ICI Seeds, Inc.

Stock 6 induced double haploidy is random

--Ming T. Chang

An experiment was designed to generate double haploids from Stock 6. Two inbreds that were derived from Oh43 and Mo17 were crossed and the F1 seeds were used as female stock for this study. These Oh43 and Mo17 derivatives are homozygous pure inbreds as shown from the isozyme test, and their genetic constitution is homozygous A b pl c1 r-g Y. These two lines carry different allelic forms at isozyme loci Mdh1, Mdh2 and Idh2 as shown in Table 1. The pollen source was BC2S2 materials that were derived from crosses between Stock 6 (A b pl C R-g y) and A B Pl C R-nj y genetic stock. The F1 was backcrossed to Stock 6 (A b pl C R-g y) twice, then selfed twice. Seeds from the homozygous ears of A B Pl C R-nj y were chosen as materials for pollen source. The seeds are white (due to y1) flint with purple color on the crown and plumule (due to R-nj). The plants are dark purple on leaves, stalk, tassel and anther (due to B Pl). The isozyme test showed that the BC2S2 Stock 6 material was still segregating at Adh1 and Pgm2 loci as shown in the Table, but 90% of the individuals were already fixed at those loci. Since the Stock 6 carries the Pgd1-22 allele and the two inbreds carry the Pgd1-11 allele, the isozyme difference can be used to identify the origin of the haploids. A total of 600 ears or 305,778 seeds were obtained. Among those, 98 double haploids were obtained. These double haploids were pure lines as examined by isozyme markers and ear to row field evaluation of their morphological uniformity. The allelic constitutions of these double haploids is summarized in Table 2. Genetic linkage distance between Idh2-Mdh2 is 1.1 ± 0.6 (Goodman, MM et al., Genetics 96:697-710, 1980). Among these 98 double haploid lines, all showed tight linkage and none were derived from a crossover between Idh2 and Mdh2 loci on chromosome 6. It is therefore concluded that Idh2 and Mdh2 are tightly linked. The distributions of allelic frequency among these double haploids were Mdh1-11 (48%) vs. Mdh1-22 (52%), and Mdh2-11---Idh2-22 (54%) vs. Mdh2-22---Idh2-11 (46%). This is almost a 1 to 1 ratio due to genetically random assortment of the chromosomes. It is therefore concluded that Stock 6 induced double haploidy is a random process.

Table 1. Genetic constitution of allelic forms of isozyme loci of the male and female materials that were used to generate double haploids.
Chromosome no. 9 1 1 8 6 6 3 1 5 6
Material Acp1 Acp4 Adh1 Mdh1 Mdh2 Pgd1 Pgd2 Phi1 Pgm2 Idh2
S6/R-nj  11 22 22 22 22 22 11 22 22 22
  11 22 11 22 22 22 11 22 11 22
Oh43 derivative 11 22 22 11 11 11 11 22 22 22
Mo17 derivative 11 22 22 22 22 11 11 22 22 11

Table 2. Genetic constitution of allelic forms of the double haploids.
Double Haploids   Genetic Constitution      
(Number) Chromosome no. 8 6 6 6
    Mdh1 Pgd1 Mdh2 Idh2
28   11 11 11 22
19   11 11 22 11
26   22 11 22 11
25   22 11 11 22

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

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