Science - USA (2022-04-08)

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orientation of the hydroxyapatite crystals,
and to determine the degree of mineral-
ization on the generation of compression
forces inside the collagen. These factors
are particularly interesting, given the
complexity and multilevel organization
of the hydroxyapatite crystals in the col-
lagen fibrils, spanning both the intra- and
extrafibrillar spaces ( 12 ). Quantifying the
contribution of prestressing to the overall
mechanical properties of bone, and how it
scales with the hierarchical organization
of the fibrils, will constitute an important
step toward understanding how the prop-
erties of the tissue arise from its compo-
sition and structure across length scales.
It will be exciting to determine whether,
and how, prestressing varies between bone
tissues with different mechanical require-
ments and across different species.
This work draws attention to a broader
perspective—namely, the large variety of
biominerals with load-bearing functions
found in nature. Enamel and dentin, which
compose the vertebrate tooth, are subject to
forces during mastication. Shells have to be
tough enough to provide protection without
fracturing, and in some cases, can withstand
large deformations ( 13 ). This raises interest-
ing questions as to whether prestresses at
the submicrometer and micrometer scales
constitute a mechanism to strengthen the
mechanical properties of other mineralized
tissues. Given the diversity of compositions,
structures, and functions of biominerals, it
is crucial to elucidate how prestressing is
enabled in each case. Hence, using advanced
correlative and in situ characterization
methods, demonstrated in this work, con-
stitutes a step change in addressing these
questions for our general understanding of
biomineralization as well as the application
of this knowledge in biomedicine, environ-
mental protection, materials design, and
engineering. j

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10.1126/science.abo1264

MICROBIOLOGY

There’s more to RNA viruses

than diseases

A simple, pervasive biological entity in the ocean

sheds light on evolution

Retroelements

Last universal cellular ancestor (LUCA)

Eukaryotes Archaea Bacteria

DNA viruses RNA viruses

By Jessica M. Labonté and

Kathryn L. Campbell

V

iruses infect and affect all domains of

life, playing roles as drivers for evolu-

tion, diversity, and global geochemi-

cal cycling. Because viruses evolve

with and depend on their hosts for

replication, they are essential in un-

derstanding the origins of life. RNA viruses

are notorious for being agents of disease

in humans and agriculturally important

plants and animals. However, because of

the focus on studying RNA viruses in pa-

thologies, there is a lack of research on their

abundance and diversity in the environ-

ment. This paucity of data has challenged

evolutionary studies aimed at determining

the origin of RNA viruses. On page 156 of

this issue, Zayed et al. ( 1 ) report the identi-

fication of thousands of RNA viruses in the

ocean. These new sequences fill previously

missing gaps, enabling the construction of a

more robust phylogenetic tree and confirm-

ing hypotheses regarding the evolution of

RNA viruses.

Every day, viral infections are estimated

to kill anywhere between 10 and 20% of all

microbial biomass ( 2 ). These infections have

impacts on microbial community composi-

tion through population control, on evolu-

tion as agents of horizontal gene transfer,

and on global geochemical cycles and nutri-

ent recycling ( 3 ). The development of next-

generation sequencing, which facilitates se-

quencing of total genetic material, provided

opportunities for discoveries. Sequencing of

the total genetic material of viruses, known

as viromics, has demonstrated that viruses

are highly diverse ( 4 ) and globally distrib-

uted ( 5 ) and play important roles in the geo-

chemical cycles ( 6 ). However, most efforts

have focused on the study of DNA viruses.

Viral genomes, especially RNA genomes, are

smaller and less stable, and difficulties in ex-

tracting high-quality viral RNA for sequenc-

ing have impaired the exploration of RNA

viruses in the environment.

INSIGHTS | PERSPECTIVES

138 8 APRIL 2022 • VOL 376 ISSUE 6589

Evolution of life and viruses are intertwined

The divergence of cellular life has led to the majority of RNA viruses infecting eukaryotes, the phylogenetic

details of which have been unclear. Zayed et al. have found the missing link between retroelements and RNA

viruses, which suggests that RNA viruses were present before the LUCA and had multiple points of origin.