In our search for the cultivars displaying a high frequency of spontaneous diploidization of haploids, the 613/2 c4 line was revealed (MNL, 1993). In 1993 the frequency of spontaneous diploidization (the ratio of the number of plants with fertile anthers to the total number of haploids) was as high as 22% in 613/2 c4 (Table 1). Some lines of the fifth cycle of haploidy were derived from 613/2 c4 by genetic marking and spontaneous diploidization. The trait under research, haploids originating in these lines, varies within the range of 11-43%. Stimulation of diploidization shows intermediate inheritance in hybrids.

Genotypes with a high frequency of spontaneous diploidization in some lines and hybrids were selected among the autodiploid lines of 1-3 cycles. In the haploids of these cultivars spontaneous chromosome doubling without a colchicine treatment was observed at a rate of 23-46%. These plants carry a vital ear and some fertile anthers or a fertile sector in a sterile tassel.

Table 1. Frequency of spontaneous diploidization of haploids in corn hybrids and their parents.
 

Origin of haploids	No. haploids	No. plants with fertile anthers	Frequency of diploidization, %
Ts8	57	3	5.3
Ts8 x 613/2 c4	89	12	13.5
Ts16	59	2	3.4
Ts16 x 613/2 c4	220	31	14.0
Intermated Mo17	170	2	1.2
Intermated Mo17 x 613/2 c4	59	8	13.6
613/2 c4	151	33	21.9

Thus, the sources of a high frequency of spontaneous diploidization could be found with a greater success within the cultivars (lines and hybrids) having passed over a haploidy state than within the lines of traditional selection. 

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