Gain efficiency in short-term testing: experimental results
USDA
1
Forest Service Forestry Sciences Laboratory, Olympia. WA, U.S.A.
AND
c.c.
LAMBETH
Container Corporation, Callahan, FL. U.S.A.
Received March
8, 1991
Accepted September
11, 1991
LI, B., WU.LJAMS. C.G., CARLSON. W.C., HARRINGTON, C.A., and LAMBETH, C.C.
experimental results. Can. J. For. Res. 22:
290-297.
1992. Gain efficiency in short-term testing:
Height growth of loblolly pine 1 Pinus raeda L.) was measured in trees subjected to one of five irrigation and fertilization
regimes in a closely spaced genetic test for 3 years. Shoot components of 3rd-year annual height increment were measured
over two contrasting treatments. Ju\'enile height and number of stem units in summer growth length in the fully irrigated and
fertilized short-term test regime exhibited (i) the highest juvenile-mature correlations (family mean correlation= 0.41-0.68),
(ii) high individual-tree heritabilities I 0.38-0.44), which were two- to three-fold higher than older tree values in a conventional
genetic test of the same families. (iifl high genetic stability across two extreme short-term test treatments (genetic correlation=
0.61-0.80), and (iv) an efficiency in genetic gain per generation of 81-87% relative to selection on height at age 8 years.
LI, B., WILLIA.\IS. C.G.. CARLSO:'\. W.C .. HARRINGTON, C.A., et LAMBETH. C.C.
experimental results. Can. J. For. Res. 22:
290-297.
1992.
Gain efficiency in short-term testing:
La croissance en hauteur chez le pin a encens (Pinus taeda L.) a ete mesuree chez des arbres ayant subi un des cinq regimes
d'irrigation et de fertilisation appliques durant 3 ans dans un test genetique a faible espacement. Les composantes de Ia pousse
annuelle a Ia 3" annee de croissance Ont ete mesurees selon deux traitements differents. Dans le test a court terme ou l' irrigation
et Ia fertilisation etaient maximales. les mesures de hauteur juvenile et de nombre d'unites de tige accumulees pendant Ia
croissance d'ete en longueur ont montre (i) les plus fortes correlations entre les stades juvenile et mature (0.41-0,68); (ii) des
heritabiJites individuelles e\evees
I
0.38-0,44) qui etaient deux a trois fois plus grandes que (eS valeurs derivees d' arbres plus
ages representant les memes families dans un test genetique conventionnel; (iii) une stabilite genetique elevee entre les deux
traitements extremes du test a court terme (correlation genetique = 0.61-0.80); (iv) une efficacite en gain genetique par
generation variant entre
81 et 87c-c par rapport a Ia selection sur Ia hauteur a 8 ans.
Introduction
Height growth of juvenile trees in short-term tests is used
to evaluate future performance in the tield because it is rela­
tively easy and inexpensive to measure and it generally cor­
relates well with later tree performance (Squillace and Gansel
1974; Lambeth 1983; Franklin 1983: Campbell et a/. 1985;
Magnussen and Yeatman 1986; Riitters and Perry 1987;
Williams 1987; Carter et a/. 1990).
However, there has been speculation that shoot components
that contribute to juvenile height, such as numbers of stem
units (NSU) and mean stem unit length (MSUL), might be
better indicators of later growth than juvenile height itself.
Shoot growth components NSU and ISUL are under mod­
erate genetic control in conifer species (Kremer and Larson
1983; Bongarten 1986; Li et al. 1991). NSU is primarily
responsible for the variation in shoot length ( Lanner 1968;
Boyer 1970).
1The use of trade names does not imply endorsement of products
used nor criticism of products not used.
zPresent address: University of Minnesota North Central Experi­
ment Station, Grand Rapids, MN 55744. C.S.A.
3Author
to whom all correspondence should be addressed.
Printed in Canada I ImprimC au Clllada
[Traduit par Ia rt!daction)
Studies relating these shoot components to later field
performance have shown some interesting but inconsistent
results. MSUL was highly correlated with tree height in blue
spruce (Picea pungens Engelm.) (Bongarten 1986), and
MSUL of fixed growth in seedlings of maritime pine (Pinus
pinaster Ait.) was significantly correlated with tree height at
6 years (Kremer and Xu 1989). However, 2nd-year annual
height increment was found to rank 8-year height performance
better than either MSUL or NSU in loblolly pine (Pinus
taeda L.) (Bridgwater 1990).
These studies did not quantify the genetic gain expected
from selection based on juvenile height versus its compo­
nents. Since many studies have shown the value of juvenile
selection in closely spaced short-term genetic tests (e.g.,
Franklin 1983; C arter et al. 1990; Williams 1986; Magnussen
and Yeatman 1986; Bridgwater 1990), it is timely to (i) define
optimum testing environment for closely spaced tests and
(ii) evaluate shoot components on the basis of genetic param­
eter estimates and genetic gain efficiency rather than on the
basis of family ranking or juvenile-mature correlations alone.
The results will provide a basis for selection-age decisions in
operational genetic improvement programs (Newman and
Williams 1991).
.93
LI ET AL.
where X;k is the maternal half-sib family mean based on the ith maternal parent and the kth sample of the environment-time
continuum. The casual components of the family mean correlation are as follows:
I
l
!
'
l
j
•
l!
1,
· COVA1A2 are the variance and covariance components, respectively, among half-sibs for seedling and field growth
where
traits measured in different environments; \-E1, \-E2, cove1e2 are the variance and covariance components, respectively, for two
random points in environment and time; Nt, N2 are the total number of individuals per family in the test(s).
l
ji
l
Thus rp r ---7 rg (genetic correlation) as N ---7
oo,
since the
environmental covariance, cove1e,. is assumed to be negli­
gible. Additive genetic variance -is the underlying genetic
basis for progeny testing, and it is the casual component of
interest to traditional conifer breeding programs that rely on
sexual recombination as a means to sequester favorable alleles.
,
The association between the two ages depends on the mag­
nitude of the additive genetic covariance, covA A common
p
to the trait(s) at two ages. Thus the phenoty ic juvenile­
mature (family mean) correlation is a simple, elegant measure
of progeny testing success in seedlings (Williams 1986).
When the number of individuals in the families is large and
testing locations and environments are random, the family
mean correlation is close to the genetic correlation (Williams
1986). The product-moment correlation coefficients were
estimated from phenotypic covariance and variance of family
means. The significance of the difference between two corre­
lations was tested by the -test (Snedecor and Cochran 1967).
Within the short-term test, the additive genetic correlation
was estimated for each trait across the two treatments (1-F
and NI-NF) to assess the stability of the juvenile traits; the
genetic correlations between short-term treatments were esti­
ratio of the gains based on juvenile trait
E
= _
,
=
­
'F
for the juvenile (J) and mature (m) traits, respectively;
root of heritabilities of the juvenile U)
and the mature (m) traits, respectively. The equation can be
h1, hm are the square
simplified with the conservative assumption that
7
[ ]
E
=
l
j
\
j
1
li
where
is the gain at mature age based on selection for the
juvenile trait;
is the selection intensity in the juvenile trait;
are the square root of heritabilities of the juvenile (j)
hi,
and mature (m) traits, respectively; rpr is the genetic corre­
lation of the juvenile (J) and maturl(m) traits;
is the
hm
=
im:
hi r-pr,.,
hm
Gain efficiency per generation (E) was compared between
height at age 8 years and juvenile traits measured in the
short-term test. Gain efficiency is considered the ultimate
''criterion of success" for early selection because it is the
aggregate value of several other criteria: degree of additive
genetic control, genetic stability across treatments, and degree
of genetic commonality with the older tree selection goal.
the least favorable treatment (NI-NF), juvenile-mature cor­
v"'
ronmental effects was adjusted according to Yamada's (1962)
methods. The genetic gain for mature performance from mass
selection based on a juvenile trait is as follows (Falconer
1981):
G
i1
Family mean correlations between juvenile traits and 8-year
height were highest for the full irrigation and fertilizer treat­
ment (1-F), which favored growth (Table 3; Fig. 1). Under
where rg is the genetic correlation; 'f is the genetic variance
among half-sib families; VFxr is the family x treatment inter­
action; t is the number of treatments.
Bias in VFxr due to heterogeneous variance and fi xed envi­
l
"'
Optimum testing environment
= ------
( v}
I
Results
VFxT
rg
and mature trait
G· ijhJhm r-pc O'pf
Gm
imh O'pf,.,
where G1, Gm are the gains at mature age based on selection
(6]
mated for fixed effects (Yamada 1962):
[4)
j
m gives the equation for genetic efficiency:
i1
O'pr
phenotypic standard deviation of the mature (m) tn'i'it. The
relations were low (Tables 3 and 4), although individual-tree
was slightly higher
= 0.38 versus 0.55;
heritability
(h2)
(h2
Table 5). Family performance averaged across all treatments
showed lower juvenile-mature correlations than the best
single treatment, 1-F (Tables 3 and 4).
Juvenile-mature correlations
Shoot growth components were measured on the third
annual increment only; in comparison, the composite trait was
limited to 3rd-year height (HT3) and its annual increment
(HT3 - HT2). Summer NSU and annual increment measured
on trees in treatment 1-F had equally high family mean cor­
relations with 8-year height (0.56 versus 0.57 for HT3 - HT2).
All other component traits exhibited lower correlations with
8-year height in the North Carolina coastal progeny tests
(Tables 3 and 4).
Juvenile height and height increments were highly corre­
lated with age 8 height; the 2nd-year increment exhibited the
highest correlation (HT2 - HTl, rpr = 0.68; Table 3; Fig. 2).
294
CAN.
I. FOR. RES. V OL . 22. 1992
TABLE 3. Loblolly pine juvenile-mature family mean correlations of (i) 1-. 2-. and 3-year
height and height increments in a short-term test with (ii) 8-year height
Treatmenr'
I-F
Composite traita
NI-NF
I-NF
I-lf2F
Nl-VzF
NCC progeny tests
HTl
0.22
0.14
HT2- HTI
HT3- HT2
0.68***
0.57***
HT3- HTl
0.37*
0.36*
0.35*
0.48**
0.16
-0.08
0.70***
0.38*
0.22
0.18
0.07
0.68***
0.69***
HT2
HT3
0.34*
0.54***
0.05
0.48**
0.33
0.34*
0.41 **
0.35*
0.25
0.28
0.06 0.34*
0.20 0.45
0.56**
0.25
0.33
Jones County test
HTl
HT2
0.30
0.52**
HT3
HT2- HTl
0.17
0.37
0.52**
0.52**
HT3- HT2
HT3- HTI
0.42
0.44
0.29
0.38
0.45
0.52**
0.37
0.41
0.20
0.19
0.29
0.34
0.53**
0.54**
0.47**
0.52**
0.20
0.32
-0.03
0.18
NoTE: Eight·year height is based on North Carolina coastal (NCC) progeny tests and on a Jones County open·
pollinated test.
•, ••. •••.
I. 2.
and 3 years, respectively.
Significantly different from zero at 10. 5. and l
"HTI, HT2, HT3. height at
le1·eis, respectively.
b See Table I for a description of each treatment.
30,-
1:
'*'
c
0
j
""
c
OS
a:
25
20
r
r
"
I
;-(
I
l
I
[x
l
0
0
)(
:
)(
!
15 i-10
3rd-year height increment. Unlike the North Carolina coastal
progeny test results, other shoot growth components (total
NSU and summer growth length) were strongly correlated
with 8-year height (Table 4). Several shoot components in
trees of treatment NI-NF also showed significant correlations
with height performance. For example. fixed growth NSU for
trees in treatment NI-NF was strongly, positively correlated
(rpr= 0.74. p < 0.01) with 8-year height for trees in the Jones
County test.
;
:<
)(
:.:
Genetic comrol ofjuvenile traits
)(
riil = 0.68*-
;.:
*
*
5
10
15
20
25
30
Rank based on a-Year Height
FIG. 1. Plot of family ranks for 2-year height increment in treatment
I-F (irrigation and fertilization) and 8 -year height in the progeny tests
of 25 open-pollinated loblolly pine families.
For example, the top five of the 25 families could have
been correctly identified at age 2 and this also held true for
correctly selecting the top 50% (Fig. 1).
The height increment for the 1st year was higher than total
height at age 1 year (Table 3), but after age 1 there was
no statistical difference between total height and height incre­
ment. Also, juvenile-mature correlations reached a plateau
after age 2 years and correlations did not increase in the
3rd year (Fig. 2).
Correlations with the Jones County open-pollinated test
provided corroborating results. Third-year increment and
summer NSU exhibited the highest juvenile-mature correla­
tions;· here, summer NSU had a higher correlation than
Total height and height increments of loblolly pine seed­
lings exhibited higher heritabilities relative to older tree
height in conventional genetic tests (Table 5). For example.
heritability in treatment I-F was 0.38 for 2nd-year height and
0.44 for its height increment. This is two to three times higher
2
than heritability at age 8 in the Jones County test (h = 0.16).
Summer shoot NSU and 3rd-year annual increment had
slightly lower heritability estimates (h 2 = 0.28 and 0.20) than
2
3rd-year total height (h = 0.39).
Stability ofjuvenile traits
Height at age 2 years was the most stable composite trait
across extreme treatments (rg = 0.78; Table 5). Total 3rd-year
height and its height increment were also consistently stable
across treatment components (Table 5).
For those components initiated and elongated in the same
season (i.e., summer growth components), MSUL and NSU
were both stable across extreme treatments (Table 5). Only
fixed growth length and NSU were unstable across treatments
(Table 5). For these shoot components, the genetic correlation
measuring F x T interaction was low and negative (Table 5).
Gain efficiency of 8-year height versus shoot components
Compared with selection at age 8 years, selection on 2nd­
and 3rd-year height and corresponding annual increments
would have doubled genetic gain efficiency per generation
(Table 6). Selection based on the best shoot component.
Ll ET AL.
0.8
TABLE 4. Loblolly pine juvenile-mature family mean
correlations of (1) 3 y
­ ear shoot composite tmits and
shoot component tmits with (ii) 8-year height
0.6
Treatmentb
Trait"
I-F
NI-NF
0
NCC progeny tests
Composite: age 3
HT3
HT3 - HT2
Component: age 3
NSU
MSUL
Fixed growth
Length
NSU
MSUL
Summer growth
Length
NSU
MSUL
0.69***
0.57***
8
0.07
-0.08
-,
0.54***
0.56***
-0.24
-0.09
0.08
-0.23
0.52*;;:
0.45
.t
.r
e
e
.)
r
y
:s
n
).
!-
iS n
t,
,l
'
!
0.55**
-0.5 1**
-0.25
-0. 12
-0.07
0.53**
0.74***
-0.34
0.52**
0.65***
-0.4 1
0. 13
0.40
-0.48
:-ISU,
••,
•••.
number of
The use of a fully irrigated and fertilized short-term test
treatment appears to be best for maximizing juvenile-mature
correlations at Fort Towson, Oklahoma. In this short-term test,
irrigation seemed to be the limiting factor; the second and
third choices were both fully watered with no or partial fer­
tilizer treatments (I-NF and I-Y2F; Tables 3 and 4 ). The short­
l
term test was located west of the loblolly pine range, where
I
drought occurs frequently, so supplemental water may be
Jl more important here than if the test were conducted along the
higher rainfall areas of the Atlantic Coast. If so, there may
not be a single short-term test treatment that is best across
t esting sites. Before using short-term testing, it may be
1
J prudent to develop a protocol for optimum short-term test
1
•
conditions in each physiographic region.
l
j
-
HT2 HT3
Height(cm)
The differences between short-term test treatments were
mainly due to magnitude of the variance and covariance com­
ponents rather than to family rank change. The genetic cor­
relations across treatments, adjusted for scale (eq. 4: Table 5),
were highly positive for height traits and summer NSU: this
suggested that differences in juvenile-mature correlations
among treatments could not have been due to family rank
change alone.
Choice of juvenile traits for early selection
Discussion
I
HTI
0.29
0. 19
summer growth NSU, was 87% as efficient as selection at age
8 years. The value is based on gain per generation so the
potential time savings provides incentive for early selection
in short-term testing.
Optimum testing environment
HTO
FIG. 2. Changes in phenotypic family correlation (r:or) as loblolly
pine seedlings grow. Correlations were calculated between juvenile
heights and 8-year height in the North Carolina coastal (NCC)
progeny tests (25 families) and in Jones County test ( 16 families).
0.52**
-0.31
MSUL, mean stem unit length.
b See Table I for a description of each treatment.
I
//
0.2
-0.2'--0
50
100
150
200
250
300
350
" HT2, HT3, height at 2 and 3 years, respectively;
i
/
p <0.05 /
,___ L.
0.03
0. 14
-0.2 1
stem units:
J
///, Jones County test
-0.24
-0. 13
-0. 13
Significantly different from zero at 5 and l% levels. respectively.
j
I
0�-----
progeny tests and on a Jones Coumy open-pollinated test.
:i
NCC Progeny tests
0. 1 1
-0.25
NoTE: Eight-yeur height is based on North Carolina coastal (NCC)
i1
0
1
0.4 1**
-0.23
Jones County test
Composite: age 3
HT3
HT3 - HT2
Component: age 3
NSU
MSUL
Fixed growth
Length
NSU
MSUL
Summer growth
Length
NSU
MSUL
,a,
.:.
04
c
295
The summer NSU was superior to both summer and fixed­
growth MSUL as a selection criterion. This does not corrob­
orate reports for temperate conifer species (Bongarten 1986:
Kremer and Xu 1989), where fixed-growth MSUL was the
best seedling growth indicator.
Species or taxa summer growth patterns may account for
the incongruent results. Blue spruce produces only fixed
growth, so there can be no comparison with summer NSU.
Maritime pine is generally monocyclic, thus it infrequently
produces summer growth. Loblolly pine, by contrast. pro­
duces 69% of the annual growth as summer growth length
(Table 2). For other conifer taxa such as Douglas-fir
(Pseudotsuga menziesii (Mirb.) Franco), summer growth is a
distinctly juvenile trait, disappearing with advancing age
(Kaya et al. 1989). In loblolly pine, summer growth contrib­
utes to annual increment until rotation age (Harrington 1991).
Juvenile selection criteria may vary with provenances (Kaya
et al. 1989), or in this case, with species.
Juvenile-mature family mean correlations increased as
seedlings increased in height in the 2nd year, then the corre­
lations plateaued (Fig. 2). There are two possible explanations
for this. First, the third growing season experienced a severe
drought, so that even trees in the irrigated portion of the study
may not have received adequate irrigation. If so, the best
treatment would have been more like the less-desirable non­
irrigated treatments (Tables 3 and 4) and juvenile-mature cor­
relations would have been unchanged or even lower in the
3rd year.
CAN. J. FOR. RES. VOL. 22. 1992
296
TABLE 5. Heritability estimates (with their standard errors in parentheses) and genetic correlations for
loblolly pine composite and component traits
Heritability estimate
Genetic
correlation
between
treatments
Treatment
I-F"
Treatment
NI -NP
I-F and NI-NF
combined
0.12 (0.17)
0.38 (0.14)
0.39 (0.13)
0.40 (0.14)
0.20 (0.09)
0.36 (0.13)
0.32 (0.12)
0.55 (0.18)
0.59 (0.19)
0.54 (0.18)
0.43 (0.15)
0.61 (0.20)
0.10 (0.07)
0.35 (0.13)
0.31 (0.13)
0.36 (0.13)
0.14 (0.08)
0.30 (0.12)
0.29
0.78
0.61
0.80
0.41
0.59
0.23 (0.10)
0.36 (0.13)
0.44 (0.15)
0.44 (0.15)
0.24 (0.09)
0.34 (0.12)
0.75
0.82
Length
NSU
0.28 (0.11)
0.30 (O.l l)
0.08 (0.06)
0.24 (0.10)
0.20 (0.09)
0.13 (0.07)
0.06 (0.07)
0.06 (0.08)
0.09 (0.05)
-0.32
-0.40
0.85
Length
NSU
0.24 (0.10)
0.28 (0.11)
0.28 (0.11)
0.38 (0.14)
0.43 (0.15)
0.38 (0.13)
0.09 (0.08)
0.20 (0.09)
0.31 (0.1 I)
-0.06
0.53
0.53
Trait0
Composite
HT I
HT2
HT3
HT2- HTl
HT3- HT2
HT3- HTI
Component: age
NSU
3
MSUL
Fixed growth
MSUL
Summer growth
MSUL
"HTI, HTI, HT3, height at l,
b See Table
1
2.
and 3 years. respectively;
The second explanation may be the static ratio of cyclic
growth to total height growth in the 3rd year. Cyclic growth,
a combination of fixed and summer growth cycles produced
after the initial seedling's bud set, has been shown to be better
correlated with mature height performance than total height
in 1st- and 2nd-year loblolly pine seedlings in greenhouse
and short-term studies (Williams 1987; Li et al. 1991). As
cyclic growth increasingly contributes to total height,
juvenile-mature correlations have been observed to increase
(Williams 1987). After the 2nd year, the ratio of cyclic growth
to height changes only slightly, and the plateau in this ratio
may also result in a plateau in the juvenile-mature correlation
in the 3rd year.
On another point, shoot components were more highly cor­
related with 8-year height in the open-pollinated Jones County
test than with performance based on the North Carolina
coastal progeny tests. The Jones County test received superior
silvicultural care compared with the North Carolina coastal
progeny tests; this was exhibited in mean tree height (8.3 m
for the Jones County test versus 6.2 m for the North Carolina
coastal progeny tests). If intensive silviculture results in more
fully expressed genetic potential, then field test quality may
be more limiting to successful early selection than is genetic
control of juvenile shoot components.
Genetic control
NSU.
number of stem units;
MSUL.
menn stem unit length.
for a description of each treatment.
Shoot growth components tended to exhibit lower herita­
bility estimates than height growth traits (Table 5). Large
measurement error was likely to be the cause, because shoot
components were more difficult to measure than total height.
There may also be a developmental basis; some fixed-growth
components exhibited negative genetic correlations across
treatments, suggesting complex interrelationships between
components as reported by Kremer and Larson (1983) for
jack pine.
The value of NSU may be underestimated in our stud:
because components were not measured in more than on
growing season. Averaging across years may be more impor
tant for genetic stability than averaging across short-term te
treatments.
Genetic gain efficiency: applications and limitations
Early selection on height increment or a combination o
height increment and summer NSU appears to yield nearly a'
much genetic gain per generation as selection on age 8 heigh·
itself for the North Carolina coastal loblolly pine provenance
There is considerable potential for time savings, but applica­
tion of these results will determine the time savings (e.g..
Carter et al. 1990). Two low-risk applications might be tl
rogue a production seed orchard or to choose which unrelatec
parents to infuse into a breeding population. Forward selec­
tion encumbers more risk of choosing incorrect selections, but
the risk-adjusted returns could still be substantial (Newmar,
and Williams 1991).
Early selection on height· increment must be monitored
each generation to ensure that breeders are selecting for the
largest increase in height increment with the least change
between bud break and final bud set (Bridgwater et at. 1985:
Williams 1986).
Conclusions
Height growth traits and summer NSU were the best
choices of juvenile traits for early selection in short-term
testing. This was based on four criteria: juvenile-mature
family mean correlations, heritability, and genetic stability
across extreme treatments, as well as an aggregate measure.
genetic gain efficiency. For these traits, juvenile-mature cor­
relations were more highly positive in one fully irrigated and
fertilized treatment than when averaged across five short-term
treatments. The best 8 out of 10 can be selected correctly on
Ll ET AL.
TABLE 6. Relative efficiency of genetic gain
(%) for loblolly pine composite and compo­
nent traits measured in a farm-field test near
Fort Towson. Oklahoma
Trait"
Gain e fficiency
per generation (%)
(treatment I-F)b
100
Base HT8
Composite
HTl
HT2
HT3
HT2 - HT1
HT3 - HT l
HT3 - HT2
Component
(total for season)
NSU
MSUL
Fixed growth
Length
NSU
MSUL
Summer growth
Length
NSU
MSUL
26
81
82
83
51
79
63
-47
-33
-17
-5
64
87
-55
NoTE: The basis for comparison is 8-year height in a
Jones County open-pollinated test. a
HTl. HT2. HT3, HTS, height at l,
2.
3. and 8 years. respectively; NSU, number of stem units; MSUL. mean
stem unit length.
b See Table
I
for a description of this treatment.
the basis of 2-year height. Individual-tree heritability was two
to three times higher in the short-term test relative to the older
tield test, and genetic stability over two extreme short-term
test treatments was high. Genetic gain efficiency per genera­
tion is over 80% for these traits relative to 8-year height.
Juvenile height growth and NSU in summer growth in a well­
watered and fertilized short-term environment appear to be
reliable early selection criteria.
Acknowledgements
We acknowledge the assistance of Leon Burris of
Weyerhaeuser Company, who installed and measured the
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