Page 11 - Asylv_molars_Pmax_revised_2014_01

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direction
of
Pmax.
Note
that
most
islands
such
as
Porquerolles,
Oléron
and
Ré
shared
a
trend

towards
broad
molars
(Fig.
2B).
In
contrast,
Sicilian
molars
evolved
parallel
to
Pmax
but
in
the

opposite
way,
with
slender
molars
than
the
Italian
stock.

Discussion

This
study
evidenced
a
complex
pattern
of
molar
shape
differentiation
in
the
European
wood
mouse

Apodemus
sylvaticus.
Main
features
are:
(1)
the
importance
of
morphological
differentiation

occurring
in
many
islands,
by
far
larger
than
any
divergence
on
continent;
(2)
a
weak
but
significant

differentiation
among
the
phylogenetic
lineages,
with
an
emerging
clustering
of
the
Westernmost

French
localities,
not
identified
until
now;
(3)
a
latitudinal
trend
in
molar
shape.
The
corresponding

directions
of
evolution
were
compared
to
the
direction
of
main
phenotypic
variance
Pmax,
in
order

to
assess
its
potential
role
in
channeling
evolution.

Stability
of
Pmax
across
populations

Pmax
was
evaluated
in
five
well-­‐sampled
populations,
documenting
the
two
main
phylogenetic

lineages
and
one
insular
situation.
Overall,
Pmax
pointed
in
the
same
direction
whatever
the

population,
supporting
the
stability
of
the
underlying
genetic
/
developmental
networks
over

evolutionary
time
scales.
This
is
in
agreement
with
previous
studies,
showing
a
conservation
of
Pmax

along
the
evolutionary
lineage
of
the
wood
mouse,
and
across
related
murine
lineages
over
more

than
10
myr
(Renaud
et
al.
2006;
Renaud
and
Auffray
2013).

Among
these
correlated
vectors,
Pmax
in
Porquerolles
appears
as
the
most
divergent.
Porquerolles
is

a
small
island
(~13
km²)
few
kilometers
off
the
French
Mediterranean
coast.
Inter-­‐specific

competition
and
predation
levels
are
decreased
on
this
islet
(Renaud
and
Michaux
2007),
factors
that

could
promote
fast
adaptation.
Effective
population
size
is
also
highly
reduced
compared
to
the

continent,
favoring
drift
cumulated
to
initial
bottleneck
effect.
Not
mutually
exclusive,
regular

immigration
from
the
nearby
mainland
may
lead
to
admixture
biasing
the
pattern
of
intra-­‐island

morphological
variance
(Guillaume
and
Whitlock
2007). All
these
effects
have
been
shown
to
cause

evolution
in
the
G
and/or
P
matrix
(Roff
2000;
Cano
et
al.
2004;
Roff
and
Mousseau
2005).
Pmax
in

Porquerolles
may
document
a
case
of
local
evolution
in
the
structure
of
the
phenotypic
variance,

promoted
by
reduced
population
size,
despite
the
reduced
genetic
differentiation
from
neighboring

mainland
populations
(Michaux
et
al.
2002).