A study of the progeny of monosomic-10 plants
--R. Lane, D. F. Weber, and M. C. Schneerman

Sears (Chromosoma 4:535-550, 1952) reported that univalent chromosomes frequently misdivide during meiosis in wheat producing telocentric chromosomes. We have been analysing progeny of maize monosomics (which have a univalent chromosome in each meiotic cell) to determine if chromosomal variants might also be recovered from them.

We (Teissonniere, Weber, and Schneerman, MNL 68:70, 1994) previously studied progeny of monosomic-4 maize plants. Monosomic-4 plants that had su on their single chromosome 4 were crossed by a Su/Su stock producing F1s that were Su/su. These F1s were reciprocally testcrossed. 15 of 49 F1s testcrossed as male parents produced ratios that were significantly different from a 1:1 ratio. Thirteen of these had more sugary than non-sugary kernels and 2 had fewer sugary than non-sugary kernels. However, none of these 49 F1s testcrossed as female parents had ratios that differed significantly from a 1:1 ratio. The fact that these differences were observed when the F1 progeny were testcrossed as males but not as females is consistent with the possibility that chromosomal abnormalities might have been present in these plants.

Here we report the results of crosses involving monosomic-10 maize plants utilizing markers on both arms of this chromosome. The monosomic-10 plants were produced utilizing the r-X1 system and selected as described by Weber (Maize Handbook, ed. M. Freeling and V. Walbot, pp. 350-358, 1994). We crossed monosomic-10 plants that were Oy Bf2 (in 10S and 10L respectively) as male parents by diploid oy bf2 female parents, and the F1s were testcrossed in the summer of 1993. Approximately 100 testcross progeny of each of 38 different F1 plants were analyzed, and 6 of the 38 testcrosses had ratios that were significantly different from a 1:1 ratio for one or both of the marker mutants as determined by a Chi-square test. The ratios in these six crosses are given below (* = significantly different from a 1:1 ratio at p < 0.05).
 

Plant	# Oy	# oy	X2	# Bf2	# bf2	X2
93-532-3	59	39	4.08*	57	41	2.61
93-532-5	58	38	4.17*	58	38	4.17*
93-532-11	63	36	7.36*	64	35	8.50*
93-533	62	37	6.31*	52	47	0.25
93-534-1	59	37	5.04*	58	38	4.17*
93-534-2	56	42	2.00	59	39	4.08*

We also crossed monosomic-10 plants that were Oy Bf2 as female parents by diploid oy bf2 male parents, and the F1s were reciprocally testcrossed in a 1991-1992 winter nursery. 45 crosses where the F1s were testcrossed as male parents have been classified, and 8 of the testcrosses had ratios that were significantly different from a 1:1 ratio for one or both of the marker mutants. Unfortunately, only a small number of progeny were available from some of these crosses. The ratios for these crosses that deviated significantly from a 1:1 ratio are given below:
 

Plant	#Oy	#oy	X2	#Bf2	#bf2	X2
H91-401-2	10	11	0.05	16	5	5.76*
H91-401-8	53	46	0.50	62	37	6.31*
H91-402-1	37	29	0.97	42	24	4.91*
H91-403-10	49	49	0.00	39	59	4.08*
H91-405-12	9	7	2.78	8	1	5.44*
H91-40 9-1	42	22	6.25*	41	23	5.06*
H91-40 9-3	43	30	2.32	45	28	3.96*
H91-41 5-6	51	33	3.86*	51	33	3.86*

These have not been corrected for small sample sizes, so deviations from a 1:1 ratio in plants 401-2 and 405-12 may be due to small sample sizes.

The reasons for these deviations from a 1:1 ratio are not known. If the univalent chromosome 10 misdivided during anaphase I of meiosis producing telocentric chromosomes for both arms, the F1 dihybrids would have telocentrics for both arms of chromosome 10 from the monosomic with dominant alleles of Oy and Bf2, and a normal chromosome 10 from the tester parent with recessive alleles of both loci as diagrammed below:

When a plant of this type is testcrossed, the telocentric chromosomes might be lost during meiosis in a portion of the cells, and one might expect that more than half of the viable gametes would contain recessive alleles for one or both loci. Because nearly all of the crosses where ratios deviated significantly from a 1:1 ratio had an excess of dominants for one or both loci, we believe that it is unlikely that these plants contain telocentrics for chromosome 10.

Another possibility is that the exceptional F1 progeny could have been trisomic for chromosome 10 (the monosomic contributed 2 chromosome 10s with dominant markers and the tester contributed one 10 with recessive markers). When such a plant is testcrossed as a male parent, a ratio of 2 dominants to one recessive would be obtained for the marker mutants (because haploid pollen grains almost always outcompete disomic pollen grains). The testcross ratios where the F1s were testcrossed as male parents could be accounted for if in this way. However, a trisomic of this type (with two dominants and one recessive) testcrossed as a female parent is expected to give a ratio of approximately 5 dominants to 1 recessive, and the testcross ratios where the F1s were testcrossed as female parents did not give such ratios. We therefore believe that the aberrant ratios where the F1s were testcrossed as female parents were not due to trisomy. 

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