41-B 4 JANUARY 2019 • VOL 363 ISSUE 6422 sciencemag.org SCIENCE
RESEARCH
OPTICS
Exceptional points in
optics
Many complex systems operate
with loss. Mathematically, these
systems can be described as
non-Hermitian. A property of
such a system is that there can
exist certain conditions—excep-
tional points—where gain and
loss can be perfectly balanced
and exotic behavior is predicted
to occur. Optical systems gener-
ally possess gain and loss and so
are ideal systems for explor-
ing exceptional point physics.
Miri and Alù review the topic of
exceptional points in photonics
and explore some of the possible
exotic behavior that might be
expected from engineering such
systems. —ISO
Science, this issue p. 42
NEUROSCIENCE
Forgetting and receptor
removal
The trafficking of AMPA recep-
tors to and from the surface
of postsynaptic membranes
regulates synaptic strength
and underlies learning and
memory. Awasthi et al. found
that the integral membrane
protein synaptotagmin-3 (Syt3)
is predominantly found on
postsynaptic endocytic zones
of neurons, where it promotes
AMPA receptor internaliza-
tion (see the Perspective by
Mandelberg and Tsien). In Syt3
overexpressing or knockdown
neurons, synaptic transmis-
sion and short-term plasticity
were unchanged. However, in
neurons from Syt3 knock-
out mice, synaptic long-term
depression was abolished and
decaying long-term potentiation
endured. In Syt3 knockout mice,
spatial learning was unaltered;
however, these animals showed
signs of impaired forgetting and
relearning during the water maze
spatial memory task. —PRS
Science, this issue p. 44;
see also p. 31
IMMUNOLOGY
Commensal-specific
T cells are flexible
Barrier tissues, like the skin, are
sites where noninvasive com-
mensal microbes constantly
interact with resident T cells.
These encounters can result
in commensal-specific T cell
responses that promote, for
example, host defense and tis-
sue repair. Harrison et al. show
that subsets of skin-resident
commensal-specific interleukin-
17A–producing CD4+ and CD8+
T cells have a dual nature:
They coexpress transcription
factors that direct antagonistic
antimicrobial (type 17) and
antiparasite and pro–tissue
repair (type 2) programs. When
skin is damaged, epithelial cell
alarmins license type 17 T cells
to turn on type 2 cytokines.
Thus, commensal-specific type
17 T cells can direct antimicro-
bial activity under homeostatic
conditions but rapidly turn on
tissue repair in the context of
injury. —STS
Science, this issue p. 43PLANT SCIENCE
Fixing photosynthetic
inefficiencies
In some of our most useful
crops (such as rice and wheat),
photosynthesis produces toxic
by-products that reduce its
efficiency. Photorespiration
deals with these by-products,
converting them into metaboli-
cally useful components, but at
the cost of energy lost. South et
al. constructed a metabolic path-
way in transgenic tobacco plants
that more efficiently recaptures
the unproductive by-products
of photosynthesis with less
energy lost (see the Perspective
by Eisenhut and Weber). In field
trials, these transgenic tobacco
plants were ~40% more produc-
tive than wild-type tobacco
plants. —PJH
Science, this issue p. 45;
see also p. 32NANOMATERIALS
Nanographenes on oxides
The growth of nanographene
islands and ribbons on metal
surfaces can be accomplished
on single-crystal metal surfaces
through carbon-carbon coupling
reactions, but the surfaces of
oxides do not assist these reac-
tions. Kolmer et al. show that
fluorinated aryl groups can be
coupled to form nanographenes
on the rutile surface of titanium
oxide. The fluorine substitution
of the aryl groups was selected
so that as the carbon-fluorine
bonds were thermally activated,
a stepwise process sequentially
added aromatic rings around
a central aryl group until it was
completely substituted. —PDS
Science, this issue p. 57MESOSCOPIC PHYSICS
A backward current
Two-dimensional materials in a
magnetic field can exhibit the so-
called quantum Hall effect. This
regime is characterized by cur-
rents running along the edge of
the sample in the “downstream”
direction determined by the sign
of the magnetic field. Lafont et
al. studied electrical transport in
GaAs-AlGaAs heterostructures,
focusing on a previously less-
studied spin-unpolarized state
in the fractional quantum Hall
regime. By considering various
experimental configurations,
they observed a component of
the charge current flowing in the
opposite, “upstream” direction.
—JS
Science, this issue p. 54NEUROSCIENCE
Another primary visual
cortex
Most functional studies in the
visual system have focused on
the cortical representation of
the geniculo-striate pathway
that links the retina to the
cortex. The parallel collicular
pathway is believed to sparsely
project throughout the visualcortex and have a modulatory
role on cortical responses to
visual stimuli. Beltramo and
Scanziani found a visual cortical
area that is entirely dedicated
to the superior colliculus. This
area can discriminate moving
visual stimuli that the “classical”
primary visual cortex cannot.
Thus, the superior colliculus, a
phylogenetically ancient struc-
ture, has its own projection in
neocortex that provides this area
with exquisite feature-detection
abilities not found in the classical
primary visual cortex. —PRS
Science, this issue p. 64CLIMATE CHANGE
Deep Pacific cooling
Earth’s climate cooled consider-
ably across the transition from
the Medieval Warm Period to
the Little Ice Age about 700
years ago. Theoretically, owing
to how the ocean circulates, this
cooling should be recorded in
Pacific deep-ocean tempera-
tures, where water that was on
the surface then is found today.
Gebbie and Huybers used an
ocean circulation model and
observations from both the end
of the 19th century and the end
of the 20th century to detect
and quantify this trend. The
ongoing deep Pacific is cooling,
which revises Earth’s overall heat
budget since 1750 downward by
35%. —HJS
Science, this issue p. 70EVOLUTION
DNA breakage and
adaptation
Adaptation to new environments
often occurs in similar ways
across different colonization
events. Stickleback fish repre-
sent a classic example of this, in
which repeated colonizations of
freshwater have resulted in the
loss of pelvic hind fins. Previous
work has shown that a pelvic
enhancer gene is involved. Xie et
al. now show that this gene lies
within a region of the genomeEdited by Stella Hurtley
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