[PDF](+1👁️) Télécharger Snake and Bird Predation Drive the Repeated Convergent Evolution of Correlated Life History Traits and Phenotype in the Izu Island Scincid Lizard (Plestiodon latiscutatus). pdf
This article is from PLoS ONE, volume 9.Abstract
Predation may create strong natural selection pressure on the phenotype and life history characteristics of prey species. The Izu scincid lizards (Plestiodon latiscutatus) that inhabit the four Japanese Izu Islands with only bird predators are drab brown, mature later, lay small clutches of large eggs, and hatch large neonates. In contrast, skinks on seven islands with both snake and bird predators are conspicuously colored, mature early, lay large clutches of small eggs, and hatch small neonates. We test the hypothesis that these suites of traits have evolved independently on each island via natural selection pressures from one of two predator regimes – birds-only and birds + snakes. Using two mtDNA genes and a nuclear lTélécharger gratuit Snake and Bird Predation Drive the Repeated Convergent Evolution of Correlated Life History Traits and Phenotype in the Izu Island Scincid Lizard (Plestiodon latiscutatus). pdf
OPEN 3 ACCESS Freely available online
•$-PLOS I ONE
Snake and Bird Predation Drive the Repeated
Convergent Evolution of Correlated Life History Traits
and Phenotype in the Izu Island Scincid Lizard
(Plestiodon latiscutatus)
Matthew C. Brandley 1 *, Takeo Kuriyama 2 , Masami Hasegawa 2
1 School of Biological Sciences, University of Sydney, Sydney, NSW, Australia, 2 Faculty of Science, Toho University, Funabashi City, Chiba, Japan
Abstract
Predation may create strong natural selection pressure on the phenotype and life history characteristics of prey species. The
Izu scincid lizards (Plestiodon latiscutatus) that inhabit the four Japanese Izu Islands with only bird predators are drab brown,
mature later, lay small clutches of large eggs, and hatch large neonates. In contrast, skinks on seven islands with both snake
and bird predators are conspicuously colored, mature early, lay large clutches of small eggs, and hatch small neonates. We
test the hypothesis that these suites of traits have evolved independently on each island via natural selection pressures from
one of two predator regimes - birds-only and birds + snakes. Using two mtDNA genes and a nuclear locus, we infer a time-
calibrated phylogeny of P. latiscutatus that reveals a basal split between Mikura and all islands south, and Miyake, all islands
north, and the Izu Peninsula. Populations inhabiting Miyake, Niijima, Shikine, and Toshima are not monophyletic, suggesting
either multiple colonizations or an artifact of incomplete lineage sorting (ILS). We therefore developed novel phylogenetic
comparative analyses that assume either a multiple colonization or more restrictive single colonization ILS scenario and
found 1) statistically significant support for the of different suites of phenotypic and life history characteristics with the
presence of bird-only or bird + snake predator assemblages, and 2) strong phylogenetic support for at least two
independent derivations of either the "bird-only" or "snakes + birds" phenotypes regardless of colonization scenario. Finally,
our time-calibrated phylogeographic analysis supports the conclusion that the ancestor to modern Izu Island P. latiscutatus
dispersed from the mainland to the Izu proto-islands between 3-7.6 million years ago (Ma). These lineages remained
present in the area during successive formation of the islands, with one lineage re-colonizing the mainland 0.24-0.7 Ma.
Citation: Brandley MC, Kuriyama T, Hasegawa M (2014) Snake and Bird Predation Drive the Repeated Convergent Evolution of Correlated Life History Traits and
Phenotype in the Izu Island Scincid Lizard {Plestiodon latiscutatus). PLoS ONE 9(3): e92233. doi:10.1371/journal.pone.0092233
Editor: Corrie S. Moreau, Field Museum of Natural History, United States of America
Received November 24, 2013; Accepted February 19, 2014; Published March 25, 2014
Copyright: © 2014 Brandley et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was partially supported by Grants-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science, and
Technology to M.H. Hasegawa (19570026, 21570024 and 24570031), and the Sasakawa Scientific Research Grant from The Japan Science Society and the Fujiwara
Natural History Foundation to T.Kuriyama. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the
manuscript
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: mbrandley@gmail.com.
Introduction
Predation may result in strong selection on prey phenotype and
life history. For example, predation by visually-orienting predators
may impose a strong natural selection pressure to evolve a cryptic
color pattern in prey species, or life history traits that improve
predator avoidance [1]. Extensive studies of poeciliid fish have
demonstrated an association between high rates of predation and a
maternal life history strategy that favors early sexual maturity and
high fecundity (i.e., clutch size) at the expense of offspring size
(e.g., [2-5]). That this life history strategy has evolved multiple
times both within and amongst species in high predation
environments is strong evidence that this life history strategy is a
response to natural selection due to predation.
Detecting this convergent evolution of life history requires
knowledge of a species' evolutionary and/or biogeographical
history. For example, if two populations share a unique phenotype,
but genetic data reveals they are also sister lineages, we could
conclude that the phenotype evolved once prior to the splitting of
the lineage. Similarly, if isolated habitats with no connectivity are
independently colonized by the same species, these populations
each become a "natural experiment" to test how species evolve in
response to different selection pressures.
Because oceanic islands form with no connection to the
mainland and are colonized only by limited dispersal, they serve
as 'blank slates' on which we can observe the varied outcome of
ecological and evolutionary processes. One such oceanic island
model system is the Japanese Izu Islands (Fig. 1). The Izu Islands
are an ideal study system because of their geologic and taxonomic
diversity, range in island size (approximately 10 to 9908 ha), and
distance of islands from the mainland (~23 to ~260 km). In
particular, the skink Plestiodon latiscutatus (formerly Eumeces okadae
[6]) inhabits all Izu Islands, yet one or more of its major predators
the Japanese weasel (Mustek itatsi), Izu Island Thrush (Turdus
celaenops), and four-lined ratsnake (Elaphe quadrivirgata) inhabits
every island (see Table 1). For example, M. itatsi is native only to
Oshima (although it was subsequently introduced to, and persists
in, Toshima, Miyake, Hachijqjima and Aogashima [8-10]); at
PLOS ONE | www.plosone.org
1
March 2014 | Volume 9 | Issue 3 | e92233
Convergent Evolutionary Responses to Predation
least one species of predatory bird (especially T. celaenops) inhabits
all the major Izu Islands [11,12]; ratsnakes inhabit most, but not
all of the islands ([12]; Fig. 1). The islands' similarity in climate and
vegetation makes them a unique test case to evaluate the effects of
different predator assemblages on the evolution of life history of
prey species. Moreover, as previous phylogeographic research has
revealed varied patterns of historical colonization of the Izu Islands
in Campanula plants [13,14], Euhadra snails [15], Apodemus mice
[16—18], and the four-lined ratsnake, Elaphe quadrivirgata [19], it is
likely that the order in which these islands were colonized would
also substantially affect subsequent ecological and phenotypic
evolution of the species community (e.g., whether the colonizer
was from a distant, mainland-adapted or closer, island-adapted
population).
Previous research has identified suites of life history traits in the
skink P. latiscutatus that appear to correlate with each island's
predator assemblage [8,9,12,20-22]. For example, skinks on
Oshima (the largest island that is nearest to the mainland and
with the most taxonomically diverse assemblage of predators)
mature earlier with a small body and lay comparatively large
clutches of small eggs [12]. Whereas skinks that inhabit bird-only
islands mature later with a large body size and lay smaller clutches
with larger eggs [12]. Moreover, there are striking differences in
color pattern between P. latiscutatus populations that inhabit snake
+ bird and bird-only predator islands. Skinks on snake + bird
islands have a dorsal color pattern with vivid yellow stripes and
pronounced blue tail in juveniles ([24]; Fig. IB), whereas skink
populations on bird-only islands are uniformly drab brown in
colour (Fig. 1C). Given the similar habitats amongst the islands,
these results suggest a causal link between the presence of
particular predators and the life history and color pattern
evolution of their skink prey.
Evidence that these phenotype and life history traits evolved
independently on two more islands with the identical suites of
predators would indicate that predator diversity was the primary
natural selection pressure. However, because phylogenetic infor-
mation was unavailable when these previous life history studies
were conducted, it was not possible to disentangle these potential
cases of repeated in situ convergent evolution via natural selection
from a pattern caused by evolution and subsequent dispersal
amongst islands. For example, if skink populations on bird-only
islands were not closely related, it would suggest that their
phenotype evolved independently on each island, thereby
suggesting convergent evolution due to natural selection pressure
by bird predators. On the other hand, if all bird-only predator
island skink populations shared a recent common ancestor
exclusive of bird + snake island populations, then one could not
distinguish the possibility of selection on life history and phenotype
from a founder effect and subsequent dispersal to other islands.
We evaluate the diversity of phenotype and life history strategies
of P. latiscutatus on the Izu Islands in a newly constructed
phylogenetic framework. We use phylogenetic comparative
methods to evaluate the relationships between a suite of life
history characteristics and color pattern and link those results to an
island's predator assemblage. Finally, because the identity of island
colonizers and their relative timing of colonization shape the
evolution of community assemblages (e.g. [23]), it is important to
uncover general biogeographic patterns in the island system. We
therefore also compare the pattern of P. latiscutatus colonization of
the Izu Islands with that of other taxa inhabiting the islands by
consulting the latest knowledge on geological history and
geochronology of island formation of the Izu-Bonin Island arc
[25-27].
Methods
Study Islands, Life History and Phenotypic Data
Collection
Life history traits and juvenile color pattern of Plestiodon
latiscutatus (formerly Eumeces okadae; [6]) were studied in ten of the
Izu Islands (Oshima, Toshima, Niijima, Shikine, Kozu, Tadanae,
Miyake, Mikura, Hachijokojima and Aogashima), in the Izu
Peninsula (Japanese mainland) and in the small offshore island
(Hatsushima) off the east coast of the Izu Peninsula (Fig. 1). These
islands, ranging in area from 10 to 9,908 ha, are all volcanic and
located off the south coast of central Japan linearly from north
(Oshima) to south (Aogashima) over the distance of ca. 230 km
(Fig. 1). The climates, under the influence of the warm temperate
water of the Kuroshio Current, are uniformly mild with average
air temperatures of 16.2-17.9 G. Because of rich annual rainfall
reaching 2000-3000 mm, the islands are well vegetated with
broad-leaved evergreen forest dominated by Castanopsis cuspidata
and Machilus thunbergii and secondary forest dominated by the
deciduous trees Alnus sieboldiana and Hydrangea macrophylla. Terres-
trial reptile and mammalian faunas of the Izu Islands and Izu
Peninsula are summarized in Table 1. Predation regime (fauna
and abundance), consequence of predatory mammal introduction,
prey resource use and other ecologically relevant information can
be found in the literature [9,12].
Protocols, procedures and methods to obtain life history data
both from the field and laboratory followed Hasegawa [9]. In
brief, life history traits were based on data from intensive mark-
recapture studies on Miyake from 1977 to 1984 [9,21], and from
less intensive mark-recapture studies conducted for the six other
insular populations (Oshima, Toshima, Shikine, Kozu, Mikura,
and Aogashima) from 1981 to 1984. The two Izu Peninsula
populations (Daiyusan and Hiekawa), an offshore island (Hatsush-
ima) and two Izu Islands (Niijima and Hachijokojima) were
studied with mark-recapture methods from intermittently from
1994 to 2012. Snout-vent length (SVL) and body mass were
measured to the nearest 1 mm and 0. 1 g, respectively. The lizards
were sexed, males were considered mature if exhibiting reddish
nuptial (red or orange) coloration around the head. Maturity and
reproductive conditions of females were determined [28] . In the
spring (April-May), gravid (reproductive) females were classified as
mature and reproductive in that year if they were either gravid or
spent; otherwise they were classified as either immature or mature,
but non-reproductive. Presence of mature but nonreproductive
females was taken as evidence of missed reproductive opportuni-
ties. Clutch size was determined from the counts of yolked ovarian
follicles, oviductal eggs, and corpora lutea in the female body
cavities and from eggs in natural nests. At least 10 gravid females
captured on each island during the late spring and early summer
were brought back to the laboratory to lay eggs. Females were
individually maintained in plastic containers with damp peat moss
and small flat stones for a nesting site. Within 1 2 h of egg laying,
the body masses of post-egg-laying (spent) females and of
individual eggs were measured to the nearest mg. SVL, tail
length, and body mass of hatchlings were measured to the nearest
mm and mg within one day of hatching. Stripe pattern and blue
tail coloration were individually scored for the hatchling lizards.
The vividness of each of the head stripe and five body stripes was
scored subjectively from absent (0), faint (1), obscure (2) and vivid
(3) for dorsal, dorso-lateral and lateral stripes both in bead and
body, and a sum of score for each stripe was calculated for each
hatchling. Thus, sums of stripe scores varied from 0 for non-stripe
to 18 for the most intensely striped individuals. For blue tail
coloration, we measured the length of pure blue colored portion of
PLOS ONE | www.plosone.org
2
March 2014 | Volume 9 | Issue 3 | e92233
Convergent Evolutionary Responses to Predation
V Aogashima 59 km
Figure 1. A. Map of the Izu Islands and nearby mainland Japan. The weasel {Mustelo itotsi), snake {Elaphe quodrivirgata), and bird {Turdus
celaenops) icons indicate whether those predators historically inhabit the island (the weasel has been subsequently introduced to Toshima, Miyake,
PLOS ONE | www.plosone.org
3
March 2014 | Volume 9 | Issue 3 | e92233
Convergent Evolutionary Responses to Predation
Hachijojima and Aogashima since the 1930s). Interior bathymetric lines indicate a depth of 100 m and exterior lines a depth of 200 m unless
otherwise specified; B. typical striped morph of Plestiodon latiscutotus inhabiting islands with snake predators (Oshima, Toshima, Niijima, Shikine,
Kozu, Tadanae, and Mikura); C. typical drab morph of the same species inhabiting bird-only predator islands (Miyake, Hachijojima, Hachijokojima, and
Aogashima).
doi:1 0.1 371 /journal, pone.0092233.g001
tail (without the anterior stretch of black body stripes), and its
proportion to the total length of tail was calculated for the
individual lizards.
DNA Collection and Sequencing
We sampled multiple individuals from each of the major Izu
Islands and the small island of Tadanae and from three
"mainland" populations including Atami, Izu Peninsula, and
Hatsushima, a continental island population off the east coast of
the Izu Peninsula (Fig. 1; Table 1). We also sampled two
individuals of P. japonicus as outgroups as numerous phylogenetic
studies have inferred it as the sister lineage to P. latiscutatus [29—
31,74].
DNA was isolated from tissue using Qiagen DNeasy columns.
We amplified two mitochondrial genes (cyt b and ND1) using
primers CB1 and CB6THR [32] for cyt b; newly developed
primers ND1-LATF, 5'-CTC TCC CTA ATC ATG CAC CCA
TTT TTC AC-3' and ND1-LATR, 5'-TGA GCT CCT TAG
TGG AGG TTG AGA TCC TG-3' for ND1; and one nuclear
gene (R35) using primers R35F and R35R [30] using standard
polymerase chain reaction (PCR) techniques (95 °C for 2 min
followed by 40 cycles of 95°C for 30 s, 55°C for 30 s and 72°C for
60 s). PCR products were cleaned using ExoSap-IT (USB Corp.
Ohio, USA). Purified templates were dye-labeled using BigDye
(Applied Biosystems, California, USA) and sequenced on an ABI
3077 automated DNA sequencer (Applied Biosystems) using the
same primers. Nucleotide sequences were examined and aligned
by eye and an open reading frame for these genes were verified
using MacClade v4.08 [33]. The sizes of the final data sets were
953 bp (cyt b), 957 bp (ND1), and 612 bp (R35) for the number of
individuals listed in Table 2. R35 sequences with two or more
polymorphic sites were phased into individual alleles using
Bayesian inference with PHASE 2.1.1 [34,35].
Time-calibrated Phylogenetic and Biogeographic
Analyses
Mitochondrial DNA. Reconstructing colonization history
requires knowledge of a taxon's phylogenetic history and age of
lineage divergences. Bayesian phylogenetic analyses assuming a
relaxed molecular clock permit the simultaneous estimation of
phylogeny, divergence time [36,37], and biogeographic history
[38] while also incorporating rate heterogeneity among lineages
and phylogenetic uncertainty (and thus, estimates of error) in the
tree estimation process. Moreover, these analyses estimate
statistical support for phylogenetic and biogeographic reconstruc-
tions by calculating Bayesian posterior probabilities.
We used beast vl.7.3 [37] to estimate the phylogeny,
divergence times, and biogeographic history using the combined
mtDNA data set (cyt b and ND1) for the 155 sampled individuals
of P. latiscutatus. Because assuming different nucleotide substitution
models for individual data partitions improves both phylogenetic
and divergence time estimation [31,39], we calculated the best
partitioning scheme and substitution models for the codon
positions of each gene using Partitionfinder v 1.0.1 [41], Partition-
finder recommended three total partitions: cyt b codon position
one, ND 1 two; cyt b two, ND 1 three; cyt b three, ND 1 one and the
substitution models TrN + G for partitions one and three, and
HKY + G for partition two.
Estimating divergence times from molecular data requires some
a priori estimate ages for at least one divergence. These are
commonly estimated by incorporating fossil taxa as age constraints
to "calibrate" the relaxed molecular clock. However, there are no
known Plestiodon fossils that can be used as calibration age
constraints. We therefore used the age distribution of the most
recent common ancestor of P. japonicus and P. latiscutatus inferred
by a multi-locus time-calibrated phylogenetic analysis of Plestiodon
[30] as our age calibration constraint. Although secondary
calibrations have been rightly criticized for potentially compound-
ing date estimation error [40], we note that Bayesian age
estimation permits the explicit incorporation of this error by
permitting age calibration constraints (rather than point estimates)
in the form of statistical distributions, thus eliminating at least one
negative feature of secondary calibrations [31]. Also, the age
distribution is broad and thus likely not overly precise (see Results).
Simultaneously with estimating phylogeny and divergence
times, we inferred ancestral biogeographic area using the discrete
traits model of Lemey et al. [38]. We coded all P. latiscutatus
individuals into groups representing each island or mainland
peninsula populations.
Each BEAST analysis was run for 10 generations and sampled
every 2000 generations. We modeled the age of the root of the tree
(P. japonicas+P. latiscutatus) as a normal distribution of ages with a
mean = 6. 3 Ma and standard deviation = 1.38 (95% CI = 3.6—
9.0 Ma; [31]) and enforced a separate lognormal relaxed
molecular clock for the cyt b and ND1 data. We otherwise used
default priors except that we modeled the mean rate of the cyt b
and ND 1 molecular clocks at uniform distributions with bounds of
0.0 and 0.1 substitutions per site. We ran eight BEAST analyses
assuming a birth-death tree prior. To determine convergence
amongst each analysis, we constructed cumulative posterior
probability plots for each clade using the cumulative function in
AWTY [42]. Stationarity was assumed when the cumulative
posterior probabilities of all clades stabilized. If posterior
probability estimates for clades were similar in the analyses, the
results were combined. We interpret posterior probabilities >0.95
as suitably strong support for both phylogenetic reconstruction and
estimation of ancestral biogeographic area [43] .
R35 nuclear locus analyses. Preliminary phylogenetic
analyses of the R35 alleles revealed very little phylogenetic
structure due to very few nucleotide substitutions (only six
parsimony-informative sites), and therefore imprecise information
about the evolutionary history of the sampled P. latiscutatus
populations. We instead visualized the genetic diversity of R35
amongst the sampled localities by calculating the allele frequencies
for each population and visually inspected them for general trends.
Comparative Analyses of Life History Traits
We performed phylogenetic comparative analyses to estimate
correlations amongst life history characteristics (correlations of
phylogenetic independent contrasts) and then linked those life
history traits to the type of predators on multiple Izu Islands
(phylogenetic ANOVA). For statistical analysis, arithmetic means
of the sampled life history and phenotype characters were
converted to independent contrasts to remove non-independence
due to phylogenetic history [44] under different phylogenetic
scenarios (see below). Correlations of independent contrasts of life
PLOS ONE | www.plosone.org
4
March 2014 | Volume 9 | Issue 3 | e92233
CO
vO
LO
v.
as
0
0
0
no
no
no
no
rM
cm
rsi
cm
s
iv
IV
LO
vO
iv
IV
LO
vo
vo
ss
Q
0
O
O
O
0
0
0
O
O
CO
0
q
q
q
q
q
q
q
q
(0
+ 1
d
0
d
d
d
d
d
d
d
c
E
c
■fl
+|
+|
+1
.fl
.fl
+1
+ 1
.fl
o
iv
01
00
CTi
fN
no
~c
w
vO
LO
LO
LO
§
LO
.0
Oi
LU
^
§
s
CO
o»
Z
0
LO
vO
CM
LO
00
00
rsi
vO
O
u>
rsi
no
LO
no
rsi
rsi
rsi
c
o>
0
3L
<u
N
to
'35
+1
CTi
iv
00
no
0
0
,_
CL
c
,— i
,— i
^
fsj
rsi
^
rsi
O
(0
+ l
+ 1
+ 1
+1
+ 1
+ 1
+ 1
+ 1
+ 1
Q_
u
4-*
(V
LO
vO
00
<—
no
fN
00
0
0
3
vd
vd
IV
IV
l<
06
06
06
o\
DIJ
u
LO
vO
as
rsi
1^
m
vO
J2
Z
rsi
no
*t
00
00
rsi
rsi
_co
_N
O
(A
£
CO
vo
00
IV
0
no
rsi
LO
rsi
"D
<U
£
+1
no
-sT
vd
no
<t
CD
c
+ l
+ 1
+ 1
+1
+ 1
+1
+1
+ 1
+ 1
Q-
0
VL
n>
(V
no
O
0
no
00
<3-
rsj
LU
in
d
d
rsi
rsi
00
d
CO
00
00
00
00
00
00
IV
00
00
n:
CO
_C
u
*<5
■M
Z
CD
O
no
IV
LO
'sf
<-
no
IV
C| —
3
0
-Q
co
*0
C
0
SD
CU
CL
'■E
0
O
0
O
0
O
0
0
O
O
ti
O
0
d
d
d
d
d
d
d
>s
a
C
+ l
+ 1
+ 1
+ 1
+ 1
+ 1
+1
+ 1
+ 1
+->
(0
00
iv
as
o\
as
LO
LO
rsi
no
Pr<
a
no
O
no
O
LO
d
d
d
d
LO
O
LO
d
LO
d
CD
_c
Cl
00
no
cm
vo
"D
Z
O
^j-
1^
rsi
C
vO
vo
iv
IV
03
9
>s
O
C
CO
d
0
<u
0
+ 1
vq
vq
,-'
fN
no
^"
co
c
+ 1
+ 1
+ 1
+1
+ 1
+ 1
tri|
n>
£
+ l
+ 1
no
rsi
vd
00
d
+ 1
q
LO
vd
q
vd
no
IV
no
00
no
0
00
00
O
Z
O
fN
rsi
CM
LO
ics
vo
vO
'sf
tis
SD
03
s
O
O
O
0
O
0
O
O
0
+->
co
ti
O
d
d
d
d
d
d
d
d
(A
C
>s
(A
flj
t\
LO
no
0
rsj
rsj
(0
a
LO
vO
LO
vO
LO
LO
imma
§
O
d
d
d
d
d
O
d
d
ns
Z
00
0
,_
no
0
j=
VO
vO
iv
IV
+->
wi
>s
"D
ngs
+->
co
£
SD
no
LO
CM
O
LO
1—
0
co
j=
chli
£
+ 1
c
+ l
+ 1
+ 1
+ 1
+ 1
+ 1
+1
+ 1
+ 1
+-»
+■»
-i
flj
00
LO
VO
rsj
LO
rsj
LO
(0
>
_C
X
oo
rsi
ro
rso
rsi
rsi
rsi
rsi
co
"D
C
_ro
TO
C
co
M—
+
O
CO
CD
a!
a;
qj
15
qj
QJ
CD
+
fT3
TO
c
to
c
TO
C
TO
C
TO
C
-Q
0
"to
sem
edai
rds
rds
rds +
rds +
rds
rds +
UJUJE
rds +
rds +
as:
Q.
CQ
CQ
CO
CQ
CQ
CQ
CQ
in
5—
0
ro
"D
QJ
ol
TO
C
E
Table 1
Populatio
Aogashima
Hachijokoji
Kozu
Mikura
Miyake
Niijima
Oshima
Shikine
Toshima
Convergent Evolutionary Responses to Predation
history traits were performed using the CAPER package in R [45]
using scripts written by M.C.B. We conducted phylogenetic
generalized least squares (PGLS) analysis of variance (phylogenetic
ANOVA) analyses [46] to determine how variation in life history
traits correlates to the presence of predators on each island after
accounting for phylogenetic relationships. There exist three major
classes of skink predators on the Izu Islands including weasels,
birds, and snakes, but we focus only on the potential influence of
snake and bird predation on life history variation. Unlike weasels
that are historically native to Oshima (they were subsequently
introduced into Toshima in the 1930s, Hachijqjima in the 1960s,
Miyake and Aogashima in the 1980s), and predatory birds that
inhabit all islands, snakes inhabit eight of the 1 1 islands sampled
for this study and likely derive from independent colonizations
[19]. Therefore, snakes offer multiple independent opportunities
to assess the affects on the presence of different predators on the
life history traits of their prey, P. latiscutatus. Moreover, snake-less
islands offer an opportunity to assess the effects of bird-only
predation on P. latiscutatus life history and color evolution.
Phylogenetic ANOVA (pANOVA hereafter) analyses were
performed using the GEIGER [47] package in R and scripts
written by M.C.B. We regressed the mean values of both hatchling
and maternal life history traits against a dummy variable coded 0
or 1 (0 = island has snakes; 1 = island does not have snakes). The
overall correlation coefficient (R) represents deviations from the
mean of the comparison group and was tested for significance
using a t-test [46].
Because both independent contrasts and phylogenetic ANOVA
analyses use phylogenetic information to remove the effect of non-
independence caused by the organisms' shared evolutionary
history, the results are therefore fundamentally reliant on the
underlying phylogeny. Inspection of the mtDNA phylogeny (Fig. 2)
reveals multiple populations that are not monophyletic including
the mainland, Miyake, Niijima, Shikine, and Toshima popula-
tions. This pattern could be indicative of multiple colonizations to
these islands (i.e., the pattern represents the true colonization
history), or it could result simply from a stochastic process where
drift has not eliminated older mtDNA haplotypes such as
incomplete lineage sorting (i.e., the pattern is an artifact of
molecular evolution). Incomplete lineage sorting is a phenomenon
where ancestral alleles (or haplotypes, in the case of mtDNA) that
are present before a lineage splits (i.e., when two or more
populations are reproductively isolated) are retained in its
descendant lineages after the divergence [48]. These ancestral
alleles will be lost to genetic drift over time and replaced by new
alleles un
Lire la suite
- 1.20 MB
- 15
Vous recherchez le terme ""
1
22
1
