The Economist (2022-02-26) Riva

76 Science & technology The Economist February 26th 2022

ment,intheformofitsNationalAcade-

miesofSciences,EngineeringandMedi-

cine,setupacommittee,co-chairedbyDr

LoandDrSanes,toinvestigatebothmat-

ters,inordertoheadofffuturetrouble.

Thiscommitteepublisheditsreport

lastyear,andDrLodiscusseditwithafel-

lowcommitteemember,AltaCharo,ofthe

University of Wisconsin-Madison. The

generaltenorwaskeepcalmandcarryon.

Butcautiously.Thepotentialbenefitsof

bothtypesofresearcharehuge.AsDr

Saneshadearlierpointedout,ailmentsof

thebrainare,collectively,thebiggestcause

ofmorbidityaroundtheworld,aswellasa

hugecauseofmortality.Butexperiments

mustbeethical.Anysignofheightened

sufferinginanimalscausedbytheirhav-

inghumanneuronsintheirbrainsneeds

tobescrutinisedcarefully.

Astobrain-organoid consciousness,

thoughorganoids’simplicityandlackof

connectiontotheworldmakesthisunlike-

ly,assembloidsmightchangethat.AsDrLo

observed,thissortofworktapsintofears

raisedovermanyyearsbysciencefiction.

Butsciencefictiondoessometimesgoon

tobecomescientificfact.

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 Jean-Baptiste 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 long-term 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 short-term effects

that resemble it do turn up from time to

time. They usually involve minor and re-

versible chemical tweaks to the dnain

sperm and eggs, or to the proteins in which

that dnais packaged into chromosomes.

These tweaks, known as epigenetic effects,

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 specific, life-saving 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 first 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 offspring of those worms.

It was. And by the offspring of those off-

spring. And by the offspring of the off-

spring of those offspring. In fact, it did not

disappear until the fifth 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 molecular-biological 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 daf-7 in a pair of

nerve cells called asineurons found near

the worm’s mouth. Not only were elevated

levels of this protein confined 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 fifth.

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, daf-7 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 asineurons. Binding between

them turns down the volume on maco-1,

which has the effect of turning up the vol-

ume on the gene which encodes daf-7 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-

rus-like 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.

aeruginosa-averse for four generations.

And it does something similar to the germ

cells inside its original host. Why the effect

persists for four generations and no longer

remains unknown. But what this elegant

piece of scienceshows is that a specific,

useful acquiredcharacteristic can, indeed,

be inherited.

Nematode progeny “remember” hostile

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 filled with lithium-rich 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 purification

is complex and inefficient. 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