estimation of heritability and response to selection for cut-flower yield in gerbera

Euphytica 30 (1981) 313-322

ESTIMATION OF HERITABILITY AND RESPONSE TO SELECTION FOR CUT-FLOWER YIELD IN

GERBERA

JAMES HARDING, THOMAS G. BYRNE and ROBERT L. NELSON

Department of Environmental Horticulture, University of California, Davis, California 95616, USA

Received II March 1980

INDEX WORDS

Gerhera jamesonii, gerbera, inbreeding depression, additive genetic variance

SUMMAR’I

Heritabilities and responses to selection for cut flower yield were estimated for a population of gerbera. Broad-sense heritabilities averaged 42 percent, but may be as high as 66 percent. Narrow sense heritabilities averaged 68 percent when based on half-sib families, and averaged 60 percent when based on parent- offspring regression. These results suggest that most genetic variance for flower yield is additive.

However, estimates of realized heritability averaged only 16 percent and inbreeding depression was estimated to be at least 38 percent, suggesting a major role for non-additive genetic variance. Nonetheless, selection for cut flower yield is expected to be successful.

INTRODUCTION

In 1930, R. A. FISHER proposed hisfundamental theorem ofnaturalselection. According to this theorem, the mean fitness of a population, F, increases over time in direct proportion to the additive genetic variance in fitness o& i.e.

dF 2. dt -oF

Fitness is defined in the Darwinian sense to indicate population survival. A population with greater additive genetic variance for fitness will, therefore, have a greater rate of increase in mean fitness than a population with lesser additive genetic variance; a population with no additive genetic variance will not respound to selection. The fundamental theorem appears to suggest that increases in fitness will be constant over time, but this is not true. The value ofo$ is a function of the frequency of genes affecting fitness. For additive loci,oi is maximum at gene frequencies of one half. When an allele that increases fitness occurs with frequency less than one half, (T; will increase until a maximum is reached, after whicho: will decrease as the less fit allele is eliminated from the population.

This theorem becomes important to the plant breeder when characters such as seed yield are considered in place of fitness. If we assume that the natural selection of Darwin and Fisher is the same process that takes place under mass selection for seed yield (Y) by the breeder, the therem becomes

dP 2 -=(T dt Y’

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J. HARDING, T. G. BYRNE AND R. L. NELSON

As the additive genetic variance in seed yield, o$ approached zero, no further gain in mean seed yield is expected. For characters having a long history of selection, it is likely that their additive genetic variances will have largely been diminished and that mass selection will be ineffective. Seed yield in grain crops, forage yield in forage crops, and oil yield in oil crops are likely examples.

The situation is not as clear in cases of domestic plants that have been selected for their ornamental value. Flower crops may not have long histories of selection for flower yield. The primary purposes of this study were (1) to estimate the level of additive genetic variance for flower yield and (2) to subject a population to mass selection for flower yield and estimate the response to selection. Gerbera jamesonii was chosen because a genetically diverse collection of stocks was available from which a population could be constructed, plants are easily grown and mass pollinations are not difficult.

MATERIALS AND METHODS

Genetic material. The germ plasm from which the Davis population was derived descended from mixed seed lots obtained from various commercial sources. Eva- luations were made of ca. 1500 plants from seed obtained from three U.S. companies; ca. 1000 plants from seed obtained from the Hebrew University, Israel; ca. 500 plants from seed obtained from a Dutch seed company; and ca. 20< plants from seeds obtained from a German seed company.

One plant was selected as particularly suited for greenhouse production; is is re- ferred to as Clone 1 by SMITH & NELSON (1967), and as Gl . Other desirable plants were chosen and crossed to G 1. From these segragating crosses were chosen a series of plants for the population, G2, G3, G4, . . . G21. An attempt was made to maximize genetic variances for as many characters as possible by selecting parents that appeared to be as divergent as possible.

Mating scheme. The 21 original parents were crossed to one another in the following polycross scheme. On a given pollination day a mass collection of pollen was obtained by harvesting flower heads from the 21 parents that showed anthers extruding from the disc flowers and shaking their pollen into a vial. No more than one flower head was used from an individual parent; parents could not be included if they did not have a flower at anthesis on that pollination day. The vial of pollen was stirred with a camel hair brush and applied with the brush to flower heads of the 21 parents that had receptive stigmata. No more than one flower head per plant was pollinated for a given pollination day; plants not having a receptive flower head on that pollination day could not be included. The process was repeated weekly until 3 pollinated flower heads were obtained on each of the parents. Consequently, each parent was crossed to three different pollen collections. Each progeny could be classified according to female parent and pollen collection. The effect of the three different pollen collections on flower productivity of offspring was tested by a standard analysis of variance for the data from the first year and was found to be not statistically significant. Thereafter, the seed obtained from the three pollinations on a parent was bulked. The practice of making three different pollinations on each parent was continued, however, because it

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SELECTION FOR FLOWER YIELD IN GERBERA

insures a more random combination of male and female gametes. Each plant had a pedigree that could be used to trace its maternal parent. Hence, individuals from the same family were half-sibs.

Annual growing procedure: Crosses were made each year during the months of April and May. Seeds from each parent were sprouted as near June 1 as possible and grown in 5.7 cm clay pots. They were transplanted into 15.2 cm clay pots during the summer and finally into 7.6 1 cans on September 1, just prior to flowering. Plants were grown in a potting mix composed of equal parts of sand, peat moss, and coarse redwood saw- dust amended with dolomite, CaSO,.MgSO,, and irrigated with a nutrient solution.

The plants were randomized on benches in a glasshouse that was maintained at 27°C maximum, during days and 21°C minimum during nights.

Each flower produced by a plant was harvested as first anthers appeared. Monthly production records were kept for each plant beginning September 1, and ending March 1 of the following year. Thus, flower number was determined for each plant during the first 6 months of production which coincided with the 6 months of lowest light intensity and shortest day length. On March 1, plants with highest yields were selected and plants with lowest yields were discarded. Population size, number of parents and selection intensities are given in Table 1.

Culling. In each generation ca. 37.5 percent of the yield selections were eliminated because they had unacceptable flower quality or short vase life. An analysis of flower quality will be presented in another paper (Drennan et al., 1980). Estimates of vase life heritability will also be presented in another paper.

Cloning of selectedplants. In each generation there were some high yielding plants that were judged to have higher flower quality than others. These were cloned by crown division and grown along with the population in each generation. These selected plants (clones) are indicated by the Prefix P.

Inbreeding. As many plants as possible were self-pollinated by crossing between flower heads on the same plant. Seedlings from these self-pollinations were designated by the prefix S and grown along with the population in each generation. Some of the more vigorous selfs were clonally propagated and grown with following generations.

Table 1. Population size, number of selected plants, and selection intensities for three generations of the Davis population of gerbera.

Generation Size of population

Number of plants selected from the population.

Selection intensity for yield’

1 164 38 44.5 2 185 49 45.9 3 196 __ _-

r Obtained by dividing the number of plants selected for yield prior to culling by the number of plants entering that generation (x 100).

Euphytica 30 (1981) 315

I. HARDING, T.G.RYRNE AND R.L. NELSON

r

Fig. 1. Distributions of 6-month flower yields in Davis for parents grown in 1972-1973 and 1973-1974, generation 1 grown in 1971-1972, generation 2 grown in 1972-1973 and generation 3 grown in 1973-1974.

Evaluation of parents and inbred lines. The original parental clones (G’s), the population, selected clones (P’s), and selfs (S’s) were grown each year in the same greenhouse at Davis, California. Clonal progeny of 22 of the selected plants, 4 of the original G stocks, and one self were also evaluated as San Jose, California, in order to compare genotypes without confounding effects of different years. Plants were grown in 15.2 1. cans and evaluated for 12 months.

EXPERIMENTAL RESULTS

Progress under selection. Frequency distributions for 6 month flower yield for generations 1, 2 and 3 are presented in Fig. 1. The original parents (G’s) were vegetatively propagated and 2 replications of each were grown at random with generations 2 and 3. The distribution of their flower yields is also included in Fig. 1. The large increase in flower yield in the first generation was not anticipated because selection had not yet been practiced. Mean flower yield continued to increase in the second generation, but dropped slightly in the third generation. Flower yields of the original parents (G’s) and the selected clones (P’s) that were grown with generation 2 in 1972-3, and generation 3 in 197334, indicate that invironmental differences between the 1972-3 and the 1973-4 growing seasons were responsible for the reduction in flower yield in generation 3. For

316 Euphyrica 30 (1981)


Fig. 2. Mean 12-month flower yields for parents, and generation 1,2, and 3 grown in Davis and for parents and a sample of selections from each generation grown in San Jest. Broken lines indicate standard deviations of estimates.

example, the mean flower yield of 26 G’s was 28.5 in 1972-3 but the flower yield of 25 G’s was only 13.6 in 1973-4. Unfortunately, these stocks were not included with the 1971-2 population so year effects could not be removed from each of the 3 generations. Seasonal confouding is eliminated from the comparison in the experiment at San Jose, where selected clones from the 3 generations were evaluated during the same year (see Fig. 2).

The 6-month means for each generation at Davis have been doubled to scale them to the 12-month yields from San Jose. The selected clones show no apparent drop in yield in generation 3 and the trend towards higher yields appears linear.

Realized heritability was estimated from h’(R) = r/d where r is the difference between the means of two generations and d is the difference between the mean of the population and the mean of the selected parents. The estimates of selection differen- tials, responses, and realized heritabilities are given in Table 2. The increase in flower yield in the second generation led to a response of ca. two flowers and a realized heritability of 33 percent. The drop in yield observed at Davis in generation 3 led to a negative response and a realized heritability of zero.

Euphytica 30 (1981) 317


Table 2. Selection statistics and estimates of realized heritability for two generations of the Davis population of gerberas.

Selection generation

Selective Response differential (d) (r)

Heritability (x 100) fh2@)1

1 6.173 f2.22 32.80 2 7.898 -0.38 <o

Estimation of broad-sense heritabilities. Heritabilities in the broad sense (HZ) can be estimated from a one-way analysis of variance. Mean squares and components of variance were estimated for between and within clone contrasts. Broad sense heritabilities were estimated from the ratio of the between-clone component of variance to the total phenotypic variance.

Twenty of the original 21 parents and six of the selected clones from generation 1 were evaluated with the generation 2 population in 1972-73. Twenty of the original 2 1 parents and 5 of the selected clones from generation 2 were evaluated with the generation 3 population in 1973-74. Six-month flower yields were taken and the results of the analyses of variance appear in Table 3. Three of the original parents (G’s), one self, and 22 selected clones were evaluated in a randomized complete block design at San Jose in 1975576. Twelve-month flower yields were taken and the results of the analysis also appear in Table 3. There was no statistically significant difference between blocks (F < 1) so this contrast does not appear in the table. Six-month yields of less than 10 flowers per plant (Davis, 1973-74) gave a broad-sense heritability estimate of about 15 %; six month yields of about 15 flowers per plant (Davis, 1972-73) gave an estimate of about 45 %, and a 12-month flower yields of about 100 flowers per plant (San Jose, 1975-76) gave an estimate of about 65 %. Thus, it appears that heritability estimates increase as mean flower yields increase.

Estimation of narrow-sense heritabilities (HSF). The population in each generation is composed of plants each belonging to a maternal half-sib family. A one-way analyses of variance can be used to estimate narrow-sense heritability from h2 (HSF) = 4c$/c$

Table 3. Estimates of broad-sense heritability for 6-month yields of gerbera grown at Davis and 12-month yields grown at San Joi.

Experiment Contrast Degrees of of freedom

Components of Heritability ( x 100) variance’ (Hz)

Davis, Between clones 25 15.64 44.59 1972-73 Within clones 26 19.44

Davis, Between clones 24 1.50 15.39 1913-74 Within clones 25 8.28

San Jo&, Between clones 25 646.05 66.33 1974-75 Within clones 75 327.96

’ Note that these values are components of variances, not mean squares.

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Table 4. Between and within family analysis and heritability estimates based on sets of half-sib families for flower yield in gerberas.

Year Between families d.f. M.S.

Within families d.f. MS.

Variance components Heritability ( x 100) 4 0,. [h’(HsF)l

1971-72 20 64.02 144 43.50 2.565 43.500 22.24 1972-73 36 136.59 148 59.82 15.355 59.822 81.69 1973-74 48 102.20 147 36.90 16.324 36.324 100.00

where 0: is the component of variance between half sib families and 0: is the total phenotypic variance (e.g. FALCONER, 1960).

The results of the analyses of variance and estimates of heritability for each of the three generations appear in Table 4. Estimates of heritability are 22 percent for generation 1, 82 percent for generation 2, and 100 percent for generation 3; the arithmetic mean is 68 percent.

Estimation of narrow-sense heritabilities (PO). The covariance between parents and their offspring can also be used to estimate narrow-sense heritabilities. Half of the additive genetic variance is expressed in the covariance between one parent, in this case the maternal parent, and its offspring. Thus, heritability can be estimated from the least squares linear slope coefficient, /3, h2 (PO) = 28 (see FALCONER, 1960). The results of the regression analysis of offspring and single parent for each generation are illustrated in Fig. 3. In the first generation the parents are the original G stocks evaluated in 2 growing seasons. Consequently, the independent variable is actually a mean. For generation 1, the correlation coefficient r = + 0.348 approached statistical significance (P = 0.06); for generations 2 and 3, the correlation coefficients were r = + 0.282 and r = + 0.285, respectively (both were statistically significant, the first at P < 0.05, and the second at P < 0.0 1). The estimates ofnarrow-senseheritability are 6559 and 58 percent.

Inbreeding depression. In order to increase levels of inbreeding, 21 of the clones (P’s) from generation 1 were self-pollinated. The flower yields of the selfed offspring can be compared with flower yields of outcrossed offspring from the same 21 parents. The results are illustratedin Fig. 4. The reduction in flower yield is 6.53 flowers, or about 38 percent (statistically significant at. 0.01). The variance among selfs is also reduced significantly and this difference was taken into account in the t-test. Of the 105 outcrossed plants graphed in Fig. 4, there were no mortalities; the estimated viability is 100 percent. Only 2 of the 28 selfs died prior to flowering; their viability was about 93 percent.

Thus, over all inbreeding depression is 7 percent for viability and 38 percent for- flower yield. If it is assumed that fitness components combine in a multiplicative manner, (0.62 x 0.93 = 0.58), the overall reduction in fitness due to one generation of selfing is about 42 percent.

For perennial species, there can be another expression of inbreeding depression. The

Euph.vtica 30 (1981) 319

I. HARDING, T. G. BYRNE AND R. L. NELSON

Fig. 3. Scatter diagrams and least squares regression lines for bivariate samples of parent and offspring means for flower yields for 3 transition generations grown at Davis.

Fig. 4. Distribution of 6-month flower yields for selfed and 1 9 II 25 33 41 19 randomly outcrossed selections grbwn at Davis. I--HONTH YIELD

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SELECTlON FOR FLOWER YIELD IN GERBERA

duration of time that a stock genotype can be successfully cloned may vary from just a few years to almost indefinitely. An attempt was made to clone 41 selfs and 38 of their outcrossed half-sibs. Of the 41 selfed clones, only 31 plants survived to flower in the second year. Therefore, important effects of inbreeding depression may be expressed after the first year of growth.

DISCUSSION

Different approaches to the estimation of heritability for flower yield in the Davis population of gerbera have resulted in somewhat different conclusions. Estimates of broad sense heritability averaged 42 percent in experiments conducted at Davis and San Jose. Surprisingly, narrow-sense heritability estimates for flower yield were greater, about 6 1 percent using the parent-offspring method and 68 percent using the half- sib family method. And finally, the mean realized heritability over three generations was 16 percent. How can these differences be explained? In the first place, it is not unusual to find variable estimates of heritability. MAURER & and HORN (1967) report an estimate of broad-sense heritability for annual gerbera flower production of 61 percent, while an estimate of 30 percent is reported by BORGHI & BALDI (1970).

It appears that lower estimates in the present study are associated with analyses where fewer flowers were produced. The estimate based on annual yield at San Jose was 66 percent, in very close agreement, with MAURER & HORN'S (1967) estimate. Another reason broad-sense heritability estimates were lower in other studies is that samples were not random. Only the relatively low yielding original parents were included, thus reducing the variance between genotypes. In the San Jose, trial high yielding selections were also included. This emphasizes the fact that heritabilities are functions of the genetic material being selected. The San Jose estimate seems to be the most representative for the Davis population. However, the broad-sense heritability estimate of 66 percent for San Jose is still no larger than the narrow-sense estimates using either parent-offspring or half-sib family methods of analysis. This suggests that nearly all of the genotypic variance is additive, and therefore selectable, in agreement with the result of MUCENIECE et al. (1978) for alkemade types in the USSR.

There are two lines of evidence that argue against this conclusion. First, the gain under selection is much less than expected with such high heritabilities. The estimates of realized heritability in this study average only 16 percent. However, evidence indicated that realized heritabilities were reduced as a result of generally low flower yields in the last growing season. The second argument against such high heritabilities seems irrefutable. Evidence has been presented that considerable inbreeding depression results from selling. This is not expected under an additive model, but suggests, to the contrary, that substantial dominance variance exists for flower yield. We cannot, therefore, assume that all of the genotypic variance is additive. It appears that (1) broad-sense heritabilities for flower yield are very high, if based on a large and variable sample of genotypes, (2) much of this genotypic variance is additive variance, but a considerable amount is also dominance variance, and (3) gains from generation to generation fluctuate widely as a result of year to year environmental influences on flower yield. The estimates do not suggest that heritabilities for flower yield are as high as the 81 percent heritability estimates reported for disease resistance in gerbera (Sparnaaij, et al., 1975).

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There has been considerable interest in the development of inbred lines of gerbera from which F, hybrids can be developed, e.g. the hybrids developed at the Station d’AmClioration des Plantes Florales (France). Since inbreeding depression has made this difficult (L. Cartes*, pers. comm.), milder forms of inbreeding than self-fertilization may be desirable (BUTTENSCH~N, 1971). HONDELMAN (1971) suggests development of inbred lines by sib crossing. If the inbreds are homozygous for specified flower color loci, they can be crossed to a non-inbred productive stock that is recessive for the specified flower color loci. Such a method avoids the problem,of low seed production characteristic of inbred lines. SHIVA (1976) reports the initiation of experiments at Sanremo designed to combine inbreeding and recurrent selection in gerbera in order to gradually eliminate the deleterious alleles responsible for inbreeding depression. The results presented here corroborate the existence of inbreeding depression and make an attempt at quantification. A single generation of self-fertilization resulted in a mean inbreeding depression of about 42 percent. However, among the 23 different genotypes selfed, there was considerable variation. Some inbreds died before they flowered, but a few appeared quite normal. One in particular, S-169, was so vigorous that it was included in the yield evaluation at San Jose. Its flower yield, although below the average of 62.5 flowers/year, was actually higher than one of the original parents (G-12). This evidence supports the assumption upon which SHIVA’S (1976) proposal is based.

ACKNOWLEDGEMENT

The authors wish to thank an anonymous reviewer for many helpful suggestions.

REFERENCES

BORGHI, B. & V. BALDI, 1970. Variabilita tra cloni di gerbera allevati in diverse condizioni ambientali. Sementi elette 6.

BUTTENSCH~N, H., 1971. Probleme der Hybridziichtung bei Gerbera. Eucarpia Meeting on Ornamentals: 99-104.

DRRNNAN, D., R. W. HODGSON & J. HARDING, 1980. Methods for selecting flower quality based on consumer evaluation. Euphytica, 30: 64-651.

FALCONER, D. S., 1960. Introduction to quantitative genetics. Oliver and Boyd, Edinburgh, 365 pp. FISHER, R. A., 1930. The genetical theory of natural selection. Dover, New York, 291 pp. HONDELMAN, W., 1971. Gerbera-ZiichtunginderForschungstellevonSenghbuschGmbH. EucarpiaMeeting

on Ornamentals : 122-l 32. MAURER, J. & W. HORN, 1967. Ergebnisse genetisch-ziichterischer Untersuchungen bei Gerbera. Gartenwelt

67: 63-64. MUCENIECE, G. YA., I. D. RASALS V. YA. DISLERS, 1978. Investigation of the inheritance of quantitative

characteristics of gerbera in diallel crosses I. Productivity of plants. Genetica 14: 251-253. SHIVA, T., 1976. Gerbera breeding - Preliminary evaluations of genotypes for improved populations of

production. Acta. Hort. 63: 177-185. SMITH, D. E. & R. L. NELSON, 1967. Gerbera production. Calif. Agric. 21(12): 7. SPARNAAIJ, L. D., F. GARRETSEN & W. BEKKER, 197.5. Additive inheritance of resistance to Phytophtora

cryptogea F%THYBRIDCE and LAFFERTY in Gerbera jamesonii BOLUS. Euphytica 24: 551-556.

* Pan American Seed Co., Bradenton, FL 33505, USA

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estimation of heritability and response to selection for cut-flower yield in gerbera

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