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Tales from the crypt(ic)

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Abstract

Neutral mutations can breathe life into evolutionary adaptation
INSIGHTS |
PERSPECTIVES
sciencemag.org SCIENCE
GRAPHIC: KELLIE HOLOSKI/SCIENCE
By Jessica A. Lee1and Christopher J. Marx2
Adaptation through natural selection
requires inherited changes in an or-
ganism’s phenotype. However, neu-
tral or “cryptic” mutations—changes
in genotype that do not affect phe-
notype—can influence adaptation
outcomes, because genotype-to-phenotype
mapping is inherently dependent on con-
text. The phenotypic consequence of a
mutation might change as a result of in-
teractions either with other mutations in
the genome (epistasis) or with the physical
environment [a genotype-by-environment
(G×E) interaction]. On page 347 of this is-
sue, Zheng et al. (1) demonstrate that the
accumulation of mutations that yield neu-
tral changes in a protein promotes faster
adaptation in an environment selecting
for a new function, and that this effect
requires the combined impact of epistasis
and G×E interactions.
The impact of neutral mutations on ad-
aptation is often framed from the pheno-
typic, rather than the genotypic, point of
view; an ancestral phenotype that remains
unchanged in the face of genomic muta-
tions is considered mutationally robust.
However, scientists have debated whether
mutational robustness spurs or suppresses
adaptation over the long term. Although a
broader array of genotypes can be tolerated
in a mutationally robust phenotype, the
mutations form a “neutral network” that
might serve as a genetic resource should
the population be confronted with a new
environment. Shielding cryptic mutations
from G×E interactions in the organism’s
original environment can allow them to
accumulate (see the figure). Theoretical
analysis has revealed that mutational ro-
bustness can either speed or slow adapta-
tion, depending on whether high-fitness
mutations are rare or common, respec-
tively, across the neutral network (2).
Until recently, there have been few ex-
perimental tests of whether or how cryptic
mutations function in the adaptation of
populations. Zheng et al. provide an empiri-
cal test to ask whether generating a broad
pool of cryptic genetic variations accelerates
and diversifies adaptive outcomes when
that population requires a specific protein
to acquire a new activity. The authors ex-
amined yellow fluorescent protein (YFP)
function in living Escherichia coli cells by
using fluorescence-activated cell sorting
(FACS) to select cells that display yellow
fluorescence in vivo. To generate cryptic
genetic variation, they mutagenized the yfp
gene, introduced the resulting pool into E.
coli cells, and then selected
for the 20% of cells with yel-
low fluorescence levels that
mirrored most closely that of
the ancestral phenotype. Af-
ter four rounds of selection,
they created a new selective
environment by switching
the FACS to select for a green
fluorescence—ancestral YFP
is weakly fluorescent at the
green wavelength—and car-
ried the cells through four
rounds of selection for the top 0.1% of YFP
variants with the highest green fluores-
cence activity.
The generation of cryptic variation in YFP
before selection for green fluorescence in-
creased the rate of adaptation compared to
control populations initiated without prior
generation of diversity. The benefit of cryp-
tic variation was most prominent in the first
round after the transition to selection for
green fluorescence. This stands in contrast
to results from the in vitro evolution of a ri-
bozyme selected for its ability to use a new
substrate, in which boosts in adaptation con-
tinued through five rounds of selection (3).
Thus, the quantitative effect of cryptic ge-
netic variation is likely to be different across
systems and selective pressures.
Zheng et al. also found that
the cryptic genetic diversity
generated in the first environ-
ment permitted evolutionary
trajectories that would not
otherwise have been acces-
sible. This crucial finding was
uncovered by reconstructing
all possible mutational inter-
mediates that preceded the
end combination, a process
that culminated in yfp genes
that expressed high green
fluorescence in the final E. coli populations.
When green fluorescence was directly se-
lected from the ancestral yfp without cryptic
variation, the network of mutational inter-
mediates almost exclusively featured steps
that, in any order, would have created cells
that survive the selection process (“benefi-
cial” mutations). This observation, along with
GENETICS
Tales from the crypt(ic)
Neutral mutations can breathe life into evolutionary adaptation
1Global Viral, San Francisco, CA 94104, USA. 2Department of
Biological Sciences, University of Idaho, Moscow, ID 83844,
USA. Email: jessica.lee@globalviral.org; cmarx@uidaho.edu
ANC C
BBC
AAC
AB ABC
ANC C
BBC
AAC
AB ABC
Under selection for yellow uorescence
(yellow glow), mutations in the ancestral
gene (ANC) that do not change the selection
phenotype (neutral; black arrows) can
accumulate as cryptic genetic variation.
When the selection phenotype is changed
(gray arrow) to green uorescence (green
glow), every path from ANC to the (brightest)
ABC variant (such as ANC A) is blocked by
deleterious mutations (red arrows).
Neutral mutations accumulated
in the AB variant enable rapid
evolution (blue arrows) to ABC
(high green uorescence).
Change
environment
E. col i
“Zheng et al.
greatly advance
understanding
of how cryptic
variation…
can aid in
adaptation.
318 26 JULY 2019 • VOL 365 ISSUE 6451
Cryptic mutations facilitate adaptation
Accumulation of mutations that yield neutral changes in a protein
promotes adaptation when selecting for a new function.
Published by AAAS
on August 1, 2019 http://science.sciencemag.org/Downloaded from
SCIENCE sciencemag.org
GRAPHIC: V. ALTOUNIAN/SCIENCE
the fact that these populations all ended up
with very similar genotypes, demonstrated a
constraint on selection. By contrast, nearly
all trajectories observed from the pool with
cryptic variation featured steps that would
have been deleterious in the environment se-
lecting for green fluorescence and, therefore,
would not have survived without the initial
generation of diversity (see the figure).
Fluorescent proteins and ribozymes ma-
nipulated under laboratory conditions rep-
resent excellent model systems; but does
evidence exist to show that cryptic genetic
variation has contributed to the evolution
of new traits throughout Earth’s history?
The answer appears to be yes. An analysis
of reconstructed evolutionary intermedi-
ates for a family of hormone receptors re-
vealed mutations in the genes that encode
these proteins that did not change activity
on their own, but were essential for the
evolution of new hormone-binding proper-
ties more than 400 million years ago (4). A
clever genetic screen even allowed research-
ers to attempt to “replay the tape” to deter-
mine the number of possible mutations
that would have set the stage for novelty to
arise without disrupting the current protein
function. They found only one amino acid
change that fit these criteria, and it was the
one known to have occurred historically (5).
By demonstrating the role of epista-
sis and the avoidance of G×E interactions
through the change of selective conditions,
Zheng et al. greatly advance understanding
of how cryptic variation—phenotypes that
are mutationally robust—can aid in adap-
tation. The authors suggest that future ef-
forts to use directed evolution for practical
purposes incorporate these principles, as is
already being done when considering the
folding stability and directed evolution of
proteins (6). From a fundamental perspec-
tive, perhaps the most important question
is whether the observations from evolving
single RNA or protein molecules also apply
at the level of the whole cell; if so, we can
expect to move toward a predictive under-
standing of these phenomena. j
REFERENCES AND NOTES
1. J. Zheng et al., Science 365, 347 (2019).
2. J. A. Drag hi, T. L. Parson s, G. P. Wagner, J. B. Pl otki n, Nature
463, 353 (2010).
3. D. P. Bendixse n, J. Coll et, B. Øst man, E . J. Hayden , PLOS
Biol. 17, e3000300 (2019).
4. E . A. Or tlun d, J. T. Bri dgha m, M. R. Re dinb o, J. W. Thor nton ,
Science 317, 1544 (2007).
5. M. J. Ha rms, J. W. Tho rnto n, Nature 512, 203 (2014).
6. P. A. Romero, F. H. Arnold, Nat. Rev. Mol. Cell Biol. 10, 866
(2009).
ACKNOWLEDGMENTS
The authors are supported by NSF MCB-1714949 (C.J.M.) and
John Templeton Foundation grant 60973 (J.A.L.).
10.1126/science.aay2727
METABOLISM
Lowering ceramides
to overcome diabetes
Lowering toxic lipid concentrations in mice has a promising
impact on obesity-associated metabolic disorder
By Christine M. Kusminski and
Philipp E. Scherer
Excess nutrient intake leads to a dis-
ruption in metabolic homeostasis.
In particular, prolonged periods of
excess glucose intake can directly
contribute to deterioration of insulin
sensitivity. Insulin is a key player in
the disposal of carbohydrates from food.
Frequently associated with this insulin re-
sistance is a dysregulation of lipid metabo-
lism that can lead to lipotoxicity, whereby
excess lipids wreak havoc on important
intracellular signaling pathways. How-
ever, it is largely unknown what types of
lipids trigger these cytotoxic effects, which
result in further deterioration of glucose
and lipid homeostasis. Concentrations of
ceramide lipids in blood plasma and tis-
sues are strongly associated with the risk
of developing type 2 diabetes (T2D), he-
patic steatosis, and cardiovascular disease,
which are caused by lipotoxicity and insu-
lin resistance (1, 2). On page 386 of this is-
sue, Chaurasia et al. (3) provide evidence
that therapeutically intervening in the ce-
ramide biosynthesis pathway in mice can
improve metabolic homeostasis.
The authors focused on a critical, rate-
limiting step in the ceramide biosynthesis
pathway. Dihydroceramide desaturase 1
(DEGS1) is an enzyme that inserts a dou-
ble bond into dihydroceramide to produce
ceramide. The authors showed that the re-
moval of DEGS1 in mice causes an increase
in ceramides lacking a double bond in
tissues and plasma. Deletion of the Degs1
gene in adult mice did not elicit any del-
eterious effects (an important consider-
ation for possible therapeutic targets, the
inhibition of which may lead to unwanted
side effects). Furthermore, complete or
partial loss of DEGS1 activity substantially
improved glucose and lipid metabolism in
mice exposed to a high-fat diet.
Chaurasia et al. showed that inhibiting
ceramide synthesis in adipocytes or hepa-
tocytes leads to a system-wide improve-
ment in metabolic parameters. This was
also observed for components of the ce-
ramide catabolism pathway, such as cell
type–specific overexpression of acid ce-
ramidase, in addition to overexpression
of adiponectin receptors (which also have
ceramidase activity) (4, 5). Together, this
reveals a strong exchange of ceramides
throughout the body, such that depleting
these lipid species in either hepatocytes or
adipocytes lowers ceramides across many
tissues involved in maintaining metabolic
homeostasis. Adiponectin is a circulating
adipokine that signals through its cognate
receptors to enhance insulin sensitivity by
reducing intracellular ceramides (6). Many
clinical studies demonstrate inverse corre-
lations between the amounts of ceramides
in plasma and adiponectin in healthy in-
dividuals or those with T2D (79) (see the
figure).
Chaurasia et al. further implicate cera -
mides in a phenomenon called selective
DEGS1
CERS6
CERS1
Ceramide
Acid ceramidase
Adiponectin receptors
Adiponectin Cellular insulin
sensitivity
Hepatic
steatosis
InEammation
Pancreatic b-cell
functionality
Pancreatic
b-cell apoptosis
NH
CH2OH
OH
O
GS
DEG
DEGS1
CERS1
CER
CERS6
CERS
26 JULY 2019 • VOL 365 ISSUE 6451 319
Intervening in
ceramide
biosynthesis
In mice, removing
dihydroceramide
desaturase 1 (DEGS1) or
the ceramide synthases
CERS1 and CERS6
reduces the amount of
available ceramide, which
improves metabolic
homeostasis.
Published by AAAS
on August 1, 2019 http://science.sciencemag.org/Downloaded from
Tales from the crypt(ic)
Jessica A. Lee and Christopher J. Marx
DOI: 10.1126/science.aay2727
(6451), 318-319.365Science
ARTICLE TOOLS http://science.sciencemag.org/content/365/6451/318
CONTENT
RELATED http://science.sciencemag.org/content/sci/365/6451/347.full
REFERENCES http://science.sciencemag.org/content/365/6451/318#BIBL
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... Neutral effects with respect to phenotypic expression may also occur for most other genetic levels and effects (Paaby and Rockman, 2014), a general phenomenon called cryptic genetic variation (CGV), which may serve as a genetic reservoir for future evolutionary change (Zheng et al., 2019). In populations of bacteria, CGV facilitated adaptation when the population faced rapid environmental change as enacted by a change in selection (Lee and Marx, 2019;Zheng et al., 2019). ...
... For the connections that remain, their mappings can be altered by epigenetic processes. When noise is introduced into the G-P map for the neurocontrollers of simulated quadrupedal robots, the resulting stochastic ontogenesis (SO), a developmental process, can have surprising and positive evolutionary consequences (Lee and Marx, 2019). As is true with populations possessing CGV, populations with SO respond better, as measured by evolutionary fitness, to changes in the environment, apparently by providing a reservoir of solutions. ...
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Cryptic alleles make a bridge for adaptation Protein function is generally constrained by selective parameters that can inhibit evolutionary potential. It has thus been difficult to determine how novelties arise. Zheng et al. allowed bacterial populations to accumulate mutations and then used directed evolution to evolve green fluorescent protein function from a gene that expressed yellow fluorescent protein (see the Perspective by Lee and Marx). Protein alternatives could evolve in cases where cryptic alleles—selectively neutral or mildly deleterious genetic variants with no apparent phenotypic differences—were present in the population. Thus, cryptic alleles provide an evolutionary bridge between diversity and selection and facilitate the generation of novel adaptations. Science , this issue p. 347 ; see also p. 318
  • J Zheng
J. Zheng et al., Science 365, 347 (2019).
  • D P Bendixsen
  • J Collet
  • B Østman
  • E J Hayden
D. P. Bendixsen, J. Collet, B. Østman, E. J. Hayden, PLOS Biol. 17, e3000300 (2019).
  • E A Ortlund
  • J T Bridgham
  • M R Redinbo
  • J W Thornton
E. A. Ortlund, J. T. Bridgham, M. R. Redinbo, J. W. Thornton, Science 317, 1544 (2007).
  • P A Romero
  • F H Arnold
P. A. Romero, F. H. Arnold, Nat. Rev. Mol. Cell Biol. 10, 866 (2009).