A maize trisome which produces hypertriploid (3n + 2) plants

A trisomic maize plant (GH 42-3), obtained from a commercial hybrid seed population, was crossed as female parent to the inbred line W22 and chromosome counts were made on squashed root tips of a sample of 90 progeny plants. The progeny consisted of diploid, trisomic or hypertriploid plants with 32 chromosomes (3n + 2):
  Chromosome number  
  20 21 32 Total
No. of plants (and %) 56 (62.2) 26 (28.9) 8 (8.9) 90

Cytological examination revealed that the chromosome involved in the aneuploid condition could be chromosome 3 or 4, as valued by relative chromosome length and arm ratio.

The morphology of the plants made for easy identification of both hypertriploid and trisomic individuals. When examined 30 days after planting, hypertriploids showed only 4 to 5 leaves:
Chromosome Number No. of Plants No. of Leaves*  Leaf Width (cm) Plant Height (cm) Days to Anthesis
20 56 7-8 12.78 (s.e.=0.2) 262.1 (s.e.=5.4) 73-76
21 26 5-6 14.32 (s.e.=0.4) 204 (s.e.=6.9) 83-87
32 8 4-5 9.65 (s.e.=1.8) 126 (s.e.=20.1) 92-99
*On the 30th day from planting.

At maturity they were considerably shorter than their diploid or trisomic sibs; moreover, hyperpolyploidy had a negative effect on leaf blade width (as measured on the 6th leaf from the base) and on the number of days to anthesis. The trisomic plants had a slower growing habit than their diploid sibs; at maturity the leaf blade width was greater in trisomic individuals which were, however, shorter than the diploid plants.

When the pollen grains were observed under the microscope after staining by Lugol's solution, striking differences were revealed among the progeny plants:
Chromosome Number No. of Plants  Frequency (%) of Unstained Pollen Grains
20 24 0.4 - 5.7
21 8 5.7 - 7.7
16 13.4 - 21.4
32 6 61.5 - 86.8

While 2n plants and one-third of 2n + 1 individuals showed a low frequency of unstained pollen grains, the remaining trisomic plants and the hypertriploid plants showed much higher values.

The seeds freely developed in both diploid and trisomic plants. In those hypertriploid plants which reached maturity, however, despite an apparently regular seed set, the majority of the seeds showed an irregular development.

The inheritance of hypertriploid induction was studied by cytological examination of root tips of a sample of 165 progeny plants obtained by crossing the trisomic plant no. 3604-12 (belonging to progeny of plant GH 42-3) as the female parent to the W.M.T. The hypertriploid-inducing capacity can be transmitted through the female at least, as shown in the following:
  Chromosome number  
  20 21 32 Total
No. of plants (and %) 126 (76.4) 37 (22.4) 2 (1.2)  165

In the progeny of this cross two classes of kernels with dark or pale aleurone were observed. Chromosome counts within these classes (see Table 5) indicate that an unidentified aleurone pigment factor is associated to this trisomic condition and may help selecting trisomic individuals through a dosage effect:
    Chromosome No.  
    20 21 Total
Aleurone color dark 3 12 15
  pale 65 9 74

Moreover, after germination of kernels, two distinct classes were observed for the phenotype of the primary root. The thick phenotype seems to have some association with the aneuploid condition:
    Chromosome No.  
    20 21 32 Total
Primary root thick 9 23 2 34
thin 117 14 - 131

Hypertriploid plants could be produced if during fertilization a 2n + 2 egg fuses with an n male gamete. Such a female gamete could possibly originate by a post-meiotic event involving the fusion of two n + 1 nuclei. The fact that eutriploid individuals were not observed in the progeny of hypertriploid-inducing trisomic plants would indicate that this phenomenon affects only the n + 1 nuclei.

Norberto E. Pogna, Nunzia Villa and Achille Ghidoni

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

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