The Economist - USA (2022-02-26)

76 Science & technology The Economist February 26th 2022

ment,intheformofitsNationalAcade­

miesofSciences,EngineeringandMedi­

cine,setupa committee,co­chairedbyDr

LoandDrSanes,toinvestigatebothmat­

ters,inordertoheadofffuturetrouble.

This committee published its report

lastyear,andDrLodiscusseditwitha fel­

lowcommitteemember,AltaCharo,ofthe

University of Wisconsin­Madison. The

generaltenorwaskeepcalmandcarryon.

Butcautiously. Thepotential benefitsof

both typesof research are huge. As Dr

Saneshadearlierpointedout,ailmentsof

thebrainare,collectively,thebiggestcause

ofmorbidityaroundtheworld,aswellasa

hugecauseofmortality.Butexperiments

mustbeethical.Anysignofheightened

sufferinginanimalscausedbytheirhav­

inghumanneuronsintheirbrainsneeds

tobescrutinisedcarefully.

As to brain­organoid consciousness,

thoughorganoids’simplicityandlackof

connectiontotheworldmakesthisunlike­

ly,assembloidsmightchangethat.AsDrLo

observed,thissortofworktapsintofears

raisedovermanyyearsbysciencefiction.

Butsciencefictiondoessometimesgoon

tobecomescientificfact.n

Intergenerationalmemory

The worm’s turn

C

harlesdarwindidnotinventtheidea

of  evolution.  But  he  did  come  up  with

the  currently  accepted  explanation,  natu­

ral selection, in which heritable character­

istics  arise  by  chance  and  are  retained  if

competition shows them to be useful. Nat­

ural selection’s success overthrew an earli­

er idea proposed by Jean­Baptiste Lamarck,

a  French  natural  historian.  Lamarck  had

suggested that characteristics acquired by

experience  during  an  organism’s  lifetime

might somehow become heritable. 

Modern  genetics  has  no  place  for  La­

marckism  as  a  long­term  mechanism,  be­

cause  it  would  involve  writing  the  recipe

for such environmentally induced changes

accurately into an organism’s dna. But oc­

casional  examples  of  short­term  effects

that  resemble  it  do  turn  up  from  time  to

time.  They  usually  involve  minor  and  re­

versible  chemical  tweaks  to  the  dna in

sperm and eggs, or to the proteins in which

that  dnais  packaged  into  chromosomes.

These tweaks, known as epigenetic effects,

tend to cause general, and not always help­

ful,  responses  to  events  like  famine,  and

persist  for  only  a  generation  or  two.  The

aaasmeeting heard, however, of an exam­

ple that has a much more intriguing mech­

anism. It encodes a specific, life­saving be­

haviour  in  a  relative  of  dnacalled  rna.

And it is passed down even unto the third

and fourth generations. 

Coleen Murphy of Princeton University

studies  C. elegans,  a  nematode  worm  be­

loved  of  geneticists  that  is,  as  a  conse­

quence,  one  of  the  planet’s  best  under­

stood  animals.  C. eleganslives  in  rotting

fruit, and eats bacteria. Among its favour­

ite prey are bugs of the genus Pseudomonas.

But  munching  these  does  not  always  go

well.  One  species,  P. aeruginosa,  is  a  dan­

gerous  pathogen,  at  least  when  the  tem­

perature  is  above  25°C.  Not  surprisingly,

worms which survive their first encounter

with  P. aeruginosain  such  circumstances

are put off by the experience. Thenceforth

they  are  repelled  by,  rather  than  attracted

to, its chemical traces.

That  makes  perfect  sense.  But  Dr  Mur­

phy, who is interested in the phenomenon

of  epigenetic  transmission,  wondered  if

this  aversive  behaviour  might  also  be  dis­

played by the offspring of those worms.

It was. And by the offspring of those off­

spring.  And  by  the  offspring  of  the  off­

spring of those offspring. In fact, it did not

disappear  until  the  fifth  generation  of

worms  descended  from  the  one  that  had

had  the  bad  experience.  By  this  time  the

ambient temperature might have fallen be­

low 25°C, making P. aeruginosaonce again

an attractive food source.

A lot of molecular­biological manipula­

tion  by  members  of  Dr  Murphy’s  team

showed that the switch from attraction to

repulsion  is  caused  by  an  increase  in  the

amount of a protein called daf­7 in a pair of

nerve  cells  called  asineurons  found  near

the worm’s mouth. Not only were elevated

levels  of  this  protein  confined  to  those

worms which were repelled by P. aerugino-

sa, but they also remained elevated for four

further  generations,  returning  to  normal,

along with the behaviour, in the fifth.

The biochemical underpinning of this,

it turned out after further rounds of experi­

ments,  is  an  rnamolecule,  p11,  which  is

produced by P. aeruginosaand taken up by

the worms. Experiments showed that after

exposure to p11, daf­7 levels in worms’ asi

neurons  went  up,  and  the  worms  then

avoided  P. aeruginosa.  Because  rnaand

dnaare chemically similar, strands of rna

can bind to strands of dnaif the composi­

tions  of  the  two  are  complementary.  And

that  is  what  is  happening  here.  Part  of  p 11

matches part of a gene called maco-1, that is

active  in  asi neurons.  Binding  between

them  turns  down  the  volume  on  maco-1,

which has the effect of turning up the vol­

ume on the gene which encodes daf­7 and

switching on the evasive response.

Somehow, therefore, p11 is being passed

from one worm generation to another. And

this seems to involve an object called a re­

trotransposon. Retrotransposons  are  vi­

rus­like structures that can copy rnainto

dna. Dr Murphy’s latest experiments show

that  worms  have  one  called  Cer 1which

does this with p11.

Cer 1thus acts as a sort of vehicle, out­

side the cell nucleus, which carries p11. It is

able, in experiments, to pass the rnaon to

other  worms,  which  then  also  become  P.

aeruginosa­averse  for  four  generations.

And it does something similar to the germ

cells inside its original host. Why the effect

persists for four generations and no longer

remains  unknown.  But  what  this  elegant

piece  of  scienceshows  is  that  a  specific,

useful acquiredcharacteristic can, indeed,

Nematode progeny “remember” hostile be inherited.n

bacteria encountered by a parent

The kids are alright

Lithiumproduction

Filter feeders

A

round 60%of  the  world’s  lithium,  a

metal in high demand for making bat­

teries, comes from evaporation ponds, like

that pictured overleaf, located in deserts in

Argentina, Bolivia and Chile. These ponds,

which can have individual areas of 60km^2

or more, are filled with lithium­rich brine

pumped from underground. That brine, as

the ponds’ name suggests, is then concen­

trated in them by evaporation, after which

it is treated to purge it of other metals, such

as  sodium  and  magnesium,  and  the  lithi­

um is precipitated as lithium carbonate. 

This  all  takes  time—often  as  much  as

two years. And the process of purification

is  complex  and  inefficient.  As  a  conse­

quence,  only  about  30%  of  the  lithium  in

the original brine reaches the marketplace. 

Two new ways of extracting lithium

from brine