C R E D I T: AVAGYA NET AL.704 5 NOVEMBER 2021 • VOL 374 ISSUE 6568 science.org SCIENCE
selectivities comparable to Birch
conditions but without the need
for condensed ammonia. —JSY
Science, abk3099, this issue p. 741EVOLUTIONARY BIOLOGY
The variable tempo
of early evolution
The Ediacaran Period, 635
to 539 million years ago, was
marked by the rise of complex
life on Earth. Yang et al. found
that Ediacaran evolution was
not slow and steady but rather
marked by intervals of rapid
diversification interspersed with
dramatic environmental change.
The authors present geochro-
nologic data that better define
the tempo of evolution and how
changes in that tempo relate to
changes in the global carbon
cycle as evidenced by carbon
isotopic measurements of
carbonate rocks in the EdiacaranNEUROSCIENCE
How to quantify local
axonal orientations
Mapping the axonal trajectories
of the brain’s white matter at cel-
lular resolution is a long-standing
goal of neuroscience. However,
existing methods for mapping
the axons are either limited to
animal studies or require highly
specialized equipment for data
acquisition and processing. Nissl
staining identifies cell nuclei
and has been used extensively
to investigate parcellations of
the cortical gray matter, but the
white matter has largely been
neglected with this technique.
Schurr and Mezer now show
that Nissl staining, together with
structure tensor analysis, can
be used to study white matter
architecture and the organization
of the glial cell framework around
axons over the whole brain. This
technique greatly advances
our knowledge regarding the
organization of glial cells and
the fine-grained organization of
axonal projections in the brain.
—PRS
Science, abj7960, this issue p. 762BIOSYNTHESIS
Big molecules build small
Actinomycete bacteria are
prolific producers of bioac-
tive small molecules such as
polyketide antibiotics. These
molecules are built by the addi-
tion of short carbon units to a
growing, protein-tethered chain,
either iteratively as in fatty acid
synthesis or in a modular fashion
by a hand-off from one distinct
enzyme complex to the next.
Bagde et al. and Cogan et al.
report structures of polyketide
synthase modules in action,
taking advantage of antibody sta-
bilization of one of the domains.
Both groups visualized multipleconformational states and an
asymmetric arrangement of
domains, providing insight into
how these molecular assembly
machines transfer substrates
from one active site to another.
—MAF
Science, abi8532, abi8358,
this issue p. 723, this issue p. 729ORGANIC CHEMISTRY
Easy aryl reductions
The Birch reduction has been
widely used for more than half a
century to achieve partial reduc-
tion of aryl rings by alkali metals
at just two diametrically opposed
carbon sites. However, the condi-
tions require condensation of
caustic gaseous ammonia. A vari-
ation developed soon afterward
by Benkeser used safer liquid
ethylene diamine but was prone
to overreduction. By diluting eth-
ylene diamine in tetrahydrofuran
solvent, Burrows et al. now obtainMicroscopy image of multicolored blood cells, genetically labeled with fluorescent proteins, that can track clonal cell lineageEdited by Michael FunkIN SCIENCE JOURNALS
RESEARCH
HEMATOPOIESISColorful clones in the blood
S
tem cells in regenerating tissues such as the blood can
acquire mutations that enable a growth advantage, increas-
ing the chance of developing cancer. It is unclear how such
diverse mutations promote clonal fitness. Avagyan et al.
generated a platform in zebrafish to label clones with unique
hues while inducing mutations in genes implicated in human blood
disorders. Mutations in some genes caused clones to expand overtime, resulting in clonal dominance. Progenitors in the dominant
clone expressed anti-inflammatory factors to resist the inflamma-
tory environment produced by their own mature progeny, leading
to a self-perpetuating cycle promoting clonal fitness. Targeting
these resistance pathways may be used to abate clonal hemato-
poiesis and prevent its associated pathology. —BAP
Science, aba9304, this issue p. 768