotypes. This frequency was not related to culture age. Most alterations involved chromosome breakage rather than changes in chromosome number. Types of alterations included heteromorphic pairs (18.1%), translocations (12.5%), addition (10.6%) or loss (1.4%) of chromosomes, and genomic doubling (2.8%). Four euploid cultures, including one with a translocation, were equally unstable (49% with alterations among 115 plants). Euploid cultures gave rise to plants with translocations (12.3%), heteromorphic pairs (8.8%), and genomic doubling (29.2%), but no single chromosome additions or losses. Plants that shared a common distinctive karyotype, such as a specific translocation, were probably derived from a common cell line. Tassels with sectors of two different karyotypes were frequent in plants regenerated from aneuploid (20%) or euploid (33%) cultures. Coenocytic microsporocytes, which lacked cell walls between nuclei, were found in plants from monosomic-2, deficient-2L, and monosomic-6 cultures. Another aberration (23% of 144 regenerants) was lack of cell wall formation after the first and (or) second meiotic division, which was often followed by nuclear fusion. Karyotypic changes observed in this study rarely involved the monosomic chromosome, which means that monosomic tissue cultures could be used to select recessive mutants. Further tests would be needed to demonstrate that the selected gene resides in the monosomic chromosome.Key words: Zea mays, monosomic, trisomic, chromosome, somaclonal variation, karyotype.