Hereditas 139: 217–222 (2003)

Genetic parameters for agronomic characteristics. II. Intermediateand advanced stages in a true potato seed breeding populationRODOMIRO ORTIZ and ALI M. GOLMIRZAIE

Centro Internacional de la Papa, Lima, Peru

Ortiz, R. and Golmirzaie, A. M. 2003. Genetic parameters for agronomic characteristics. II. Intermediate and advancedstages in a true potato seed breeding population. — Hereditas 139: 217–222. Lund, Swden. ISSN 0018-0661. ReceivedApril 28, 2003. Accepted November 4, 2003.

The aim of this research was to determine the genetic variation available in some important characteristics for true potatoseed breeding in intermediate and advanced stages of a breeding population developed by the Centro Internacional de laPapa. A factorial mating design was used to calculate variance components and heritability at both selection stages. Fourmales were crossed with four females within each set (4 for intermediate stage and 5 for advanced stage) and theirresulting offspring tested across two contrasting locations. Tuber yield and set were the only common characteristicsrecorded in both selection stages. In the intermediate stage early development characteristics (seed germination plus rootand internode length) were also measured. In the advanced breeding material, vine earliness and other reproductive traits(days to flowering, flowering intensity, style length and pollen production) were scored. The heritability for tuber yield(0.35) and tuber set (0.32) in the advanced selection stage was higher than in the intermediate stage (0.26 and 0.13respectively), which suggest that recombination through more cycles of recurrent selection brought untapped variation forboth characteristics in this breeding material. Significant additive genetic variation, and thereby high heritability, wasobserved for internode length (0.52) in the intermediate selection stage, and for days to flowering (0.53) in the advancestage. Progress through selection may be expected for both characteristics. The heritability for pollen production wasintermediate (0.23), while it was low (i.e.�0.10) for the other characteristics recorded in both populations.

Rodomiro Ortiz, International Institute of Tropical Agriculture, c/o L.W. Lambourn & Co., Carolyn House, 26 DingwallRoad, Croydon, CR9 3EE, England, UK. E-mail: r.ortiz@cgiar.org

True potato seed (TPS) may be an option in locationswhere pests impede farmers to access local pathogen-free potato propagules or costs are high for expensiveimported tubers (ORTIZ 1997; SIMMONDS 1997). Fur-thermore a few hundred grams of TPS can replace 2t of tubers for planting 1 ha. However, TPS mayrequire some extra labor (nursery care and trans-planting) for growing seedlings and longer time (15–20 extra days) for tuber maturation. Selecting forgenotypes showing the many desired traits of TPScultivars also requires screening of large populationsizes.

A breeding program requires a limited number oftargeted traits and their grouping into primary andsecondary goals for selection. The success for improv-ing sustainability in the crop regarding these traitswill also depend on the genetic variance and the meanin the source population (BOS and CALIGARI 1995).There is a rising debate concerning genetic erosion inthe breeding populations but facts are difficult toprovide, particularly when well-trained professionalbreeders use selection schemes that keep enough ge-netic variation in their breeding materials, becausethey are aware of the benefits for such an approach.Thus, this research investigated the heritability fora few traits (in early and advance plant growth)

relevant to the genetic improvement of potato inintermediate and advanced stages of selection in abreeding population propagated by true seed, whichwas developed for broad adaptation to the tropics bythe Centro Internacional de la Papa (CIP, Lima,Peru).

MATERIAL AND METHODS

The breeding populations developed at CIP for thelowland tropics provide the source material for ge-netic improvement of TPS at this Institute. The initialbreeding gene pool included commercial cultivarsfrom group Tuberosum, a few Andigena landraces,breeding clones from Neotuberosum (Andigena pop-ulation selected under long-days) and others derivedthrough backcrosses from hexaploid S. demissum,and some wild and cultivated diploid species –mostly S. chacoense, S. microdontum, S. sparsipilum,S. phureja and S. stenotomum (GOLMIRZAIE et al.1991). Most of the breeding work was undertaken atSan Ramon (11o08�S, 800 m, humid mid-altitude onthe eastern Andes slopes with temperatures rangingon average from 18.6 to 31.2°C and on average totalrainfall of 1400 mm for the 6-month rainy season and400 mm for the other 6-month drier season). The best

R. Ortiz and A. M. Golmirzaie218 Hereditas 139 (2003)

clones selected at this location were the further testedat Yurimaguas – a warmer Peruvian environment(5o41�S, 180 m, hot lowlands in the Amazonas junglewith temperatures ranging on average from 20.6 to31.3°C and on average total rainfall of 400 mm forthe very short 3-month cropping season). Selectedclones, at both San Ramon and Yurimaguas, werethe initial source for CIP TPS breeding populationthat was further broadened by introducing mostlyTuberosum cultivars from Europe, North Americaand a few selections from Argentina, Brazil andSouth Asia. Thereafter, recurrent selection methodswere used to advance the TPS breeding population atCIP. This report includes two breeding cycle stages:intermediate (i.e. breeding cycle ensuing after crossesbetween selected clones from CIP lowland breedingpopulation and foreign Tuberosum cultivars orbreeding clones), and advanced, second cycle of re-current selection, ensuing from the intermediatebreeding cycle stage.

The two breeding cycles included in this investiga-tion were grown at contrasting Peruvian locations: LaMolina (12o05�S, 240 m, coastal desert with tempera-tures ranging on average from 16.1 to 21.6°C andalmost nil rainfall (5 mm for the 6-month wintercropping season), which needed therefore supplemen-tary sprinkler irrigation), Huancayo (12o07�S, 3280 m,highlands with 21.8°C as average temperatures and662 mm rainfall for the 6–7 month cropping season)and San Ramon. For the intermediate breeding cyclestage, 16 random male clones were crossed with 16random female clones in four sets of four parents (i.e.64 full-sib hybrid offspring) each following a designII or a factorial mating approach (COMSTOCK andROBINSON 1952). For the advanced breeding cyclestage another set was added; i.e. 80 full-sib hybridoffspring.

In all experiments, true potato seed (TPS) of eachoffspring were planted in flats at a greenhouse nurs-ery. Before sowing, 200 seeds were soaked in a solu-tion of 1500 ppm giberellic acid for 24 h, andthereafter all seeds were sown in plastic trays contain-ing a 1 sand: 2 peat: 1 soil (by volume) substrate.When the seedlings were 3 to 5 cm high, they weretransferred into peat pots and grown to about 15 cmtall, when 40 full-sib seedlings were transferred aftereight weeks to single-row plots in the field. Theexperimental field layout was always a randomizedblock design with a maximum of three replications.Each replication had rows spaced at 0.75 m and 0.30m between plants within the same row. The daybefore transplanting to the field a sample of 20full-sib seedlings were measured for root and in-ternode length in the intermediate breeding cycle.

Because a large number of offspring was sampledfrom the breeding population, half-sib offspring wereincluded in the same set. The common charactersrecorded in both breeding cycles were tuber yield perplant (kg) and tuber set (number of tubers per plant).In the intermediate breeding cycle there was an inter-est to correlate seedling traits and tuber characters,hence, seed germination (%), root length (cm) andinternode length (cm) were measured with the aid ofa caliper. Plant survival (%), and using a 1-(worst) to9-scale (best) other characters such as vine earliness,days to flowering, style length and pollen productionwere scored in the advance breeding stage, in whichthe breeders are more interested in reproductive traitsassociated to TPS productivity per se.

The analyses of variance for factorial (design II)and mating design combined across environmentsand sets followed ORTIZ and GOLMIRZAIE (2002).Because different sets of parents were used as malesand females the analysis of variance for each designincluded a source of variation due to sets. However,the expectations of the mean squares of males, fe-males, and their interaction are the same for each set.

RESULTS

The coefficient of variation (CV, %) for the combinedanalysis of variance for the intermediate breedingcycle was low for seed germination and root length,indicating that both characters were measured accu-rately (Table 1). The CV was intermediate for in-ternode length and tuber set and slightly higher fortuber yield per plant (a highly variable character thatneeds reps for a precise assessment). Locations wereindeed contrasting environments as noted by therespective significant source of variation for all traits,but tuber set, in this intermediate breeding cyclestage. Tuber yield at la Molina, the cooler environ-ment, was more than 2-fold larger than at SanRamon.

Narrow-sense heritability was low for seed germi-nation and root length (h2�0.10) in this intermediatebreeding cycle (Table 2). A low additive genetic vari-ation viz. a viz. the non-additive genetic variation(almost twice larger), and a significantly larger (al-most thrice) additive genetic variation× location in-teraction accounts for low h2 in seed germination,which was already high – on average, in the breedingpopulation (�90 %). Similar results were reported inan advanced cycle of selection in a NeotuberosumTPS breeding population (THOMPSON et al. 1983).

Though the non-additive genetic variation was nilfor root length, the significant additive genetic varia-tion× location interaction led to a low h2 for thischaracter and for tuber set; i.e. the interaction was

Genetic parameters for agronomic characteristics. II 219Hereditas 139 (2003)

Table 1. Analysis of �ariance of design II in an intermediate true potato seed population pooled o�er sets anden�ironments (San Ramon and La Molina).

SeedSource of variation Root lengthDegrees of Internode Tuber set Yield plant−1

freedom lengthgermination

%+ cm cm c kg1 71040.96** 9500.26** 261.69* 12.40 17.98**Locations (L)

3525.51 455.68 28.053 30.80Sets (S) 0.1941219.09** 69.37 16.18 75.53S×L 0.7363

92.41 66.66 8.5516 23.63 0.032Replications/S×L1140.16 27.74Males (M)/S 35.72* 52.15 0.445121753.25 82.69 31.8412 35.51Female (F)/S 0.091

36M×F/S 492.40 33.18 9.35 8.32 0.03812M/S×L 809.38 31.91 10.04 58.01** 0.230

1235.92 106.59* 22.98*12 22.43* 0.067F/S×LM×F/S×L 36 405.55 43.68 9.79 10.30 0.021

59.46 30.08 9.94240 8.14Pooled error 0.0199.46 10.17 21.67Coefficient of variation (%) 20.25 24.47

* and ** indicate significant at 5 % and 1 % respectively.+ after logarithmic transformation.

Table 2. Variance components, genetic �ariances and heritability (h2) of agronomic and tuber characteristics in anintermediate true potato seed population (San Ramon and La Molina).

Root length Internode length Tuber setItem Yield plant−1Seed germination

% cm cm c kg10.1637 0.2637 1.0883 0 (=−) 0.0082Males (M)/S

0 (=−) 0.3875 0.627517.9367 0.0003Female (F)/S0 (=−) 0 (=−) 0 (=−)M×F/S 0.002814.47500 (=−) 0.0208 3.975833.6525 0.01742M/S×L

69.1975M/S×L 5.2425 1.0992 1.0108 0.0038115.3633M×F/S×L 4.5333 0 (=−) 0.7200 0.0007

0.5275 2.9517 1.255046.5508 0.0152�2A

86.8500 0 0 0 0.0170�2D

7.4628 2.2400 9.4933128.7911 0.0421�2AL

692.1800�2DL 27.2000 0 4.3200 0.0040

0.0231h2 0.5153 0.1319 0.26020.0840

�2A=additive variance; �2

D=non-additive variance; �2AL=additive-by-location variance; �2

DL=non-additive-by-locationvariance.

almost 15-fold and 8-fold larger than the additivegenetic variation for each character, respectively.Hence, additive genetic variation though importantfor both root length and tuber set, needs to capitalizeon selection within each contrasting environment formaking breeding gains, particularly if indirect selec-tion for tuber yield through tuber set will be under-taken as suggested earlier by THOMPSON andMENDOZA (1984). Tuber yield per plant showed anintermediate h2 in this intermediate breeding cyclebecause both additive and non-additive genetic varia-tion were almost the same, and the additive geneticvariation× location was almost 3-fold larger for thischaracter in this intermediate breeding cycle.

The high h2 for internode length, in which thenon-additive genetic variation was nil, suggests that

recurrent selection will improve and sustain steadygenetic gains for this character. Multiple correla-tion analysis showed however, that there was verylow and non-significant association between earlystages of growth and agronomic characteristics(GOLMIRZAIE and SERQUEN 1992). Hence, early se-lection for growth characters such as root or in-ternode length as well as seed germination wasdiscarded for TPS breeding.

The coefficients of variation were high for the fieldexperiment in the advance breeding stage; only theCV for days to flowering was low and intermediatefor flowering intensity and style length (Table 3). Thelocations for testing this more advance breeding cyclewere indeed contrasting for all traits but plant sur-vival after transplanting. Tuber yield and set har-

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Table 3. Analysis of �ariance of design II in an ad�anced true potato seed population combined o�er sets and en�ironments (San Ramon and Huancayo).

Vine earlinessPlant survival Yield plant−1 Tuber set Days to Flowering Style length PollenDegrees ofSource ofvariation intensityfloweringfreedom production

(1–9 scale; 1=worst; 9=best)ckgc367.50** 45571.52** 169.22**Locations (L) 4.22** 21.68**1 28397.63 93.98** 35620.22**

5.20 242.97 43.22** 4.38** 7.05*933.06**0.78**Sets (S) 4 354.740.92 16.73 29.32** 0.29 3.28S×L 4 411.94 0.17 152.832.72 7.84 1.97 0.55 1.7757.560.0544.7720R/S×L

0.04 257.58 3.51 27.25 5.98 0.56 1.8615Males (M)/S 59.173.40 102.21 8.53 1.31 4.79*142.950.12100.2315Females (F)/S1.79 8.20 1.77 0.39 1.46M×F/S 45 39.09 0.03 43.361.79 23.52 4.51** 0.68 1.80128.20**0.0547.8415M/S×L

106.05**F/S×L 1.90 19.76 6.01** 1.16* 2.0615 72.35 0.06*1.19 6.69 1.07 0.48 1.2141.48M×F/S×L 0.0331.5445

31.99Pooled error 1.15 5.38 1.02 0.37 1.17300 17.34 0.0314.6 22.12 27.25 28.25 4.84 17.00 13.5 30.3Coefficient of

variation (%)

* and ** indicate significant at 5 % and 1 % respectively.

Table 4. Variance components, genetic �ariances and heritability (h2) of agronomic and tuber characteristics in an ad�anced true potato seed populationtrue potato seed population (San Ramon and Huancayo).

Source of Pollen productionPlant survival Yield plant−1 Tuber set Vine earliness Days to flowering Flowering intensity Style lengthvariation

c kg c (1–9 scale; 1=worst; 7=best)

Males (M)/S 0.1575 0 (=−) 5.3125 0.0467 0.0925 0.0321 0 (=−) 0 (=−)Females (F)/S 0.8471 0.0025 1.4592 0.0375 3.3725 0.0758 0.0100 0.1033M×F/S 1.2583 0 0.3133 0.1000 0.2517 0.1167 0 (=−) 0.0417M/S×L 1.3583 0.0017 7.2267 0.0500 1.4025 0.2867 0.0167 0.0492F/S×L 3.4008 0.0025 5.3808 0.0592 1.0892 0.4167 0.0567 0.0708M×F/S×L 4.7333 0 3.1633 0.0133 0.4367 0.0167 0.0367 0.0133

�2A 1.1703 0.0050 13.3344 0.1017 6.7622 0.1381 0.0200 0.1789

�2NA 7.5500 0 1.8800 0.6000 1.5100 0.7000 0 0.2500

�2AL 6.3628 0.0083 23.1061 0.2094 4.6922 1.3856 0.1222 0.2311

�2NAL 28.4000 0 18.9800 0.0800 2.6200 0.1000 0.2200 0.0800

h2 0.0404 0.3529 0.3206 0.0978 0.5273 0.0789 0.0791 0.2295

�2A=additive variance; �2

D=non-additive variance; �2AL=additive-by-location variance; �2

DL=non-additive-by-location variance.

Genetic parameters for agronomic characteristics. II 221Hereditas 139 (2003)

vested at Huancayo were higher than at San Ramon,a location where tuberization can be affected bywarm temperatures.

Heritability was low (h2�0.10) for plant survivalafter transplanting (a trait with an outstanding per-formance in both locations, style length and floweringintensity) and average to high in both environments,and vine earliness towards late maturing (Table 4).For these characters the additive genetic variation×location interaction was higher (2- to 10-fold) thanthe additive genetic variation per se. The non-additivegenetic variation was nil for style length and 5-foldhigher than additive genetic variation for vine earli-ness and flowering intensity. The results indicated thesignificant influence of this kind of genetic variationfor both characters in this advanced breeding cycle.Therefore, new sources of variation may be needed tobroaden the genetic diversity for improving furthersome of these characteristics showing a low heritabil-ity, in this breeding population.

Rapid progress may be achieved for days to flower-ing by selecting early flowering individuals in thisadvanced breeding cycle and for their further use asTPS parents. Indeed they can transmit this desiredattribute easily to their offspring as suggested by theh2 for this character, for which the additive geneticvariation was the highest among all. Pollen produc-tion showed an intermediate h2 because of the almostequal size for additive and non-additive genetic varia-tion and the additive genetic variation× locationinteraction.

The h2 was slightly higher for both tuber charactersin which the non-additive genetic variation was nil(tuber yield) or about 6.5-fold smaller than the addi-tive genetic variation (tuber set), though the additivegenetic variation× location interaction was thehighest (almost two-fold viz. a viz. additive geneticvariation) for tuber characters, corroborating the re-sults observed in the intermediate breeding cycle forsame characters. However, the h2 for tuber traits wassmaller than that reported in a heterogeneous TPSbreeding population (h2�0.6) by THOMPSON andMENDOZA (1984), who calculated the additive geneticvariation as four times the covariance of half-sibs,which can be inflated by some fraction of the digenicvariation included in the pooled variance among par-ents (WRICKE and WEBER 1986). The distinct h2 fortuber characters between these reports may be alsoascribed to gamete phase disequilibrium between locibecause this will not be completely removed after onegeneration of random mating in a tetrasomic poly-ploid, which leads to a temporary reduction of thegenetic variability.

DISCUSSION

The h2 for tuber characters was higher in the ad-vanced breeding cycle than in the intermediate breed-ing cycle. This result indicated that recurrent selectionwith progeny testing does not lead to genetic erosionin TPS breeding and that recombination in advancecycles of selection can tap more variation for bothcharacteristics in this breeding material. Such an ap-proach can be further improved by determining het-erotic groups to subdivide the still variable andheterogeneous TPS breeding population originatingfrom a wide genetic background. Reciprocal recur-rent selection can assist in this endeavor to keepdistinct sub-populations completely separate. Theparental value of clones from each sub-populationmay be easily assessed by using clones from the othersub-populations as testers. In this way, TPS breederscan identify the best clones in each sub-population tocontinue improving genetically within populations bycrossing the selected clones. Best clones can be usedfor producing new inter-population hybrid TPS culti-vars ensuing from intermating selected clones thatshow superior combining ability after populationcrosses. This breeding approach capitalizes not onlyon additive genetic variation but also digenic geneticvariation in tetrasomic polyploid potato, because, asindicated by ORTIZ and GOLMIRZAIE (2002), gametesof this species –due to tetrasomic inheritance, trans-mit a small fraction of these interaction effects totheir offspring.

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