Partially inbred maize lines in topcrosses and hybrid performance

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INTRODUCTION
In a breeding program, at least four steps are involved to obtain hybrids, namely: the selection of populations; the acquisition of lines; the evaluation of their combining ability; and extensive testing of the hybrid combinations achieved (Paterniani & Campos, 1999).Nevertheless, the selection of parental lines should be made according to methodologies that allow their evaluation in different combinations.It is important to point out that one of the main problems faced by maize breeders who work with line hybrids is the evaluation of the parent lines.For a high number of lines, the assessment of all possible crosses is impractical (Paterniani et al., 2010).
Various techniques and genetic designs can be employed in maize breeding, including diallel crosses, which is a methodology used to select genotypes with a high combining ability and allelic complementarity.Since breeding programs involve a high number of genotypes, partial diallels and topcrosses have been given preference in the evaluation of specific sets of crosses (Rodovalho et al., 2012).
The topcross method, though apparently simple, has a certain difficulty in defining the adequate tester for the group of lines to be evaluated and its interpretation (Paterniani & Miranda Filho, 1987).On this point, Hallauer et al. (2010) state that broad genetic base testers are helpful in obtaining the estimates of the general combining ability, while those of narrow genetic base are useful in estimating the specific combining ability.
The topcross hybrids of partially inbred lines are alternatives to reducing time and cost to obtain hybrids, because the production system needs a smaller number of successive self-fertilizations and a smaller area to obtain and multiply the lines, reaching the market at a faster time, maintaining greater yield when compared with inbred lines (Ferreira et al., 2009;2010).
Thus, the goals of this study were to verify the potential of S 2 strains for synthesis of synthetic varieties and to obtain experimental information on yield and agronomic potential of topcross hybrids.

MATERIAL AND METHODS
This work was carried out in two steps.The first step was to obtain topcross hybrids by crosses between the S 2 families and the tester, and the second one comprised the assessment of topcross hybrids in field trials.
The inbred progenies utilized in the study were from the population consisting of the "Creole" maize variety called Movimento de Pequenos Agricultores-MPA (Small Farmers Movement), which was self-fertilized in the firstcrop corn 2012/2013, generating 95 S 1 families.In the second-crop corn 2012/2013, the inbred progenies were evaluated and the inbreeding depression of each of these progenies was estimated.Negative values of more than 80% of inbreeding depression were noticed for grain yield, with an average depression of 53.75% (Mendes et al., 2013).
Eighteen S 1 families were selected, which showed less inbreeding depression for grain yield, which were sown in the second-crop corn 2013/2014 for the second cycle of self-fertilization, generating the S 2 progenies.In accordance with the seed production, 75 S 2 progenies were selected to compose the group of partially inbred lines (75%).
The tester used was the F 2 generation of commercial hybrid AG6040, considered to have a broad genetic base.The seeds of the 75 S 2 progenies used were sown in a fivemeter row for the crosses.One line of the tester was sown every three lines.Planting was done in an isolated field and, at the time of male flowering, the detasseling of the S 2 family was done, so that there was pollen only of the tester, which was the male parent.The seeds of the 75 topcross hybrids were harvested individually, for the experimental evaluation.
For the second stage, the experiment to assess topcross hybrids was conducted in the second crop 2014/ 2015, in a randomized complete block design with four replicates.The plots were four-meter rows spaced 0.90 m between rows and 0.20 m between plants.Two commercial hybrids, planted in the region, were interspersed as controls (AG7098 PRO and SHS5050).The experiment was sown in February 2015, and the recommended cultural management for the maize culture were adopted to conduct the experiment.The following traits were assessed in each plot: the number of days for male flowering (MF) and for female flowering (FF), and for a sample of five plants, the plant height (PH) and the ear height (EH) were measured in meters.After cropping, five ears randomly obtained from each plot were used to assess the ear length (EL) in centimeters and the ear diameter (ED) in centimeters.The ear yield, in kg.ha -1 (EY), and the grain yield (GY), in kg.ha - 1 , were evaluated by the total weight of the plot corrected for 13% humidity and adjusted for stand variation.Stand correction was using the covariance method described by Vencovsky & Barriga, (1992).The analysis of variance, for the measured traits (PH, EH, EL, ED), was performed with means of plots and, for EY and GY, with the total of the plot.
The general combining ability (g i ) based on the statistical genetic model was also estimated, as mentioned by Ferreira et al. (2009): which, m = overall mean; g i = effect of the general combining ability of i line; e ij = medium experimental error.
The g i was obtained according to the following expression: Scott & Knott test was used to group the means with at 5% probability.The statistical analyses carried out at all stages were made with the Genes program (Cruz, 2013).

RESULTS AND DISCUSSION
The coefficients of experimental variation (Table 1) were, in general, of low magnitude for all traits, ranging from 1.89 to 16.91.Thus, it is deduced that there was good experimental accuracy for the traits evaluated.These results are in line with what has been reported in the literature for the maize crop (Guimarães et al. 2011;Mendes et al., 2013).For the source of variation hybrids, a relevant difference was seen for all the traits assessed.It is concluded, then, that there was variability among the assessed hybrids, which allows selecting topcross hybrids with the best agronomic performance.Table 2, referring to the means of the 75 topcross hybrids and the two controls, displays that, regarding control 1, hybrid AG7098, the traits related to yield (ear yield and grain yield) showed, for all topcross hybrids, magnitudes below the referred hybrid, while in the other traits there is equality for some of the topcross hybrids compared and inferiority for others, which can be favorable, as it is the cycle and height of the plant.Concerning control 2, hybrid SHS 5050, topcross hybrids presented equality in means or even Rev. Ceres, Viçosa, v. 67, n.3, p. 216-222, may/jun, 2020 superiority.For the plant height trait, the results agree with the ones verified by Ferreira et al. (2009), who noticed that, in general, the plant height of topcross hybrids was low compared to that of control hybrids, which may be due to the smaller size of the tester.For the yield trait, the results do not match those obtained by Ferreira et al. (2009) and Marcondes et al. (2015), in which the topcross hybrids were classified in the group of the most productive, possibly because they were using lines with higher levels of inbreeding (S 3 and S 4 ), respectively.Nonetheless, the comparison with commercial hybrids should be treated with caution, as it only indicates a reference of the topcross hybrids to be tested with the one that is being marketed.
Among the topcross hybrids, the progenies that had reduction in the number of days for male and female flowering simultaneously were 2, 20, 31, 41, 43, 47 and 66, with a mean of approximately two days less than the overall mean of the progenies.With respect to the ratio between plant height and ear, the topcross hybrids with the lowest ratio were 5, 24, and 45, meaning that these progenies have the lowest ear height in relation to the total plant height, and, in general, the lower the ear height, the lower the probability of the plant breaking below the ear.
The topcross hybrids that presented the highest ear diameter were 21, 25, 55, and 72; those that showed the highest ear length were 50, 51, 68, and 74.These progenies may be interesting in the breeding process, as the length and diameter of the ear is generally well related to yield.These results underline the significance of evaluating the value of each partially inbreeding line by using the topcrosses, as already related by Arnhold et al. (2009).These authors, by studying the performance association between S 3 families and their topcross hybrids of pop corn, observed that it is not possible to recommend the replacement of the topcross selection by the selection per se of inbreeding families in pop corn, since, for the grain yield and expansion capacity traits assessed, the low correlation does not justify the selection only by the performance per se.
In relation to the grain yield of 16 higher yielding topcross hybrids, they were statistically equal to the SHS 5050 hybrid, which is used in the market.There are some topcross hybrids with a yield above 7,000 kg.ha -1 .
Table 3 provides estimates of the effects of the general combining ability (GCA) for the assessed traits of the 75 maize S 2 lines.Given that the lines were crossed with a 1466.80 (line 32) to 1881.81 (line 67), demonstrating successful possibilities in selecting lines for the synthetic production.Thus, for grain yield the following lines should be recommended: 67, 14, 44, 69 indicating a GCA higher than 1700.00kg.ha-1 .These same genotypes could be used, in their hybrid form, with the AG6040 for base population yield for selection purposes, and they could be used both for future yielding of an open pollinated variety and for extraction of lines to obtain hybrids.Estimating the effects of the general combining ability for grain yield in maize, Scapim et al. (2008) and Clovis et al. (2015) also identified high and positive values, suggesting the feasibility of selecting inbreeding lines with yield potential by means of topcrosses.Barreto et al. (2012), also assessing the ability to combine maize S 2 families by using testers, found significant effects of GCA.Both studies confirm the results found in this study.

CONCLUSIONS
It is concluded that topcross hybrids of partially inbreeding maize lines have good yield potential, being equivalent to one of the commercial hybrids used in the trial, and may be an alternative for the production of synthetic or base populations for selection purposes.
The 67, 14, 44, and 69 lines stand out for presenting the best grain yields.It was also possible to identify good agronomic behavior of the other traits evaluated.
g i = effect of the general combining ability of the lines; c i = mean of each hybrid; c = overall mean of topcross hybrids.