2011 SOUTHEASTERN NATURALIST 10(4):751–760
No Evidence of Local Adaptation in Uniola paniculata L.
(Poaceae), a Coastal Dune Grass
Cara L. Gormally1,* and Lisa A. Donovan1
Abstract - Studies of local adaptation generally investigate plants growing in relatively
stable habitats. We asked whether populations of the long-lived clonal grass Uniola
paniculata (Sea Oats) are locally adapted to microhabitats in the southeastern US coastal
dunes, a habitat characterized by dynamic environmental gradients spanning relatively
small distances. Although vegetative zonation is well characterized across these gradients,
little is known about intraspecific evolutionary responses of species spanning
the gradients. Plants from the foredune and backdune areas of the gradient (less than 10 m
and 40–60 m from the shoreline, respectively) were reciprocally transplanted into experimental
plots in both habitats. Although foredune plots were washed away by storms
before harvest, the foredune plants demonstrated no early advantage in stem diameter
or height growth, and thus there was no support for local adaptation in foredune plants.
In the backdune plots, the backdune plants demonstrated no early growth advantage,
and additionally demonstrated no advantage in survival, nor in growth or total biomass
of surviving plants at harvest. Thus, there was again no support for local adaptation. In
frequently disturbed environments such as the coastal dunes, plants may be more likely
to respond with phenotypic plasticity than through local adaptation.
Natural selection can drive local adaptation when plant species encounter
heterogeneous environmental conditions (Clausen et al. 1940, Hereford 2009,
Turesson 1922), both at the landscape level across the span of their ranges, as
well as at finer, microhabitat scales (Antonovics and Bradshaw 1970). Local
adaptation is the association of certain genotypes with particular habitats, with
the locally adapted genotypes outperforming “foreign” genotypes (Kawecki and
Ebert 2004, Linhart and Grant 1996). Most field-based reciprocal transplant
experiments of local adaptation target plant species growing in relatively stable
environments characterized by differences in abiotic factors such as heavy metals,
fertility, herbicides, elevation, light, temperature, and soil-water availability
(Leimu and Fischer 2008, Linhart and Grant 1996). However, much less is known
about local adaptation of plant species located in frequently disturbed habitats,
particularly for long-lived clonal plants (Galloway and Fenster 2000, Gregor
1930, Leimu and Fischer 2008, Miller and Fowler 1994).
Coastal sand dunes are spatially and temporally heterogeneous, in part due to
frequent disturbances that can result in the removal of low-lying foredunes, partial
destruction of larger dunes, the erosion of dune profiles and even complete habitat
destruction (e.g., submerging of an entire barrier island) (Claudino-Sales et al.
1Department of Plant Biology, University of Georgia, Athens, GA 30602. 2Current address
- School of Biology, Georgia Institute of Technology, 310 Ferst Drive, Atlanta, GA
30322. *Corresponding author - email@example.com.
752 Southeastern Naturalist Vol. 10, No. 4
2007). In addition to physical disturbance, environmental conditions—including
salt spray, sand accretion, and soil nutrients—may differ dramatically across distances
as small as 0.5 m, driving changes in plant community composition and
species’ distributions (Barbour et al. 1999, Boyce 1957, Cowles 1899, Doing 1985,
Maun and Perumal 1999, Oosting and Billings 1942, Smith and Smith 2001, Wagner
1964). Plants in these habitats demonstrate intraspecific variation in morphological,
physiological, and phenological traits (Gormally and Donovan 2010, Jerling
1985, Wagner 1964). Among the grasses studied, as the distance from the shoreline
increases, plant size generally decreases, and proportions of flowering, seeding, and
establishment decline (Gormally and Donovan 2010, Jerling 1985, Wagner 1964).
While extensive ecological research has been conducted in order to understand the
role of abiotic conditions in species zonation and primary succession on coastal
dunes (Barbour et al. 1999, Boyce 1957, Cowles 1899, van der Valk 1974, Wilson
and Sykes 1999), we know relatively little about the evolutionary responses of plant
populations to the environmental heterogeneity of coastal dune systems.
Tests of local adaptation in coastal dunes are affected not only by the dynamic
coastal dune environment, which is frequently altered by sand erosion and accretion,
but also by the growth habit of dune plants, most of which are long-lived and
clonal (Cheplick and White 2002, Gray 1985, Knight and Miller 2004). Vegetative
reproduction may reinforce local adaptation, through the level of clonal diversity
as well as through clonal growth patterns, e.g., the establishment of independent
daughter ramets in the same habitat as the maternal plant. Alternatively, guerrilla
clonal growth patterns—spreading rather than clumping placement of ramets—
may prevent the association of particular genotypes with certain habitats, decreasing
the potential for population differentiation and local adaptation.
Phenotypic plasticity is another response to heterogeneous environmental conditions
(Schlichting and Pigliucci 1998, Via and Lande 1985), which may evolve when
selective pressures fluctuate frequently (Schlichting and Pigliucci 1998). The ability
to respond plastically is a means to deal with environmental variability, as adjusting
its phenotype in response to particular environmental cues may allow an individual
to maximize its fitness potential (Schlichting and Pigliucci 1998). Disturbances to
the coastal dune habitat are frequent relative to its plant inhabitants’ long life spans.
Erosion can result in the removal of foredune habitat, exposing plant populations
previously situated on the backdunes to the unprotected foredune conditions. Alternately,
sand accretion can result in the building of new dunes along the shoreline, so
that established plant populations experience changed conditions when new dunes
act as buffers from salt spray and sand movement. If the time scale of disturbance is
shorter than the lifespan of an individual genet, one individual may produce multiple
phenotypes—through clonal reproduction of daughter ramets—in response to fluctuating
environmental conditions (van Kleunen and Fischer 2001). Thus, processes
such as phenotypic plasticity may facilitate an individual’s success in environments
which experience frequent disturbances.
We used Uniola paniculata L. (Poaceae) (Sea Oats) to investigate populationlevel
evolutionary responses, since its natural history, as a rhizomatous perennial
grass which reproduces both clonally and sexually, is representative of many dune
plant species (Wagner 1964). Uniola paniculata is a federally protected plant due
to its role in stabilizing dune habitats, and is frequently used in dune restoration
2011 C.L. Gormally and L.A. Donovan 753
projects. Uniola paniculata is the dominant plant species on the primary dunes
where it occurs, from Virginia southward along the Atlantic coast to the Bahamas
and along the Gulf coast to Veracruz, Mexico (Wagner 1964). On the dunes,
U. paniculata’s habitat spans a localized dune gradient from the dynamic foredunes
(embryonic dunes) to the older, stabilized backdunes situated farther inland.
We investigated whether foredune and backdune populations of U. paniculata
were locally adapted to microhabitats across its shoreline-to-landward range on
the coastal sand dunes. In a previous study, we documented intraspecific trait
variation in populations of Uniola paniculata located in the dynamic foredunes
and on the stabilized backdunes that mirrored the underlying habitat variation
(Gormally and Donovan 2010). However, that study could not identify the underlying
process driving this variation. A previous allozyme study demonstrated
that there is no strong regional pattern of genetic structure among populations of
U. paniculata, but fine-scale clonal structure and diversity vary widely, perhaps
resulting from localized microhabitat differences (Franks et al. 2004). In this
manipulative study, we asked whether populations of U. paniculata were locally
adapted to foredune and backdune microhabitats as evidenced by ramets transplanted
from their source habitats outperforming ramets from the other habitat.
Field Site and Species Description
All fieldwork was conducted on the coastal sand dunes at the National Estuarine
Research Reserve System (NERRS) on Sapelo Island, located on the Atlantic
coast of Georgia. Sapelo Island beach dunes are protected through the NERRS, as
well as through the Georgia Coastal Ecosystems Long Term Ecological Research
Network (GCE-LTER). Thus, these dunes are ideal settings to document the evolutionary
responses of natural coastal dune plant populations, as the dune habitats
are not currently impacted by anthropogenic development or foot traffic.
Vegetation on the Sapelo Island dunes is typical of southeastern Atlantic coastal
dunes, consisting primarily of clonal perennial grass and vine species. Uniola
paniculata reproduces both vegetatively and sexually, although vegetative reproduction
may contribute more to fitness than seed production, as a high percentage
of ovules are aborted (Hester and Mendelssohn 1987). On Sapelo, U. paniculata’s
growing season begins in late March, when plants develop new leaves to replace
the previous year’s leaves, and ends in October, and plants are quiescent during the
winter (Wagner 1964). On average, annual precipitation is 115 cm, with about 80%
occurring during the growing season (2000–2005 Georgia Coastal Ecosystems
Long-Term Ecological Research network data). From July 2006 until November
2008, conditions in coastal Georgia ranged from a moderate to severe drought (National
Oceanic Atmospheric Administration 2009).
Reciprocal transplant plots
We conducted two reciprocal transplant experiments, designated as Summer
2006 and Fall 2007 (referring to the date plants were transplanted into experimental
plots). The foredune habitat was defined as the first 10 m closest to the shoreline,
754 Southeastern Naturalist Vol. 10, No. 4
and the backdune habitat was defined as the farthest inland edge of the species’
range, based on previous characterization of the environment at this study site
(Gormally and Donovan 2010). The foredune habitat is characterized by greater
levels of soil boron, potassium, magnesium, manganese, as well as higher soil salinity,
pH, and sand accretion, than the backdune habitat, which experiences less
sand movement in terms of accretion and erosion and is consequently a more stable
habitat (Gormally and Donovan 2010). For both the Summer 2006 and Fall 2007
experiments, we set up three 3-m2 reciprocal transplant plots in each habitat (foredune
and backdune), with the plots located at least 300 m apart within each of the
habitats (corresponding to 3 of the transects in Gormally and Donovan 2010). Plot
locations in the backdune habitat varied from 40 to 60 m inland from the foredune,
according to the presence of the species, which varied with island geomorphology
(Stallins and Parker 2003). Existing vegetation was cleared from each plot at least
four months prior to transplanting. Plots were weeded to remove other vegetative
growth throughout the course of both experiments.
The U. paniculata ramets were collected from the foredune and backdune
habitats and are referred to as the foredune (FD) and backdune (BD) populations,
respectively. To ensure that ramets represented distinct genets, they were
always dug up from locations separated by at least 3 m (Franks et al. 2004). When
collected, the ramet consisted of a tiller—an aboveground stem formed from an
axillary bud on a maternal ramet—and a variable amount of roots. For the Summer
2006 experiment, we collected 412 total ramets in June 2006 (n = 207 FD ramets
and n = 205 BD ramets). Ramets were treated with Hormex to induce rooting,
fertilized once with Jack’s Peat-Lite fertilizer (NPK 20-10-20; 200 ppm N), and
planted in 25.4-cm3 pots containing sand. Plants were maintained in an open-air
shaded greenhouse on Sapelo to facilitate root growth and minimize environmental
or maternal effects (Roach and Wulff 1987). In August 2006, the surviving ramets
were transplanted into experimental plots, 30 ramets per plot (n = 89 FD ramets and
n = 91 BD ramets), for a total of n = 180 ramets in the Summer 2006 experiment.
Plants were watered for two weeks following transplant to promote establishment.
We measured the stem basal diameter and stem height (from the base of the plant
at the soil surface, to the uppermost node) of the Summer 2006 ramets at transplant
(August 2006) to account for initial size, and then at six months (February 2007)
to assess growth. The Summer 2006 foredune plots were completely washed away
during storms in April 2007. Backdune plots remained, and surviving ramets were
measured for height and stem diameter at 1 year (August 2007), and aboveground
biomass was harvested at 2 years (September 2008). At harvest, ramets were assessed
for stem diameter, height, and number of tillers, i.e., number of new stems.
Biomass was weighed, after drying at 60 °C.
The Fall 2007 experiment was set up after the Summer 2006 foredune plots
were washed away. For this experiment, we adjusted the experimental design to
include more ramets and collected two ramets per genet in order to place the same
genotype into plots in each habitat (360 ramet pairs for a total of n = 720 individual
ramets). Ramets were treated with Hormex to induce rooting and fertilized
once with Jack’s Peat-Lite fertilizer (NPK 20-10-20; 200 ppm N). The ramets
were initially planted in 25.4-cm3 pots of sand and maintained in the University
of Georgia Plant Biology greenhouses in Athens, GA, for approximately 4.5
2011 C.L. Gormally and L.A. Donovan 755
months to facilitate root growth and minimize environmental or maternal effects
(Roach and Wulff 1987). Ramets were watered regularly. In November 2007,
the 326 surviving ramets (161 FD and 165 BD ramets) were transplanted into
experimental plots, and were watered for two weeks to promote establishment.
We measured the wet weight of the Fall 2007 ramets at transplant to account for
initial size. The Fall 2007 foredune plots washed away during storms in December
2007, so no growth measurements were possible for these plots. The Fall
2007 backdune plots remained and surviving ramets were harvested after 1 year
(September 2008). At harvest, ramets were assessed for stem diameter, height,
number of tillers, and dry biomass (dried at 60° C).
For the Summer 2006 experiment, we compared survival and growth of FD
and BD ramets in foredune and backdune plots at 6 months, and in backdune
plots at 1 year and at harvest. Survival was analyzed with a chi square (χ2) test.
Growth traits (stem diameter, height, and biomass) of surviving ramets met assumptions
of normality and heterogeneity and were analyzed with analysis of
covariance (ANCOVA), with source population as the main predicting variable.
For the Summer 2006 experiment, stem diameter or height at transplant was included
as the covariate. For the Fall 2007 experiment, wet weight at transplant
was included as the covariate. To determine whether the amount of vegetative
reproduction, as indicated by number of tillers, differed between populations, we
used analysis of variance (ANOVA).
Summer 2006 experiment
At six months after transplantation into reciprocal transplant plots, there was
no evidence that either FD or BD ramets were locally adapted to their respective
home habitats, using non-destructive growth measurements of height and stem
diameter (Tables 1 and 2). Although there was a significant population effect for
stem diameter in the backdune plots (Table 2), with FD plant stem diameter having
increased more than that of BD ramets during this interval, this is opposite
of the pattern that would support local adaptation in the backdune habitat. Nor
was there evidence of differential survival in either habitat (foredune: χ2 = 0.37,
df = 3, n = 89; backdune: χ2 = 0, df = 3, n = 90).
After a year of growth in the backdune plots, there were no differences between
FD and BD ramets for survival (χ2 = 0.79, df = 3, n = 90), height or stem
diameter (Table 2). At harvest, only 15 ramets were alive (7 FD and 8 BD ramets),
and there was no differential survival of FD and BD ramets (χ2 = 0.00891, df = 3,
n = 90). There were also no differences in size of surviving FD and BD ramets
at harvest: stem diameter, height, total dry biomass, and vegetative reproduction
(estimated as number of tillers produced).
Fall 2007 experiment
At harvest, only 13 ramets (7 FD and 6 BD ramets) remained alive in the
backdune plots, with no differential survival by population (χ2 = 0.05829, df = 3,
756 Southeastern Naturalist Vol. 10, No. 4
n = 156). At harvest, there were no differences between populations in stem diameter
or height, total dry biomass or number of tillers produced (Table 3).
Our study contributes to the small but growing body of evidence that clonal
plants are more likely to respond through the strategy of phenotypic plasticity
than local adaptation in frequently disturbed habitats. Using measurements of
growth in the field and, most significantly, survival in the field during an extreme
drought, we found no evidence that populations of U. paniculata were locally
adapted to specific microhabitats across a shoreline-to-landward environmental
gradient. This finding helps to explain the results from a previous study that
showed that FD ramets were morphologically and physiologically different (larger
in height, stem diameter, number of nodes, with higher aboveground tissue
concentrations of N and K) and more likely to flower than ramets located farther
inland (Gormally and Donovan 2010). The phenotypic variation detected in the
previous study mirrored the underlying environmental variation, with increased
soil salinity, sand accretion, soil pH, and concentrations of soil nutrients (B, K,
Mg, and Na) in the 10-m interval closest to the shoreline. In order to demonstrate
that populations were locally adapted, transplant populations would have to outperform
populations from other sites in the reciprocal transplant plots located in
Table 1. Uniola paniculata growth for Summer 2006 experiment foredune plots measured at 6 months
after transplant. Least square means (LSmeans) ± 1 SE are presented. (*** signifies P ≤ 0.0001).
BD ramets FD ramets (initial size) Population
Height (cm) 14.173 ± 0.952 13.422 ± 0.967 F(1,62) = 45.44*** F(1,62) = 0.29
Stem diameter (cm) 2.450 ± 0.165 2.898 ± 0.187 F(1,68) = 2.05 F(1,68) = 3.17
Table 2. Uniola paniculata growth (measured in cm) for Summer 2006 experiment backdune plots
measured at 6 months, 1 year, and harvest (2 years). LSmeans ± 1 SE are presented. (* signifies P <
0.05, ** signifies P < 0.01, *** signifies P ≤ 0.0001).
BD ramets FD ramets (initial size) Population
Measurements at 6 months
Height 11.821 ± 1.086 13.058 ± 1.073 F(1,82) = 39.26*** F(1,82) = 0.61
Stem diameter 2.327 ± 0.187 3.051 ± 0.182 F(1,81) = 0.69 F(1,81) = 7.34**
Measurements at 1 year
Height 53.872 ± 2.691 54.228 ± 2.812 F(1,72) = 9.08* F(1,72) = 0.01
Stem diameter 3.808 ± 0.264 3.774 ± 0.272 F(1,67) = 0.03 F(1,67) = 0.01
Measurements at harvest
Biomass 9.899 ± 1.752 10.431 ± 1.874 F(1,12) = 1.01 F(1,12) = 0.04
Height 50.333 ± 3.817 57.705 ± 4.083 F(1,12) = 1.79 F(1,12) = 1.72
Stem diameter 3.370 ± 0.323 3.838 ± 0.426 F(1,10) = 0.02 F(1,10) = 0.65
Tillers 2.375 ± 0.676 2.429 ± 0.723 n/a F(1,13) = 0
2011 C.L. Gormally and L.A. Donovan 757
their source habitat (Linhart and Grant 1996, Kawecki and Ebert 2004). However,
there was no evidence from our reciprocal transplant experiments to support the
hypothesis that the trait variation previously documented in naturally occurring
populations was due to local adaptation or genetic differentiation of any kind.
Early growth measurements from both the foredune and backdune plots
provided no evidence of population differentiation. At harvest, there was no differential
survival by population in the backdune plots. Though the plots in the
backdune habitat were relatively unaffected by the nor’easters, mortality was
substantial, with <10% of ramets from both the Summer 2006 and Fall 2007
experiments surviving to the time of harvest. Growth and survival following transplantation
were likely negatively impacted by the moderate to extreme drought
conditions which lasted from May 2006 to August 2008. Nearly 30% of the ramets
in the Summer 2006 experiment died during the six-month period of May–December
2007, characterized as a severe to extreme drought, according to the NOAA
Palmer Drought Index (National Oceanic Atmospheric Administration 2009)
Our results are consistent with the few studies that explicitly test for local
adaptation of a coastal perennial. Populations of Triplasis purpurea (Walter)
Chapm. (Purple Sandgrass) located 15 m and 80 m from the shoreline, were not
locally adapted (Cheplick and White 2002). Foredune and mature dune populations
of Ammophila arenaria (L.) Link (European Beachgrass) were not locally
adapted (Gray 1985). Although a study of the coastal perennial vine Hydrocotyle
bonariensis Comm. Ex Lam (Largeleaf Pennywort) did find evidence of local adaptation,
these populations were adapted to microhabitats located at high and low
dune heights (Knight and Miller 2004), different from the microhabitats tested in
our study. Additionally, local adaptation of H. bonariensis appeared to be associated
with interactions of the surrounding vegetation at each microhabitat (Knight
and Miller 2004).
Most dune plant species are clonal perennials, propagating both by sexual and
vegetative reproduction. Though clonal reproduction does not include recombination,
clonal plants are not less genetically diverse (Hamrick and Godt 1989).
One expectation is that clonality would increase the likelihood of local adaptation
through the reduction of gene flow and increased placement of a genet’s ramets
into preferentially locally adaptive sites. However, clonality may reduce local
adaptation if genets are adapted to historical environmental conditions, so that
populations might be genetically differentiated, but not locally adapted to their current
habitat, but instead to past conditions (Callaghan et al. 1996). Local adaptation
Table 3. Uniola paniculata measurements at harvest (1 year) for the Fall 2007 experiment. LSmeans
± 1 SE are presented. (* signifies P < 0.05).
BD ramets FD ramets (initial size) Population
Biomass (g) 4.756 ± 0.80 3.22 ± 0.738 F(1,10) = 7.66* F(1,10) = 1.91
Height (cm) 33.259 ± 7.743 32.564 ± 7.144 F(1,10) = 1.33 F(1,10) = 0
Stem diameter (cm) 2.686 ± 0.395 2.259 ± 0.365 F(1,10) = 3.22 F(1,10) = 0.60
Tillers (#) 1.333 ± 0.182 1.143 ± 0.169 F(1,10) = 0.05 F(1,10) = 0.41
758 Southeastern Naturalist Vol. 10, No. 4
may also be constrained when strong gene flow prevents differentiation, when
selection is constrained by low amounts of genetic variation, or when natural selection
fluctuates due to strong spatial variability. Given the changeable nature of the
coastal sand dune environment, one genet might experience multiple environments
over the course of its life. Under fluctuating environmental conditions, plasticity
may be more likely to have evolved than local adaptation (Mitchell-Olds 1992,
Richards et al. 2010). In a greenhouse study of the clonal salt marsh perennial Borrichia
frutescens L. (Bushy Seaoxeye), Richards et al. (2010) determined that trait
variation along a salinity gradient was due to phenotypic plasticity. In this common
garden study, plants from microhabitats along the salinity gradient did not respond
differentially to salinity treatments; instead, plants responded plastically for traits
measured. It seems likely that the trait variation we previously documented (Gormally
and Donovan 2010) was a result of phenotypic plasticity rather than local
adaptation (Schlichting and Pigliucci 1998, Via and Lande 1985).
Our study highlights some of the challenges implicit in addressing the question
of how long-lived plants respond to selection in a variable environment that
may change through the course of an individual’s lifetime. Since U. paniculata
is a long-lived perennial grass, identifying the environments to which these
populations may be adapted is difficult. On the coastal dunes, selective pressures
may fluctuate frequently, particularly due to periods of sand accretion and erosion,
sometimes resulting in the removal of foredunes, exposing populations of
plants previously situated on the backdunes to increased disturbance from sand
accretion and erosion. Further, as a clonal plant capable of sexual and vegetative
reproduction, U. paniculata may respond to its environment at both the level of
the genet—through the processes of local adaptation or phenotypic plasticity—
and the ramet—through physiological integration, developmental plasticity, and
selective placement of daughter ramets (van Kleunen and Fischer 2001). We used
ramets rather than seedlings due to concerns about seedling survival following
transplantation, but it is possible that seedlings might respond differently than
ramets to transplantation and might differ in average lifespan. Additional knowledge
about the average lifespan of an individual ramet, as well as the average
lifetime of a genet, would enhance the study of evolutionary responses of plant
populations to the environmental heterogeneity of coastal dune systems.
Our finding of no evidence of local adaptation in U. paniculata may have
positive implications for conservation and restoration efforts. Despite dramatic
environmental differences across the coastal dunes, we found no evidence that
populations are locally adapted across dune microhabitats. This means that plant
material for this species can be sourced without concern for the visible intraspecifi
c variation that exists in these microhabitats, since genotypes from each
microhabitat should be equally likely to flourish in any habitat. However, studies
of populations across the species’ entire range are needed in order to understand
larger geographic patterns. Current conservation and restoration considerations
underscore the necessity for acquiring a better understanding of the evolutionary
responses of natural plant populations on the coastal dunes, despite the challenges
implicit in addressing these questions.
2011 C.L. Gormally and L.A. Donovan 759
The authors wish to thank the following people for help with fieldwork: Beau Brouillette,
Jason Bonner, Kate Seader, Allison Hennigan, Anna Johnson, Nicole Umberger,
Meredith Barton, Victor Thompson, Scott Gevaert, Katherine Hale, Anna Harvey, Mike
Boyd, Samantha Carvalho, Patrick Gormally, Maggie Kilgo, and Haley Zapal. Research
at the Sapelo Island National Estuarine Research Reserve (SINERR) was facilitated by
the research reserve coordinator, Dorset Hurley, and by Jon Garbisch at the University of
Georgia Marine Institute. Tom Patrick at the Georgia Department of Natural Resources
Wildlife Resources Center provided invaluable help in the permitting process for ramet
collection. We thank Jim Hamrick and Eleanor Pardini for insightful comments throughout
the experimental design process. We thank SINERR (NOAA) (NA07NO54200039),
Sea Grant (NA04OAR4170033), and the Georgia Botanical Society for financial support.
This is contribution number 988 from the University of Georgia Marine Institute.
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