GENETIC PARAMETERS AND GAIN EXPECTED FROM DIRECT SELECTION FOR RESISTANCE TO SYLVESTRELLA RATZ. (: ) IN PINUS PZNASTER AIT., USING A FULL DIALLEL MATING DESIGN

M. Kleinhentz', A. RaffinZ& H. Jactel'

" Laboratoire d'Entomologie forestikre, INRA, BP 45, F-33611 Gazinet Cedex, France ''. Laboratoire d'Am6lioration et GCnCtique des Arbres Forestiers, INRA, BP 45, F-33611 Gazinet Cedex, France

Received October 27, 1997; accepted June 26, 1998

ABSTRACT

A complete diallel progeny test of Maritime from the Landes provenance was carried out on 14- and 16- year-old trees (1994,1996)for sensitivity to the stem borer () and tree diameter, using a full diallel mating design. The family effect was significant and the genetic contribution of the parents almost entirely additive for both resistance and diameter. Heritability estimates for resistance in 14- and 16-year-old pine were 0.08 and 0.09 respectively. Strong differences among families for resistance was shown by a high coefficient of variation (2.32 and 1.68 for DS94 and DS96 respectively). Tree susceptibility was positively and genetically correlated with tree diameter. Expected genetic gains for tree resistance and correlative genetic loss for tree diameter growth were estimated. Age and pruning effects on tree sensitivity were included to define proper time for selecting resistant genotypes.The possibility of integrating D. s~~lvestrellaresistance in the Maritime pine breeding program is discussed.

Key-words , Dioryctria sylvestrella, diallel, selection, resistance, genetic parameters

INTRODUCTION BERT 1984). In France, since the 1960s, Maritime pine has been selected for total height (h2 - 0.4), stem Maritime pine, Pims pinaster Ait., is the primary straightness (h2 - 0.2), wood density (h2 - 0.6), fine- coniferous species in France (MERZEAU1995) in terms ness of the branches (h2 - 0.2) and absence of forks (h2 of surface area (1.4 million ha) and harvest yield (5 - 0.1) in a recurrent breeding program, which has led millions m3 per year). It is also one of the most inten- to the creation of improved varieties (Baradat and sively managed commercial species. JACTEL et al. Pastuszka 1992). Selection for stem borer resistance (1994,1996a) demonstrated that forestry practices has been included occasionally in this program. Only which favour tree vigour, such as fertilizing, intensive trees that have shown a high level of stem borer attack thinning and low-density planting, or tree wounding, as have been eliminated in mass selection. induced by pruning, may in turn increase the damage by This paper aims to examine the possibility of Dioiytria sylvestrella, one of the major maritime pine integrating D. sylvestrella resistance in the Maritime pests (MENASSIEU& LEVIEUX1990; CARISEYet al.. pine breeding program at the Forestry Research Station 1994). A host-selection model for the stem borer has of INRA Bordeaux. The genetic parameters of resis- been proposed: susceptible trees would develop a tance (i.e combining ability, reciprocal effects, additive primary attraction to the stem borer, mediated by some and dominance variances, heritability, and gain predic- volatile attractants that originate from the constitutive tion) were estimated, using a diallel mating design, oleoresin exuding from the pruning wounds or bark since it provided excellent estimates of combining cracks (JACTELet al. 1996a). ability, reciprocal effects and expected gains ( FINS et The high cost of intensive management and the al. 1992). This study also investigated the relationships, potential value of wood yield and quality dictate that at the genetic level, between stem borer attack and tree stem borer control should be of high priority. Since diameter, and assessed the effect on wood production chemical control is impossible - the is hidden when selecting for the stem borer resistance. All under the bark for more than 11 months a year -, the parameters were estimated at two different tree ages in selection of pest-resistant trees would appear to be a pruned and unpruned trees, in order to assess the effect promising strategy. Actually, when combined with of age and wounding on the expression of tree suscepti- good forest management, breeding for resistance to bility and consequently to determine the appropriate has proven to be very feasible (ZOBEL& TAL- age for selection.

O ARBORA PUBLISHERS M. KLEINHENTZ ETAL. : GENETICPARAMETERS OF RESISTANCE TO DIORYCTRIASYLVESTRELLA

MATERIALS AND METHODS where ijk = kthreplicate of the cross between the i"' female and jthmale; Sij=S,,, IJ = general mean level, 1, Plant material and measurements (A,) = general combining ability (GCA) of the irh(fh) parent, S,, = specific combining ability (SCA) of the Data were obtained from a Maritime pine full diallel cross between the ilh and jLhparent, 6, (6, ) = general cross (including both reciprocal and selfed crosses) reciprocal effect (GRE) due to the ithfemale(jth male), between twelve parents. Twenty nine combinations ri, = specific reciprocal effect (SRE) due to the cross were missing. The 12 parents were "plus trees" pheno- between the irhfemale and thejhmale, e,, = error term. typically selected for stem growth and straightness in the local provenance of the Landes de Gascogne and Variance components grafted in clonal archives. The diallel was located in Cestas (Gironde, France) Additive and dominance variances are: on a semi-humid podzolic soil. It was established in the 2 2 2 2 autumn of 1982, with 2-year-old seedlings. Spacing oA = 4oAe and o, = 40,,, was 4m between rows and l.lm between individual trees ie 2272 trees.ha-'. The phenotypic variance is: The experimental design consisted of 74 incomplete randomized blocks. Each block comprised 16 plots with 2 to 4 trees. Each family was repeated 4 to 15 times. Genetic and environmental correlations In October 1994, 3618 trees were sampled for D. sylvestrella infestation and diameter at breast height Genetic and environmental correlations were carried (1.30m). In May 1996, one tree per plot giving a total out with OPEP using a multicharacter analysis of of 1006 trees was pruned to increase tree sensitivity. variance and covariance. The pruning consisted of the removal of two live whorls. In October 1996, all the 3618 trees were Coefficient of genetic prediction (CGP) arld relative sampled again for D. sylvestrella infestation. genetic gain estimates

Data analysis and estimation of genetic parameters The coefficient of genetic prediction (CGP) (BARADAT 1976) was computed as a generalization of the heri- For each plot (2 to 4 trees), the mean diameter (DBH) tability concept (GALLAIS1973). CGP and relative and the percentage of attacked trees (DS94 and DS96 genetic gain estimates were calculated according to the in 1994 and 1996, respectively) were calculated. All formulae given by BARADATand DESPREZ-LOUSTAU plot percentage values were transformed by the arc sin (1997). Three different types of selection were simu- square root transformation (DAGNELIE1973) and the lated: mass selection, selection on progeny test (selec- statistical analyses were performed on both percentage tion on AGC) which is currently used to create new and transformed values. Only analyses based on varieties of Maritime pine and individual-family percentage are presented as the conclusions were combined selection which is used to manage the similar. breeding population. Analysis of variance for block and family effects The simulations were carried out using 50 individu- was carried out with the OPEP software (BARADAT als per family. Two selection intensities were used: 5% 1989; BARADAT& LABBE1995) derived from 'Hender- and 50% corresponding respectively to the rates used in son 111' model (SEARLE1971). Data were adjusted for new variety creation and in population management. the block effect, prior to the decomposition of family effect (diallel analysis). RESULTS

Diallel analysis In 1994, 10.5% of the trees were infested. Two years later and after pruning, 16.8% of the trees were in- Analysis of the diallel (selfed combinations were not fested. considered) was carried out with OPEP using the Analysis of variances indicated highly significant random model given by GARRETSEN&KEULS (1977). variation among families for both resistance and The random model is: diameter (Table I), which justifies the following genetic analyses. As the block effect was highly signifi- cant in all of the traits, data were adjusted in further calculations. Due to sampling methods, age and prun- ing effects could not be separated. Table 1. Analysis of variance for the percentage of infested trees in 1994 and 1996 (DS94 and DS96) and the diameter (DBH) in the diallel cross of Maritime pine.

Mean square F test Source Prob (5%)

Family (random)

Block (random)

Error

Table 2. Analysis of the genetic effects for the percentage of infested trees in 1994 and 1996 (DS94 and DS96) and the diameter (DBH) in the diallel cross of Maritime pine.

Traits Effect DS 94 DS 96 DBH

Mean 10.5 % 16.8 7% 12.0 cm

F 5.64 7.20 16.55 GCA Prob (9%) (0.001) (< i 0-l) (< 1o-') General combining ability % of phenotjpic variance 1.96 2.27 3.36

F 0.62 0.58 0.45 SCA Prob (% (93.8) (96.0) (99.5) Specific combining ability g%, of phenotypic variance 0 0 0

F 0.89 1.1 1 2.52 GRE Prob (%) (55.7) (38.2) (2.26) General reciprocal effects % of phenotypic variance 0 0.1 1 1.52

F 1.85 I .65 2.10 SRE Prob (%) Specific reciprocal effects (0.28) (1.30) (0.04) % of phenoiypic variance 43.41 37.26 50.29

Genetic parameters of Maritime pine resistance to less that 4% of the total phenotypic variance whilst the D. sylvestrella: relationship with diameter reciprocal components (GRE+SRE) accounted for 37.4% (DS96) and 51 2% (DBH) respectively. Recipro- The study showed a complete lack of significance of cal effects were included in the phenotypic variance, as specific combining ability effects (SCA) for both tree components of the environmental variance, before the diameter and resistance to D. sylvestrella ( DS94 and calculation of heritabilities and coefficient of genetic DS96) coupled with consistent highly significant prediction. general combining ability effects (GCA) (Table 2) The heritability estimate for DBH was moderate indicating strict additive effects for both characters. The (h2,,=0.13) and estimates for Maritime pine resistance general reciprocal effects (GRE) were only significant to D. sylvestrella were slightly lower: 0.08 and 0.09 for for DBH. The specific reciprocal effects were signifi- DS94 and DS96 respectively (Table 3). The CGP cant for all characters: DBH, DS94 and DS96 (Table estimates between DBH and tree sensitivity were 2). GCA and SCA variance component accounted for positive and quite low, 0.06 and 0.08 for CGP (DBH, o ARBORA PUBLISHERS 149 M. KLEINHENTZ ET AL. : GENETICPARAMETERS OF RESISTANCE TO DIORYCTRIASYLVESTRELLA

Table 3. Coefficients of genetic prediction sensu stn'cto. Values in the diagonal represent the heritability. -- DS94 DS96 DBH - DS94 DS96 DBH

Table 4. Additive (upper half) and total (lower half) genetic correlations between diameter and susceptibility to D. sylvestrella in the diallel cross of Maritime pine.

DS94 DS96 DBH

DS94 DS96 DBH

Table 5. Environmental (upper half) and phenotypic (lower half) correlations between diameter and susceptibility to D. sylvestrella in the diallel cross of Maritime pine.

-- DS94 DS96 DBH

DS94 DS96 DBH

DS94) and CGP (DBH, DS96) respectively. The values Because tree infestation by D. sylvestrella was of the phenotypic coefficient of variation were respec- recorded twice, the risk of selecting genotypes in tively 2.32 and 1.68 in DS94 and DS96. families not attacked in 1994 but attacked in 1996 could Tree sensitivity exhibits an important and positive be assessed. To simulate a selection strategy, families genetic correlation with DBH (Table 4), increasing with were arranged first in increasing order of infestation tree age (0.55 and 0.74 for DS94 and DS96 respec- level in 1994 and secondly in decreasing order of mean tively). The environmental correlations between tree diameter (Fig. 2). According to the new level of infesta- resistance and DBH were moderate but positive (Table tion in 1996, the number of families outside the selec- 5). tion interval was calculated. The risk of choosing unattacked genotypes that subsequently reveal their Determination of genetic gains estimates in resis- susceptibility is high when selecting resistant trees in tance to D. sylvestrella. Consequences for tree 1994: respectively 22.4% and 60% of selected families diameter would be misclassified using selection rates of 5% and 50%. Important relative genetic gains can be obtained for Maritime pine resistance to D. sylvestrella (Fig.l). DISCUSSION When using a selection on progeny test (value of the progeny to select the parents) with a selection rate of Heritability estimates 5%, which is the case in producing new varieties, the expected gains could reach 95% based on DS94 crite- The estimate of the Maritime pine diameter heritability ria. Gains would be slightly lower with DS96. However given in the present paper is generally consistent with the associated loss on DBH would be close to 10%. those reported in ? pinaster at the same age by DUREL Using a scenario of breeding population manage- (1990) (h2 = 0.1) and COSTA& DUREL(1996) (h2~0.2), ment (individual-family combined selection with a rate in Pinus radiata, by DEANet al. (1983) (h2=0.23), of 50%), the gains in tree resistance would reach 3 1% BURDONet al. (1983) (h2 0.14) and MATHESON or 25% based on DS94 or DS96 respectively. The = &RAYMOND(1984) (h2 = 0.18), and in Pinus elliottii genetic loss on DBH would then be close to 5%. -20 INDIVIDUAL AND FAMILY MASS SELECTION ~0~61~~~SELECTION SELECTION ON PROGENY TEST

-20 1 MASS SELECTION lNDIVIDUAL AND SELECTION ON PROGENY TEST COMBINED SELECTION Figure 1. Genetic gain estimates (relative 96) expected in Maritime pine resistance to D. sylvestrella and associated genetic loss in tree diameter according to the type and the date of selection using a selection rate of (a) 50% and (b) 5%. Engelm. by DETERS (1996) (1x2 0.2). The low for the resistance could be partly explained by the heritability of diameter may be due to environmental presence of reciprocal effects which contributed up to effects, competition effects and location of resources 37.4% of the phenotypic variance. General reciprocal (LEMOINE1980; VON EULERet al. 1992). For Maritime effects were low with significance only for diameter pine resistance to D. sylvestrella, heritabilities were indicating the absence of maternal effects in the trans- slightly lower than for diameter (h' - 0.09). In a mission of Maritime pine resistance to D. sylvestrella. previous study based on a Maritime pine progeny test Specific reciprocal effects for the three characters were (factorial design) of 11- and 12-year-old trees, heri- highly significant. Their presence should be solved tability estimates of the resistance ranged from 0.07 to before the integration of the resistance to D. sylvestrella 0.09 (SUERON1982). BARADAT& DESPREZ-LOUSTAU in the Maritime pine breeding programme. No obvious (1997) found a heritability estimate of 0.06 for 3-year- explanation of such effects was readily apparent. They old Maritime pine resistance to twisting rust (Melanzp- might result from environmental effects although no sora pinitorqua) on the same diallel mating design than method of adjustment allowed for the elimination of studied here. The heritability estimates of tree resis- these effects. SAMUEL(1991) found the same genetic tance to insects or pathogens are usually low to moder- effects for diameter of 15-year-old Sitka (Picea ate. Individual-tree heriiability for Sitka spruce resis- sitchensis [Bong.] Carr.) in a diallel cross: presence of tance to the white pine weevil was -0.40 (KINGet al. GCA effects, absence of SCA effects, presence of 1997) and CILASet al. (1995) estimated the heritability reciprocal effects but these last effects accounted for a of Theobronza cacao L. resistance to Phytophthora sp. smaller proportion of the total variance. to be 0.13 in a diallel mating design. The heritability of resistance increased slightly with In our study, diameter at breast height and Maritime the age of trees. According to the model of tree infesta- pine resistance to D. sylvestrella at the 14- and 16-year- tion given by JACTELet al. (1996a), pine attractiveness old stage presented highly significant GCA effects could be enhanced by factors which increase the resin associated with an absence of SCA effects, indicating flow. In our study, tree pruning (JACTELet al. 1994) that the genetic variance for these characters is almost and two years growth - which involves bark cracking- entirely additive. The low value of heritability estimates (JACTELet al. 1996a) may have resulted in a higher

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M. KLEINHENTZ ETAL. : GENETICPARAMETERS OF RESISTANCE TO DIORYCTRIASYLVESTRELLA 1 g lnfestatlon level ln 1996 , a 11 I+II 1 lnfestat~onlevel ln 1994 ~ M #inill

Figure 2. Comparison of the infestation level by Dioryctria s~lvestrellain 115 Maritime pine families at two different tree ages (14- and 16-year-old trees). Families were arranged in increasing order of infestation at 14 years and in decreasing order of mean diameter. 5% and 50% correspond to the two selection levels used to simulate a selection strategy. DS94: Percent of attacked trees at 14 years; DS96: Percent of attacked trees at 16 years. expression of the tree sensitivity. However, plus trees are not related, and were initially selected on stem form and volume, excluding severely Relationships between tree diameter and sensitivity attacked trees (ILLY1966). Thus a random association to D.sylvestrella between low resistance and good growth was not favoured. The genetic correlations between tree diameter and Environmental correlations between tree susceptibil- sensitivity to D. sylvestrella, were large and positive ity to D. sylvestrella and tree diameter were positive indicating that faster growing families have a lower and moderate, suggesting that environmental effects on level of genetic resistance. The correlation level in- the expression of the genes controlling these traits were creased with the age of the tree, indicating that the role similar. Environmental conditions are considered to of diameter in host selection increases with age. A largely affect the diameter growth. According to the genetic correlation may have three causes: pleiotropy, host selection model given by JACTELet nl. (1994), tree linkage (that a genetic mapping is about to assess), and vigor favors tree susceptibility to D. sylvestrella. Then, statistical association without genetic significance. environmental conditions which affect tree vigor, might Pleiotropy would be consistent with the host selection indirectly affect tree susceptibility in the same direc- model proposed by JACTELet al. (1994): radial growth tion. and consequently bark cracking and tree attractiveness would depend on the same group of genes. This case Expected gains (and to a Iesser extent linkage) is unfavorable for breeding, as the selection for global tree resistance to In spite of low heritabilities, high relative expected D,sylvestrella would systematically and negatively genetic gains could be obtained when selecting for affect wood production. Statistical association could resistance to D. sylvestrella, due to large phenotypic also be the cause of the genetic correlation. in a popula- coefficients of variation. High gains were also found by tion originating from twelve plus tree parents only: BARADAT& DESPREZ-LOUSTAU (1997) for Maritime genes controlling tree sensitivity and radial growth may pine resistance to twisting rust on the same diallel have been associated at random in those parents. progeny test. The mean expected gain was equal to 36% for a varietal creation (selection on progeny test at a 5% SCA (i.e. entirely additive genetic contribution of the rate) and to 12% for breeding population management parents), the acquisition of new results would appear to (combined selection at a 50% rate). Associated genetic be easy to obtain in the progeny test. The main limit of loss in tree height at 5 years was estimated between 4% a direct selection would be the negative effects on to 1I%, but was inconsistent with other results (positive volume and the time of selection. Because the expres- genetic correlation between juvenile height and twisting sion of tree susceptibility depends on the natural rust resistance). Futhermore, juvenile height has a weak infestation and increase with age, breeders would have predictive value for adult volume. Thus, BARADAT& to delay selection after 15 years to decrease the error DESPREZ-LOUSTAU(1997) suggested including the made in genotype choice. Pruning trees may represent resistance to twisting rust in the management of the an effective way to speed up this process. Because tree Maritime pine breeding population without any increase resistance to D, sylvestrella and radial growth are in the program cost. In our study, a direct selection for negatively correlated, indirect selection using traits resistance would induce an associated loss in tree unrelated to stem volume should be investigated. diameter up to lo%, which represents a 20% decrease JACTELet al. (1996b) related Maritime pine sensitivity in volume at constant height, at an age where volume is to D. sylvestrella to the proportion of some monoter- a good predictor of adult growth (KREMER1992). If penes in the wood resin. Since the oleoresin composi- such a direct selection of a resistant-specific variety can tion was stable in time and space within the same tree not realistically integrated into the Maritime pine and variable between pine trees (JACTELet al. 1996b), breeding program, it indicates the necessity to take into these compounds may yield suitable markers in an account this criteria in the usual selection, to avoid an indirect selection program for D. sylvestrella resistance increased tree sensitivity to D. sylvestrella in breeding in Maritime pine, provided that the heritability of the population or varieties selected for stem volume. D. terpene profile in the wood resin can be demonstrated. sylvestrella resistance could then be integrated into the It would present the advantage to decrease notably the usual selection index, with the coefficient fixed so as to age of selection from 15 years to 7 or 8 years since obtain an optimum compromise between genetic gains terpene composition in the oleoresin is stable at this on all characters. age.

Time of selection REFERENCES

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