Our preliminary findings have demonstrated that recurrent selection involving haploid plants seems to be quite efficient in the improvement of synthetic populations (Chalyk, ST and Rotarenco, VA, MNL 73, 1999). Selection efficiency in this case is associated with the fact that only additive and epistatic gene effects are expressed in haploids; increasing the frequency of favorable alleles is the basis of the improvement of heterogeneous populations for the traits selected. In our opinion, this specific feature of haploids might be utilized to evaluate breeding and genetic values of populations as well as to assess selection efficiency.

The comparison of haploids demonstrated the importance of additive and epistatic gene effects in the development of the trait. This information might be especially useful in breeding programs and in the prediction of selection efficiency.

The study was aimed at the comparison of the haploid plants derived from different cycles of haploid recurrent selection for some quantitative traits. Simultaneously, we conducted an experiment to assess the genetic results of selection in two diploid populations (SP and SA) for five traits. The goal of the experiment was to obtain estimates of additive variance for each cycle of selection (C0,C1,C2) in both populations. The values of variance among half-sib families in each selection cycle was multiplied by four to estimate values of additive variance.

Table 1 shows the values of five traits of haploids resulting from three cycles in the SP population. Haploids of the later selection cycles (C1 and C2) significantly exceeded those derived from the initial population (C0) for most traits. The findings demonstrated the changes in the population due to selection, i.e. the changes in the frequency of the favorable alleles possessing additive and epistatic effects.

Table 1. Trait means and coefficients of variation in haploids derived from three selection cycles in the SP population.
 

Traits	Selection Cycles	Traits Means, cm	Coefficient of Variation (%)
Ear Length	SPC0	7.03±0.3	29.2
	SPC1	7.85±0.4***	23.4
	SPC2	7.66±0.3***	25.3
Plant Height	SPC0	47.12±1.8	29.2
	SPC1	50.13±2.1***	26.5
	SPC2	56.47±1.9***	23.2
Height to the ear	SPC0	6.64±0.5	60
	SPC1	7.56±0.8***	70.6
	SPC2	6.76±0.6	64.3
Leaf Length	SPC0	27.19±0.7	20.5
	SPC1	28.67±1.8***	23
	SPC2	29.19±0.8***	20.5
Leaf Width	SPC0	4.07±0.09	16.8
	SPC1	4.15±0.1**	18.6
	SPC2	3.89±0.1	20.3

**,*** The differences from the haploids of the initial population (C0) are significant at 1% and 0.1% significance level, respectively.

The material was evaluated under extremely adverse environmental conditions for maize production, occurring in Moldova in the year of 2000. There was no rainfall during two months (May and June). This significantly reduced the total yield level of the SP and SA diploid populations (Tables 2 and 3). The unfavorable growing conditions are likely to have caused a considerable reduction in the variation between the initial population and subsequent selection cycles, especially in the SP population.

Table 2. Trait means, coefficients of variation and estimates of additive variance of three selection cycles of the SP diploid population.
 

Traits	Selection Cycles	Trait Means	Coefficient of Variation	Additive Variance
Productivity	SPC0	64.4±2.7	25.2	1033.9
	SPC1	64.1±1.7	17.4	365.8
	SPC2	66.9±2.7	19.6	732.3
Ear Length	SPC0	13.3±0.3	12	9.9
	SPC1	14.2±0.15**	6.8	2.64
	SPC2	13.8±0.2	7.8	4.95
Ear Diameter	SPC0	3.8±0.03	5.5	0.17
	SPC1	3.8±0.01	3	0.03
	SPC2	3.9±0.04*	5	0.16
Number of Seed Rows	SPC0	12.7±0.13	6.2	2.2
	SPC1	13±0.1	5.2	1.1
	SPC2	13.6±0.2***	6.7	3.5
Plant Height	SPC0	115.8±2.5	13	1161.9
	SPC1	125.3±1.6	8.5	458.4
	SPC2	138.6±1.7*	6.8	471.4

*,**,*** The differences from the initial population (C0) are significant at 5%, 1% and 0.1% significance level, respectively.

Table 3. Trait means, coefficients of variation and estimates of additive variance of three selection cycles of the SA diploid population.
 

Traits	Selection Cycles	Trait Means	Coefficient of Variation	Additive Variance
Productivity	SAC0	54.02±1.5	17	264.5
	SAC1	70.5±1.8***	16.2	420.4
	SAC2	65.9±2.3***	23.2	865.7
Ear Length	SAC0	13.8±0.14	6.5	2.6
	SAC1	14.06±0.2	7.7	3.95
	SAC2	14.2±0.15***	7.2	3.7
Number of Seed Rows	SAC0	3.5±0.02	3.7	0.05
	SAC1	3.7±0.02***	3.6	0.05
	SAC2	3.62±0.04**	7.3	0.26
Number of Seed Rows	SAC0	12.2±0.09	4.8	1.04
	SAC1	12.5±0.09*	4.6	0.25
	SAC2	12.5±0.1*	5.5	1.64
Plant Height	SAC0	120.3±1.4	7.5	319.5
	SAC1	123.7±1.2	6.5	249
	SAC2	135.3±1.8***	9	624.3

*,**,*** The differences from the initial population (C0) are significant at 5%, 1% and 0.1% significance level, respectively.

The values of the additive variance and corresponding coefficients of variation were higher in the initial SPC0 population than in the SAC0 population (Tables 2 and 3). Theoretically, selection would be expected to be more effective in the SP population, but the three years' trials show that selection was considerably more efficient in the SA population compared to the SP population. Judging by the additive variance in subsequent selection cycles (C1, C2) of both populations, a supposition might be made that the SP population was losing its potential for further significant improvement, while the variation among the half-sib families was increasing in the SA population, which indicates that further selection might be promising.

The fact that two synthetic populations differ in their responsiveness to the same selection method is a common phenomenon, observed by many researchers. We would like once more to underline some discrepancy between the prediction of the selection results in populations based on the assessment of half-sib families and observed results. Our method of estimation, as well as the extreme environmental conditions of the year could be responsible, but the literature suggests that such results have been quite usual.

Coefficients of variation for plant height and ear length of the haploid population were greater than those of the half-sib families in the same selection cycles. Since genetic variation in the haploid population was expressed mostly by the non-allelic gene effects. This significant difference between the haploid and diploid population is likely to be the main reason for the discrepancy mentioned above. Based on the result of this comparison we may suggest that the variation among the half-sib families in each selection cycle in this case, did not quite reflect the variation among the genes possessing non-allelic effects. Hence, the additive variances obtained due to this variation could not reflect the differences for these gene effects between both populations and selection cycles in these populations.

In our opinion, a comparative analysis of populations at the haploid level to determine their genetic structure and to assess changes occurring in them might be promising. The necessity of special crossing for progeny testing is not necessary. A simple comparison of the values of haploid traits might provide the information on the efficiency of the selection, while the variations of the traits might prompt whether further selection is promising.

Additionally, haploid plants may be used to make it easy to obtain estimates of the heritability in the narrow sense. Haploids, as has already been mentioned, do not possess intra-allelic gene interactions and effects are based only on the additive and epistatic contributions of alleles. In haploid populations, narrow and broad sense heritabilities are equivalent in the absence of epistasis.

In our viewpoint, use of haploid plants might make the studies in population genetics significantly easier.
 
 

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