There is no information about the variation in chiasma frequency among and within populations of maize. So, an exploratory study was done by analyzing cytological material already in existence in the laboratory. It is known that chromosome knobs influence genetic recombination and special chromosomes (abnormal 10 and B type) affect both the genetic recombination and the chiasma frequency (Kikudome, Genetics 44:815-831, 1959; Rhoades and Dempsey, Genetics 53:989-1020, 1966; Ward, Can. J. Genet. Cytol. 18:479-484, 1976).

Cytological materials of six collections were selected: three with high and three with low knob content belonging to three races: Cacahuacintle (Mexico 7 and one of unknown origin); Chalqueño (Mexico 208) and Conico (Guanajuato 30, Mexico 3 and Oaxaca 377). Of these collections, the one of Chalqueño and two of Conico (Guanajuato 30 and Oaxaca 377) have many knobs (an average of 30.2% of the known knob positions), and the other three possess few knobs (6.6%). Guanajuato 30 was known to have plants with abnormal chromosome 10 and B chromosomes. These collections had been grown in the experimental field of the Colegio de Postgraduados located in Montecillo, State of Mexico during the years of 1990, 1991 and 1993.

Data on chiasma frequency were obtained from 40 meiotic metaphase I cells from anthers of at least two flowers for each plant, using the traditional propionic-carmine squash technique. The number of plants analyzed varied for the different collections (Table 1). Total and terminal chiasmata were registered for each bivalent in every cell; by differences between these data the interstitial chiasma frequencies were obtained.

The mean total, interstitial and terminal chiasma frequencies between the two groups of collections were not statistically different; however, among and within the collections these frequencies were significantly different. These results seem to indicate, first, there is not any general effect of chromosome knobs on the formation of chiasmata and if there is any influence, probably this is related to the local chiasmata distribution on specific bivalents or chromosome arms where the knobs are localized; second, it seems that different plants have distinct chiasma formation capacity, depending on their particular genotype (Table 1).

Table 1. Mean total chiasma frequencies per plant in six maize collections.
 

	Many knobs			Few knobs
Plt.	Mex. 208	Oax. 377	Gto. 30	Mex. 3	Mex. 7	Cac. O.D.
1	20.07	20.50	22.07	19.90	21.40	20.35
2	19.87	20.17	21.87	19.85	21.32	20.25
3	19.77	20.02	21.30	19.72	20.65	20.07
4	19.70	20.00	21.07	19.67	20.47	20.02
5	19.67	19.85	20.92	19.57	20.30	19.87
6	19.62	19.55	20.92	19.42	20.07	19.87
7	19.45	19.55	20.90	19.25	20.07	19.72
8	19.45	19.47	20.87	19.25	20.05	19.62
9	19.40	19.42	20.60	19.15	19.30	19.55
10	18.87	19.27	20.37	19.10	19.15	19.52
11	18.72	19.15	20.15	18.77	19.10	19.50
12	18.55	19.15	19.97	18.47	19.07	19.42
13	18.27	19.07	19.92	16.72	19.00	19.20
14	18.37	19.77			18.57	18.82
15	18.35	19.67				18.72
16		19.47				18.70
17		19.37				18.45
18						17.42

The collection that showed the highest mean total chiasma frequency was Guanajuato 30. In this collection two plants were found having a heterozygous abnormal chromosome 10, and one plant with 1B and three with 2B chromosomes. Since it is known that these special chromosomes increase the chiasma frequency of the plants where they are present, it would be logical to expect that all the plants having these chromosomes should be grouped among those showing the highest chiasma frequency within the sample analyzed. As shown in Table 2 this is not the case. This suggests that the increasing effect of special chromosomes has a limit that is established by the specific chiasma formation capacity of any plant which, in turn, depends on its genotype, as concluded above.

Table 2. Relationship between mean total chiasma frequency per plant and abnormal chromosome 10 and B chromosomes in the collection Guanajuato 30 of the race Conico of maize.
 

Plant	Mean Total Chiasmata/Plt.	Stat. Dif.1	Abnormal Chrom. 10	B Chrom.
1	22.07	a	Het.
2	21.87	ab		2B
3	21.30	abc
4	21.07	bcd
5	20.92	bcde	Het.
6	20.92	bcde		2B
7	20.90	bcdef
8	20.87	cdef
9	20.60	cdefg
10	20.37	defgh
11	20.15	defghi	2B
12	19.97	efghi	1B
13	19.92	fghi
14	19.77	ghi
15	19.67	ghi
16	19.47	hi
17	19.37	i

¹Tukey test P < 0.05

When the correlations were calculated between the total and the interstitial chiasma frequencies in each of the six collections analyzed, it was found that the three collections with high knob frequencies also showed high r values (0.93 in Oaxaca 377, 0.81 in Guanajuato 30 and 0.73 in Mexico 208). In those with few knobs, these values were much lower (Cacahuacintle of unknown origin, 0.61, Mexico 3, 0.58 and Mexico 7, 0.48). One problem in these results is that not all the collections were planted in the same year and field plot, so they grew under different environmental conditions and probably are not completely comparable. However, there are two cases that, separately, seem to show the aforementioned relationship to be valid since the collections comprising each case were grown in the same year and in contiguous plots: 1) Mexico 208 with many knobs, and Mexico 3 and 7 with few knobs, grown in 1991; and, 2) Guanajuato 30 with many knobs, and Cacahuacintle, of unknown origin, with few knobs, planted in 1993 (Table 3).

Table 3. Correlations between total chiasma frequencies and the corresponding interstitial and terminal ones in different maize collections.
 

		Chiasma Frequency Correlation
Planting		Total Interstitial		Total Terminal
Year	Collection	r	r2	r	r2
1990	Oaxaca 377	0.93	0.86	-0.85	0.73
1993	Guanajuato 30	0.81	0.66	0.57	0.33
1991	Mexico 208	0.73	0.53	0.02	0.0004
1991	Mexico 3	0.58	0.34	0.36	0.13
1991	Mexico 7	0.48	0.23	0.10	0.01
1993	Cacahuacintle (unknown origin)	0.61	0.37	-0.30	0.09

In spite of these limitations, a tentative hypothesis was developed for explaining these results: the chromosome knobs, depending on their size, interfere to a certain degree with the chiasma terminalization, probably by slowing down the process. It is known that the chromosome knob heterochromatin is of the constitutive type that usually appears compacted in all cellular stages. Possibly this characteristic of the knobs causes the interference of chiasma terminalization by presenting some resistance for the homologous chromatids united by a chiasma to become "free" and permit their separation.

Even though the described data and their interpretations might be interesting, there is a need for further well-planned experiments that provide more acceptable evidence to strengthen the proposed ideas. 

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