The genetic of resistance against Phoracantha semipunctata

The genetics of resistance against Phoracantha semipunctata attack in Eucalyptus
globulus in Spain
By F. Soria and N.M.G.Borralho
1) ENCE, Crta. Madrid Huelva KM 630, Apdo 223, Huelva 21080, Spain
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2) Cooperative Research Centre for Temperate Hardwood Forestry, University of Tasmania, GPO Box 25212, Hobart TAS 7001, Australia
Abstract
A study on the inheritance of resistance against Phorancantha semipunctata (eucalypt
longhorn borer) was conducted in a progeny trial established in south Spain, involving a
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large Eucalyptus globulus base population. The results showed small race effects and a low
heritability (h2=0.19) for survival against Phoracantha. The local land race was not
significantly different from the mean of the Australian seedlots, but the two selected clones
were significantly more resistant and growing faster. Level of Phoracantha attack was
poorly but positively correlated with growth at both race and genetic level.
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Keywords: Heritability, genetic correlation, drought, Eucalypt longhorned borer, Spain
Introduction
In many areas of Iberia and Mediterranean basin, characterized by low rainfall and extended
dry summers, the productivity of eucalypt plantation can be considerably reduced because of
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the impact on survival of Phoracantha semipunctata, a cerambycide beetle native from
Australia.
So far the main control measure has been sanitation cuts and trap logs, but this is costly and
only partially effective. There has been reported differences in susceptibility to Phoracantha
between different eucalypt species (Hanks et al. 1993, Hanks et al. 1995), but the magnitude
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of genetic variation within E. globulus, the most important eucalypt species in Spain, has not
been previously described. This papers looks at the level of genetic control of Phoracantha
attack and its relationship with growth on a large E. globulus base population trial
established in a severely infected area in the South of Spain.
Material and Methods
Genetic material
The families used in this study were obtained from a range-wide collection of 256 openpollinated families covering the entire natural range of Eucalyptus globulus subsp. globulus
5
in Australia (collection details are published in [Jordan, 1994 #71]). Cuttings of two plustrees from ENCE breeding program, and one commercial seedlot were also included in the
trial. The plus-trees have been selected on the basis of their superior phenotype for growth
and health, in highly affected areas in the region.
Trial site and design
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The trial is located in the SW of Spain, in the province of Andaluzia, near Huelva (lat
3734’N; long 650’W; alt 70m). Climatic conditions are typically Mediterranean with
around 500 mm annual rainfall, a mean annual temperature of 17.5C, and less than 10 days
of frost per year. However, the climatic conditions prevalent during most of the trial were
generally drier than average: the total rainfall in 1992, when the trial was 2 years old, was a
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record low 250 mm. Soil is a distric cambisoil, with a hard-pan horizon at around 50 cm
deep. The soil was cultivated prior to planting using a mould-board plough to 60 cm. Plants
were established in a 3 x 3.5m spacing, and fertilized at planting. In the commercial
seedlings, a slow release fertiliser was also applied to the paper-pot medium. Weeds were
controlled one year after planting by discing.
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Families were grouped in five sub-lines, according to Table 1. Families were then
randomized within each sub-line, in three non-contiguous two tree plots per block. There
were 4 blocks per subline. The two clones, propagated from stem cuttings, and one
commercial seedlot were included in all blocks across the five sublines. [Jordan, 1994 #71]
and [Dutkowski, 1997 #81] classification was used as a basis to group the different families
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into native races. The two clones and the commercial seedlot were grouped in a local landrace.
Table 1. Distribution of races across the five sublines (SL1 to SL5), and total number of
families and trees per race in the trial.
Measurements
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The measurements include five year old diameter, height and presence or absence of
Phoracantha. Tree health was assessed according to a Status and Cause code, as detailed in
Table 2. Trees which were which have been attacked or killed by Phoracantha or by fungus
(Status 02 and 03 and Cause 04, 08 and 09) were scored as 0, whereas those which showed
no sign of attack were given a score of 1. Although fungus and Phoracantha attack was
10
scored separately, in practice the two effects seem to be associated or difficult to separate.
Trees dead or affected by other causes (e.g. frost, wind, animals), were excluded from the
analysis.
Subline
Race
0. Land race + clones
1. Lavers Hill
2. OtwayRangesW
3. OtwayRangesE
4. Strzelecki
6. S.Gippsland
7. FlindersIs
8. Cape Barren
9. Clark Is.
10. St. Helens
11. NE Tasmania
13. Tasman Peninsula
15. SE Tasmania
16. Dromedary
17. Jericho
18. South Tasmania
19. Dover
22. West Tasmania
23. King Is.
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1
X
X
X
2
X
3
X
4
X
5
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
No. Fam
3
5
41
28
32
1
35
28
3
3
5
5
26
4
5
9
4
10
7
No. Trees
351
68
629
408
568
10
510
451
45
58
90
51
503
50
94
129
46
132
118
Table 2. Classification of status and cause of damage in the trial.
Status
01
02
03
04
99
Broken top
Sick
Dead
Leaning
None of the above
Cause of damage
01
02
03
04
05
06
07
08
09
10
99
Water runnoff
Waterlogging
Frost
Drought (in association with Phoracantha)
Cattle
Rats
Wind
Phoracantha
Fungus
Machines
None of the above
5
10
15
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Statistical analysis
Estimates of variance and covariance for height, diameter and Phoracantha were obtained
by Restricted Maximum Likelihood (REML), using an Average Information algorithm
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([Gilmour, 1995 #15]). For estimation of variance components, the commercial controls and
clones were removed from the analysis and analysis was carried out within a set, based on
the following linear model:
y = u + set + rep/set + race + fam + plot + error
where y is the vector of observations for height, diameter and attack by Phoracantha, set is
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the effect due to sets (fixed), rep the effects due to blocks (fixed), fam is the effect due to
open pollinated families (random), race is the effect due to races (random) and plot is the
effects due to plots (random). Standard errors for variance ratios were obtained directly
from the inverse of the mixed model equations. Additive variance was assumed to be equal
to 2.5 the family variance. Given 0/1 nature of the survival trait, heritability and phenotypic
correlation estimates for attack by Phoracantha were in a binomial scale and were converted
to a liability scale according to ([Olausson, 1975 #72]).
The data used in the estimation of random (BLUP) and fixed effects (BLUE) included the
Australian open pollinated families, and all unimproved seedlings and the two clones,
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assumed unrelated and coming from the same local land race. The model differed slightly in
that no account was made of the set effects:
y = u + rep + race + fam + plot + error.
The two clones and the unimproved seedlot were present in all reps and sets, and are
expected to provide the necessary links between sublines, hence allowing the analysis to
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account for genetic differences between families across sublines.
Results and Discussion
The trial had poor growth and survival. Around 10% of the trees were dead or badly
affected by frost and of the remaining trees, used in the analysis, 59% were dead or severely
affected by Phoracantha. Such levels are not uncommon in many dry areas of Spain and
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Portugal ([Lencart e Silva, 1994 #64]), particularly following a number of dry years. The
mortality due to Phoracantha varied across the trial, with mean block incidences ranging
between 42 and 82%. Height at five years was 5.7  1.4 m, and diameter was 6.2  1.9 cm.
This is lower than reported growth rates in progeny trials of similar origin and age in higher
rainfall areas in central and south of Portugal ([Borralho, 1992 #14], [Araújo, 1996 #23]),
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and North of Spain (Soria et al. 1997), and amongst the lowest site indices from forest
inventory data ([Tomé, 1994 #73]). It reflects the particularly poor years experienced by the
this trial prior to the 5 year old assessment.
Race effects
The means for diameter, height and level of Phoracantha incidence are listed in Table 3. The
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local land race grew marginally better (2%) than the mean of all Australian races, excluding
Wilson Promontory (Table 2). The best native races for growth were St. Helens (Tasmania),
Strzelecki and West Otway Ranges (Victoria) and South Tasmania, with growth rates
significantly better (at 1% level) to the unimproved land race.
A few native races were also more resistant than the unimproved land race to Phoracantha.
The best races were Cape Barren Island and Flinder Island from the Furneaux group of
islands in Bass Straight, with 45 and 50% incidence, respectively. King Island, NE Tasmania
and West Coast Tasmania were the least resistant (around 75%).
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Table 3. Least-square means for height, diameter and percentage of trees dead or seriously
affected by Phoracantha at five years, for the Australian races, the unimproved land-race,
the two clones from ENCE breeding program, and the best 5 families in the native collection
for each trait.
Race
0. Land race
Clone A
Clone B
Commercial
1. Lavers Hill
2. OtwayRangesW
3. OtwayRangesE
4. Strzelecki
5. Wilsons Promontory
6. S.Gippsland
7. Flinders Island
8. Cape Barren
9. Clark Island
10. St. Helens
11. NE Tasmania
13. Tasman Peninsula
15. SE Tasmania
16. Mt. Dromedary
17. Jericho
18. South Tasmania
19. Dover
22. West Tasmania
23. King Island
Best 5 Aust. Families
Height
m
Diam
cm
Phor
(%)
6.76
6.53
6.00
5.75
6.27
5.98
6.30
4.33
5.95
5.62
5.50
5.14
6.40
5.99
5.67
5.97
5.39
5.56
6.29
6.21
5.61
6.00
7.00
7.26
7.02
6.51
6.38
5.76
6.39
6.79
4.70
6.45
6.23
6.22
5.70
6.83
6.31
6.14
6.42
5.88
6.13
6.67
6.67
5.96
6.24
7.73
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29
56
54
56
64
53
69
58
51
43
55
64
73
63
61
60
55
67
66
70
79
31
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The two plus trees, which were selected on the basis of superior phenotype for growth and
health in affected areas in South of Spain, were clearly above the trial mean for growth and
resistance against Phoracantha. The clones grew on average 12% better for height and
diameter, compared with unimproved seedlings and the mean of Australian races, and had a
mean Phoracantha incidence of 28%, a substantial gain from the 56% observed in the
seedlings.
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Genetic parameters
Variance components due to race, additive genetic and plot effects are listed in Table 4, for
height, diameter and Phoracantha attack. Race effects account for 17% of the total
variation in height, 9% in diameter and 4% in Phoracantha. Within-race heritability for
height and diameter was 0.23 and 0.16 respectively, somewhat lower than previous
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estimates from the same population in Australia ([MacDonald, 1997 #75]), but agreeing well
with [Borralho, 1992 #37] and [Araújo, 1996 #23] results in Portugal and (Soria et al.
1997) in Spain. The results could suggest that growth in this type of environments may be
less heritable than elsewhere, although the poor connectedness between the five sublines and
the large block sizes may have also affected the accuracy of heritability estimates. The
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heritability for Phoracantha attack, given in the underlying scale was low (h2L=0.19). To
our knowledge, this is the first reported heritability estimate for Phoracantha resistance in
eucalypts.
Table 5. Variance components and heritability estimates, given on the underlying scale based
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on an intraclass correlation amongst open pollinated sibs of 0.4.
Variance components
Trait
Race
Fam
Plot
Height
0.325
0.178
0.123
Diameter
0.320
0.230
0.200
Phoracantha
0.010
0.011
0.007
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Heritability adjusted to the liability scale.
Error
1.258
2.880
0.206
h2 (s.e.)
0.23 (0.056)
0.16 (0.049)
0.19 (0.022) 1
The genetic and race correlation amongst the three traits is listed in Table 5. Height and
diameter were highly correlated at both genetic and race levels (r G=0.93 and rR=0.97).
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Similar findings were reported for the subspecies in Portugal ([Borralho, 1992 #14]) and
Australia ([Volker, 1990 #69], MacDonald et al. 1997). Genetic correlation between growth
traits and Phoracantha was positive but low and, as expected, associated with large
standard errors (rG = 0.30  0.17 and 0.32  0.20, for height and diameter respectively).
Table 5. Genetic (above diagonal) and race correlation (below diagonal) for height, diameter
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and Phoracantha attack.
Height
Height
Diameter
Phoracantha
0.96  0.03
-0.02  0.36
Diameter
Phoracantha
0.93  0.03
0.30  0.17
0.32  0.20
0.19  0.37
Corresponding race correlations for height and diameter were less interpretable (r R=-0.02
and 0.19, respectively; Table 5), but suggest a near zero genetic relationship between
growth rate and resistance against Phoracantha at the race level.
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There is some chance that the correlations presented here may be affected by unbalance in
the data. The majority of the tree scored as affected by Phoracantha had no assessment for
growth. Of the 4236 trees included in the analysis, 2024 were measured for growth but only
202 of them were also considered affected by Phoracantha. Therefore, most trees affected
did not have any growth measurement. If the two traits are genetically correlated, there is an
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indirect selection for growth due to the mortality caused by Phoracantha. In theory, this
problem is expected to reduce in absolute terms the estimated covariance between the two
traits ([Villanueva, 1990 #77]), so the magnitudes of the correlations reported here should
be seen as an upper limit of the true estimates. In conclusion, correlations between growth
and resistance against Phoracantha are likely to be low.
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Implications for breeding
The differences observed in resistance to Phoracantha between races and families have
important implications for breeding decisions. Recently, [Chambers, 1997 #78] showed that
with mortality reducing the stocking to 50% of its optimum level, as observed in our trial,
survival becomes four times more important than growth of the survivors, when the
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objective is to maximise gains in volume per hectare. In conditions were cause of death can
be attributed to a specific cause, such as with Phoracantha here, selection criteria should
therefore focus on improving the resistance against Phoracantha even if only modest gains
are achieved in the growth rate of survivors. At the race level, Strzelecki and Otway Ranges
(West) from Victoria seemed to have the best combination of growth and resistance against
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Phoracantha. On the other extreme, West Tasmania and King Island had poor growth and
were particularly susceptible to Phoracantha. These races are expected to be ill-adapted to
Spanish conditions - the two are genetically close ([Jordan, 1994 #71]) and from the wettest
range of E. globulus distribution. However, the NE Tasmania races, and particularly St.
Helens, were highly susceptible to Phoracantha. This is a surprising result since NE
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Tasmania has been amongst the fastest growing and more drought resistant races in
Phoracantha-free conditions in Western Australia ([Kube, 1995 #70], [Dutkowski, 1995
#67]). St. Helens was also amongst the fastest growing race in our trial but its high
susceptibility to Phoracantha attack makes it less attractive for future infusions. This
suggests that resistance mechanisms against the insect may be distinct from those against
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drought in the absence of the insect. The result, however, contradicts early findings by
[Hanks, 1995 #79] which reported a strong correlation, albeit at the species level, between
dry conditions in the seed source, and higher ex situ Phoracantha resistance characteristics.
Despite the race differences, most of the genetic variation is found within a race. For
example, selecting the 5 best Australian families on the basis of progeny information alone is
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expected to reduce the incidence of Phoracantha from 58% as in the unimproved seedlings,
to around 31%. Similar gains were found for the two clones used in the trial and previously
selected for they superior growth, form and health in highly affected plantations. They were
15% better than average for growth and only 28% of the cuttings were damaged or killed by
the insect.
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Expected gains from family selection seem therefore to be of the same order of magnitude to
those from mass selection and propagation. While mass selection is known to be clearly less
accurate, it is also associated with much larger selection intensities. This is expected to be
the case since the plus-trees were amongst the very few survivors in highly affected
plantations. The result confirms the effectiveness of ENCE’s plus-tree selection and
propagation program to improve growth and in particularly insect resistance under local
conditions.
Because of the binary nature of the trait, there is little chance of further improvements from
within family selection. This would require, for example, having several ramets per clone and
5
select them on average incidence, or establish large family blocks in highly infected areas
where incidence levels could approach a high selection intensity (of say 90%). These are
important operational limitations when considering the selection against Phoracantha.
Conclusions
So far E. globulus programs have concentrated on volume per tree and, in some cases,
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wood density ([Borralho, 1993 #34], [Greaves, 1997 #35]). However, given the large
impact of Phoracantha in the mortality of plantations in dry areas in the South of Spain,
breeding programs should consider including Phoracantha damage as a major selection trait.
According to our results, important gains in survival and growth of survivors have already
been achieved from mass selection, but further improvements could be realised as a result of
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moderate heritability and considerable genetic variation at the race and family levels.
Unfortunately, resistance against Phoracantha seems to be poorly correlated with growth,
so selection programs relying on growth alone are not expected to improve the resistance
against the insect. Furthermore, one of the fast growing races was also amongst the most
susceptible to the insect.
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The relative importance of each trait in future selection decisions depends on the impact of
Phoracantha across the whole plantation estate and the ability to carry out effective within
family selection for Phoracantha, not the least to have progeny trials being established in
highly affected areas. The results on heritability and genetic correlations with growth
reported in this paper provide important basic information to model such decisions.
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