ESTIMATES OF GENETIC PARAMETERS FOR TASSEL CHARACTERS IN MAIZE (Zea mays L.) AND BREEDING PERSPECTIV

TITLE:

ESTIMATESOFGENETICPARAMETERSFORTASSELCHARACTERSINMAIZE (ZeamaysL.)ANDBREEDINGPERSPECTIVES

AUTHOR: I.O.GERALDI J.B.MIRANDAFILHO RVENCOVSKY

Maydica XXX (1985): 1-14

ESTIMATES OF GENETIC PARAMETERS FOR TASSEL CHARACTERS IN MAIZE (Zea mays L) AND BREEDING PERSPECTIVES (')

1.0. GERALDI (?), J.B. MIRANDA FILHO, R. VENCOVSKY

Universidade de São Paulo - ESALQ - Depto. Genética, C. Postal 83, Piracicaba, 13400, SP, Brasil

Received May 15, 1983

ABSTRACT - Three broad base maize (Zea mays L.) populations were used to study yield and tassel characteristics respective to genetic parameters estimation and breeding perspectives. The less improved populations showed lower yields and higher tassel weights, while the highly improved population and check hybrids showed higher yields and lower tassel weights. The coefficient of heritability (plant basis) was relatively low for yield (Y), ie. 6.1% on the average for three populations, but were relatively high for tassel characteristics: 36.1% for tassel weight (TW), 45.8%, for tassel branch number (NB) and 28.8% for tassel length (TL) on the average. The additive genetic correlation (plant basis) between Y and TW was non-significant for all populations, but showed a negative trend (fy =- 0.14). On the other hand, a high and significant correlation showed to exist between Y and NB (fy, = 0.65), and between TW and NB (f, = 047). The correlation between TW and TL showed low consistency among populations. The coefficients of partial genetic correlation were consistently positive for the combinations (Y, TW)-NB and (TW, NB) - Y, and consistently negative for the combination (Y, NB) TW. Results led to conclude that NB is primarily correlated with Y and that the negative correlation between TW and Y results from the positive correlation between NB and TW. The expected gain and correlated response from selection were investigated by considering three seclection schemes, which indicate that selection toward the decrease of the number of branches per tassel will lead to additional increase in yield. Because of the high heritability of tassel branch number, selection for this trait can-be easily and effectively performed through visual evaluation withouth the need of tassel remotion. Some practical limitations are discussed for the suggested selection schemes.

KEY WORDS: Maize; Tassel weight; Tassel branch number; Ideotype.

INTRODUCTION

Maize (Zea mays L.) is among the cultivated crops which have undergone

(1) Adapted from a thesis submitted by the senior author in partial fulfillment of the M.S. requirements.

(2) Research Geneticist (EMBRAPA) at Department of Genetics, ESALQ/USP.

GERALDI, J.B. MIRANDA FILHO, R. VENCOVSKY

intense selection, Recently, there have been some interest for selection of traits associated with high cfficiency in energy conversion toward maximum vield, Traits such as plant height, ear height, leaf area, leaf number, among others, are components of the plant architecture that may affect plant efficiency (Moss and MusGravE, 1971). Another important trait related to maize plant cfficiency is the tassel size, that affects grain yield, either physiologically (competition for photosynthates) or physically (shadding effect), as pointed out by several authors (GROGAN, 1956; HUNTER et al., 1969; SANFORD el dl., 1965; BUREN et al., 1974; Mock and ScHueTtz, 1974).

The effect of detasseling of maize plants has been studied since the last part of the nineteen century. After detasseling became an usual procedure for hybrid seed production, many studies were carried out in order to investigate its effect on grain yield. Some results have shown a small and non-significant effect (DuneaN and WoopwortH, 1939; KiesseLBACH, 1945; GROGAN, 1956), but a significant decrease in yield was observed by pulling some leaves with detasseling (DUNGAN and WoopworTH, 1939; KIESSELBACH, 1945; HUNTER et al., 1973). On the other hand, GroGan (1956) found a considerable increase in yield after normal detasseling under less favorable environmental conditions. The shading effect of the tassel was studied by HUNTER et dl. (1969), concluding that a rather small increase in yield is possible by eliminating the shading effect, mainly under high planting density. The use of male sterility has shown that its effect on grain yield is similar to physical detasseling, mainly under adverse conditions (Duvick, 1958; JosePHSON and KINCER, 1962; BRUCE et al., 1966). Other studies have shown that for malesterile plants, as compared with normal fertile plants, a smaller amount of nitrogen (SANFORD et al., 1965), dry matter (GROGAN et al., 1965) and carbohydrates (CRISWELL et al., 1974), were found in the tassel in a certain period of the plant development.

In general, results have shown that detasseling and male-sterility are factors that tend to increase tolerance for high planting densities, mainly by decreasing the number of barren plants (DuNcaN et al., 1967; BureN et dl., 1974). Mock and Buren (1972) found no strong association between yield and tassel dry weight under low planting density; on the other hand, the higher yielding inbred lines were associated with lower tassel dry weights under high planting density. Under similar conditions, BUREN et al. (1974) detected high positive correlations between dry tassel weight and barrenness. SmiTH e al. (1982) found negative genetic and phenotypic correlations between tassel branch number and grain yield, while positive correlations were found between tassel branch number and barrenness. Such correlation coefficients were higher in magnitude at high planting density.

GENETIC PARAMETERS FOR TASSEL CHARACTERS 3

Therefore, there is ample evidence that tassel size is an important trait related to maize plant cfficiency, Several authors have suggested selection for smaller tassels, partial detasseling in the field and use of male-sterility as means to increase grain yield and mainly to increase tolerance to high planting densities (DuNCAN et al, 1967; HUNTER et al., 1969; BurEN et al., 1974: Mock and ScHuETz, 1974). From a breeding viewpoint, selection toward smaller tassels deserves attention, for which additional information on the inheritance and breeding potential of tassel characters are of prime interest. Mock (1979) reported a substantial predicted gain for increasing yield and decreasing barrenness under selection for decreasing tassel branch number Our purpose in this study was to investigate the genetic structure of three broad base populations respective to yield and tassel characteristics, and breeding perspectives.

MATERIAL AND METHODS

In the present study, the following three populations were used: "Dent Composite-B and "Flint Composite-B , two broad base composites, obtained by intercrossing of several materials from South and Central America and Mexico; and Centralmex-VI, an improved variety, which has undergone six cycles of among and within half-sib family selection. The origin of these populations was described by PaTERNiANT (1968). Two commercial double cross hybrids (H-7974 and AG-152) were used as checks,

The material used in this study comprised 200 halfsib families, obtained through open pollination in isolated blocks, from each population. Yield trials were carried out in 1975 at two locations (Piracicaba with two replications; and São Simão, with one replication) in the State of Sao Paulo, in two 10x 10 triple lattice experiment for each population. Plots were 10 meter long, rows spaced 1.0 m apart, with 25 hills and two plants per hill after thinning.

After pollination had been completed, a sample of twenty tassels was collected from each plot, which were labeled and storaged under natural conditions with free air movement in order to permit moisture homogeneity. Then each set of 20 tassels was weighted. A random sample representing 20% of the progeny set and ten tassels per progeny was taken and for each tassel the following traits were measured: tassel weight (TW), number of primary tassel branches (NB), and tassel lenght (TL, lenght of the principal axis). Yield data (Y) were based on plot totals for ear weight, which were corrected for stand and moisture variation, thus resulting ear weight of 50 plants at 0% moisture,

The analysis of variance was performed according to two models: set 1) model for triple lattice design, involving the whole set of progenies, for the traits Y and W, based on plot totals; and set 11) model for randomized block design with three replications, for the traits TW, NB and TL, which were based on plot totals of individual measurements from the sampled set of progenies. For both analyses the within plor variation was included in the model, in addition to the effects of progenies (p,)

GERALDI, J.B. MIRANDA FILHO, R. VENCOVSKY

replications (b;), and experimental error (intrablock error for lattice design) as Isol.}rce of variation. For the joint analysis, within each population, results of the two lattices were pooled.

For both analyses, a random model was assumed and the expected valuezs of mzeazn squares for progenies (M,), error (M,) and within plots (M,) were: E(M,) = no?%, + Ze + mro?.; EMM,) = no?, + nã?%; and E(M,) = ¢2; where n and r are the numfer of plants per plot and replications, respectively. Components o2, o2, an.d a2, stand' or progenies, error and within plot variances, respectively, and were estimated as linear functions of mean squares. For yield (ear weight) the within plot variance was not estimated, but was obtained approximately through the relation 42, = 1052, (GARDNER, 1961). )

The analysis of covariance between traits was similar to the analysis of variance, respective to degrees of freedom and sources of variation. Mean products were obtained as proposed by KEMPTHORNE (1966). Estimates of the components of covariance were obtained in the same way as for the components of variance. When the sample size (m) for trait x is different of the sample size (n) for trait y, then the expected mean products for m < n are: E(P) = m Cov, + mn Cov, + mnr Cov,; E(P) = m Cov, + mn Cov,; and E(P;) = Cov,, where P,, P, and P, stand for the mean products for progenies, error and within plots as source of covariation (GERALDI e al., 1978). The within plot mean product (P;) is obviously obtained with the smaller sample size (m) for both traits. The following estimates were obtained for each trait (x or y) or combination of traits (STEEL and ToRRIE, 1960; FALCONER, 1964; KEMPTHORNE, 1966; VENCOVSKY, 1978a), on a plant basis: additive genetic variance: 62, = 4 &2; additive genetic covariance: CovV, =4 COV,; phenotypic variance among plants: 8% = 62, + &2, + &2,; phenotypic covariance among plants: COV, = COV, + COV, + COV,; coefficient of heritability (narrow sense): h? = 62,/62,; additive genetic correlation: n = COV,,, /(G- )s phenotypic correlation: 1, = COV,_Y/(ãPl-&W); partial additive genetic correlation: fm = En (R, 1)1 / [(112,,) (112, )1"; expected gain from selection: G, = icó / correlated response from selection: R, = ic.COV,, /&, The phenotypic correlations involving yield on the other hand, were expressed on a family mean basis, and were estimated taking the covariance term as P /mnr and the variance terms as M, / m? and M,, /0% for traits x and y, P,,, being the mean product relative to progeny totals, and M,, and M,, the corresponding mean squares. For G, and R,, four selection procedures were considered, for each of which the constant factor c and the phenotypic variance of selection units (02%,) are given as follows

(VENCOVSKY, 1978a):

Components of g2 Selection procedure

1. Mass selection for both sexes (')

2. Selection among half sib families (both sexes)

3. Phenotypic selection within HS families

sex only)

4. Stratified mass selection within male rows in the recombination block (2)

(") For mass selection 6%;, = 02,

(2) Selection for tassel traits; male rows represent 2 bulk of selected half sib families.

GENETIC PARAMETERS FOR TASSEL CHARACTERS 5

For practical purposes, any selection scheme may involye one or more of the selection procedures as given above. Three selection schemes were considered to increase Y and decrease NB: 1 - mass selection (20%) for NB; II - sclection among (20%) and within (10%) halfsib families for Y; III - same as scheme II plus mass selection (50%) for NB in the male rows of the recombination block.

RESULTS

Mean values and coefficients of variation due to error for yield and three tassel characters are shown in Table 1, on a plant basis. The less improved populations (Dent CompositeB and Flint Composite-B) showed lower yields than the highly improved population (Centralmex-VI) and checks.

Tassel weight, on the other hand, was higher for the less improved populations. For tassel branch number and tassel lenght, populations Dent Composite-B and Centralmex did not differ greatly, with an average of 18.1 and 18.7 branches per tassel, respectively, with lenght of 65.5 cm for both populations. Such values differed slightly from those for Flint Composite-B (22.4 branches per tassel with lenght of 63.2 cm).

TABLE 1 - Means and coefficients of variation due to error for yield (Y) tassel weight (TW), tassel branch number (NB) and tassel lenght (TL) in the joint analysis of experiments for three populations.

Means are expressed in g/plant for Y; g/tassel for TW; and em/tassel for TL; check means (set-I) were 142.1 g/plant for Y, and 6.2 g/tassel for TW,

GERALDI, ].B. MIRANDA FILHO, R. VENCOVSKY

Estimates of additive genetic variance and cocfficient of heritability are shown in Table 2. Populations differed very little respective to additive genetic variability for tassel weight. Greater differences were observed for vield and tassel branch number, indicating that the variety Centralmex showed higher genetic variability for those traits, followed by Flint Composite-B. On the other hand, for tassel lenght Centralmex showed less variability, while Dent Composite-B showed the higher estimate for additive genetic variance. The coefficient of heritability estimates were relatively low for yield (6.1% on the average for the three populations), and Dent Composite-B showed the lowest estimate. Tassel characteristics showed a relatively high heritability. There is, however, some variation for the coefficients of the same trait among populations.

TABLE 2 - Estimates of additive genetic variance (0?,) and coefficient of beritability (h?) on individual plant basis for yield (Y), tassel weight (TW), tassel branch number (NB) and tassel lenght (TL) for three populations.

Coefficient of heritability

Set-]: whole set of progenies; Set-Il: random sample of 20% of progenies.

The additive correlation between yield and tassel weight was nonsignificant for all populations (Table 3) but showed a negative trend

TABLE 3 - Estimates of coefficients of additive genetic correlation, phenotypic correlation and partial (1%t order) additive genetic correlation between the traits yield (Y), tassel weight (TW), tassel branch number (NB), and tassel lenght (TL) in three maize populations.

(") Estimated from the pooled analysis of the three populations.

(2) *, **: stand for significant level at P < 0.05 and P < 0.01 respectively, according to t-test;

(3) Phenotypic correlation based on family means; all other values are based on individual plants. other estimates are non-significant.

GERALDI, J.B. MIRANDA FILHO, R. VENCOVSKY

(f = 0.14). On the other hand, a high and significant correlation showed to exist between yield and tassel branch number (fx = 0.65) and between tassel weight and tassel branch number (;x = 0.47). The correlation between tassel weight and tassel lenght showed some variation among populations and the low precision of the estimates precluded any generalization on the genetic association of these traits. For some combination of traits, the phenotypic correlation coefficients differed in magnitude from the additive genetic correlation coefficients, showing that the environmental correlation may affect at some extent the genetic association between traits.

The coefficients of partial genetic correlation were consistently positive among populations for the combinations (Y, TW) -NB and (TW, NB)-Y. Trait after dot indicates the fixed one, according to Yule s notation. The positive value for the combination (Y, TW) NB contrasted with the negative values of the coefficients for total additive correlation, showing that such a genetic association is affected by the third character (tassel branch number).

On the other hand, coefficients for the combination (TW, NB) are consistently positive for either total or partial genetic correlation and apparently is not affected by the third trait (yield). A similar result is observed for the

Selection schemes: 1 - mass selection (209%, both sexes) for NB; 11 - selection among (2096, both sexes) and within (10%, one sex) halfsib families for Y; 111 - same as scheme 11 plus mass selection (50%) for NB in the male rows of the recombination block.

TABLE 4 - Expected gain per cycle on yield and tassel branch number according to three selection schemes when selection is for increasing yield and decreasing the number of branches per tassel, in three maize populations.

combination (Y, NB) whose coefficients were consistently negative for both total and partial genetic correlation.

Because of the high negative correlation between tassel branch number and vield, selection schemes were considered such that selection criteria were based on increasing yield and decreasing tassel branch number (Table 4).

Table 4 shows the expected gain on yield and tassel branch number in percent of the population mean for each selection scheme. For selection scheme (I), the expected gain ( 15.8% per cycle, on the average) was relatively high for tassel branch number as well as the correlated response on yield (7.9% per cycle). Because of the high coefficient of heritability for tassel branch number, mass selection was the most effective among the selection schemes, as far as indicated by the expected gain. For scheme (II), there will be an increase in yield and a decrease in tassel branch number at lower rates (2.9% and 3.5%, respectively) than for scheme (I). However, the efficiency of the method may be increased by introducing a mild selection (50% selection intensity) for tassel branch number in the male rows of the recombination block (scheme III), thus increasing the expected gain for yield from 2.9% to 4.0%, on the average.

DISCUSSION

The analysis of variability for tassel characters indicated a good amount of genetic variability that can be used for selection, as indicated by the estimates of the coefficient of heritability for weight, branch number and lenght, which were 36.1%, 45.8%, and 28.8%, on the average for the three populations. For tassel branch number Mock and ScHuEeTZ (1974) and SMITH et al. (1982) also obtained a high coefficient of heritability. Therefore, the first conclusion is that simple selection methods may be effective for changing the mean of tassel characteristics. On the other hand, the low coefficient of heritability for yield, (average of 6.1% in this study), indicates that progeny tests are required for selection to be effective.

The purpose of the present work was not to investigate only on the possibilities of selection for tassel traits, but also on their relation with the maize plant efficiency, for which yield must be considered jointly. In this respect, the negative coefficients of additive genetic correlation between yield and tassel weight and between yield and tassel branch number must be taken into account. They are indicative that the most efficient maize plants must have smaller tassel sizes, as already pointed out by some authors

GERALDI, J.B. MIRANDA FILHO, R. VENCOVSKY

(Mock and BUREN, 1972; BUREN el al., 1974). For tassel weight, the results herein obtained does not indicate a clear evidence of its genetic association with yield, under the particular experimental conditions used, which, among other factors, was characterized by a panting density of 50,000 plants per hectare. This correlation would be possibly higher under more severe competition as consequence of a high plant density, as verified by Mock and BUREN (1972), BUREN et al. (1974) and SMITH el al. (1982). It is relevant to notice that the tassel weight includes not only the reproductive but also the s of the tassel. This could partly explain its low correlation vegetative portion: s shown with yield. Nevertheless, a slight evidence of that association wa: among populations, because the less yielding one (Dent Composite-B and Flint Composite-B) showed higher tassel weights while Centralmex and both hybrid checks showed higher yields and smaller tassel weights.

The other tassel characteristic, number of branches, was found to be highly correlated with yield. Furthermore, the coefficients of partial genetic correlation indicated the number of branches as the primary trait correlated with yield and that the negative correlation between yield and tassel weight is a consequence of the positive correlation between tassel weight and tassel branch number. Therefore, tassel weight will be positively correlated with yield when the influence of tassel branch number is eliminated. Negative correlations between yield and tassel branch number also were observed by SMITH ef al. (1982).

Little information has been reported on the correlation between yield and tassel characteristics. In spite of that, tassel branch number can be viewed as a prolificacy on the male side, which is negatively correlated with yield, contrasting with the positive correlation between yield and prolificacy on the female side. The negative coefficient of additive correlation obtained (ra = 0.65) was as high in magnitude as the positive correlation between yield and female prolificacy (Suanpr and ComproN, 1974). It must be pointed out that the tassel branch number was evaluated in this study as the number of primary branches, such that the parameters would be better estimated if the total branch number was considered.

Tassel size is a trait related to maize plant efficiency and must be considered for the obtention of an ideal plant or «ideotype» (Mock and Pearce, 1975). Previous reports and the results obtained in the present study suggest that selection for decreasing tassel size may be introduced as a criterion for increasing plant efficiency toward maximum grain production. For such an objective, selection for tassel branch number is potentially more effective because of its high heritability and its high genetic correlation with

GENETIC PARAMETERS FOR TASSEL CHARACTERS 11

yield. FALCONER (1964) point out that indirect selection through a correlated trait may be more effective than direct selection if the correlated trait shows a substantially higher heritability than the primary trait and if the additive genetic correlation between them is also high. In this sense, downward selection for tassel branch number may be more effective for increasing yield than selection for tassel weight or even direct upward selection for yield, as far as indicated by the expected correlated response for the first cycle of selection. In addition, selection for tassel branch number can be based on visual selection without the need of measuring and any destructive procedure for tassel evaluation (Mock and ScHuETZ, 1974).

It was shown that for the populations under study, mass selection for , both sexes for tassel branch number, with 20% selection intensity, would result in an average gain of 7.9% vyield increase per cycle. Mass selection for prolificacy (5% selection intensity) was reported by LonnquisT (1967), where the correlated response on yield averaged 6.28% per cycle over five cycles; under similar conditions, the direct selection for yield resulted in 3.80% gain per cycle. The high correlation between yield and prolificacy was the base for the effectiveness of indirect selection.

The possibility of phenotypic selection for tassel branch number may turn feasible some combined selection schemes. Thus, combining downward selection for tassel branch number (50% selection intensity in the male rows of the recombination block) with selection among and within half-sib families (modified ear-to-row selection) for higher yield, the expected gain on yield would increase in about 40 percent as compared with selection for yield alone. Nevertheless, for such a comparison, one considers that each plant has an equal contribution of pollen (male gametes) for the next generation. However an equal contribution of male gametes is not expected under random mating, and the less efficient plants with larger tassels would be more prolific as male parents, thus contributing to decrease the realized gain. Therefore, selection for decreasing tassel size would be more advantageous as can be inferred from the comparison of the expected gains. On the other hand, the expected gain based on tassel branch number was calculated with the actual number of branches per tassel, so that visual evaluation of the tassel size would result less acurate and the expected gain would probably be overestimated. Other factors such as environmental effects and genotype x environment interactions were not taken into account but may introduce additional bias in the comparisons.

The problem of unequal contribution of male gametes is not particular to the selection schemes so far considered. Unequal pollination may be included

GERALDI, J.B. MIRANDA FILHO, R. VENCOVSKY

eness of selection, for the instances where the realized gain is low eventhough the expected gain is promising. Probably it may .ªl,m be a cause of low effectiveness of mass selection for traits of low heritability such as yield, where the low progress from selection would be compcnsa?led negatively by unequal pollination; obviously, other factors must be taken into account. Such suppositions reinforce the need for selection toward smaller tassels. The results would be more appatent if selection is for both sexes, by detasseling the undersirable plants before pollen shedding. Selection for both sexes also can be petformed through biparental crosses by hand pollinating the selected plants. If an equal sample of seeds is taken from each selected ear (biparental progenie) then each male parent contribute equally with male gametes to the next generation and the effective population size is kept at a higher level as compared with randomly sampled male gametes (VENCOVSKY, 1978b; HALLAUER and MIRANDA FiLHO, 1981).

as a cause of low effectiv

Evidently, there may be several selection schemes for tassel size, from which some were considered in the present study. The purpose, however, was not to make comparisons between methods based on the expected gain, but to show the feasability and expected advantages of selection for tassel size. Each method has its own intrinsic limitations and selection intensities were taken according to a practical viewpoint. For example, selection for tassel size in the male rows of the recombination block in the modified ear-to-row selection, seems to be feasible because the experimental techniques are not changed but just better explored. This detasseling procedure will enhance the effectiveness of the ear-to-row method. Another possibility is to combine mass selection for tassel branch number with a low selection intensity in the period from tassel emergence to pollen shedding, with usual mass selection for yield.

When considering selection for tassel size, the choice of an appropriate selection intensity is an important task. Selection pressure cannot be so high as to compromise an effective pollination. For example, a selection intensity of 20% (elimination of 80% of plants with larger tassels), as we hav; considered for mass selection, would require a practical evaluation relative to effectiveness of pollination. Also, a strong selection for tassel size in male row of the recombination block of selected families, would decrease the effective population size and affect the pattern of pollination and recombination, Such effect will be reinforced if one considers that the selected plants are just those with smaller tassels. For this reason, a selection intensity of 50% within male rows was considered in this study, but it also would require a better approximation based on experimental evidences.

Í W(.- have so far presented some evidences that selection for tassel size is pracnf*n y feasible with some increase on the effectiveness of selection for hxgh? r yields. However, due to the high heritability of tassel branch number, a I_RFI_ and immediate response from selection is expected, but genetic variability is expected to decrease fastly in a few generations. Also in such a combined selection, criteria and the balance and intensity of selection must be eventually changed in advanced cycles for maximizing yield in a long term program.

'!'hc authors grntefully. acknowledge Dr. Ernesto Paterniani for the valuable suggestions and discussion on several important aspects of this paper, and for the encouragement in developing rescarches directed towards more efficient corn plants.

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