Science - USA (2021-07-09)

154 9 JULY 2021 • VOL 373 ISSUE 6551 sciencemag.org SCIENCE

By F. Keith Barker

S

ensory systems evolve to enable or-

ganisms to detect cues pertinent to

survival. The diversification of these

systems is a critical aspect of the ad-

aptative radiation of animals—that

is, how an ancestral species rapidly

diversifies into a large number of morpho-

logically diverse descendant species. Birds

are the most diverse clade of terrestrial ver-

tebrates, at more than 10,000 currently rec-

ognized species. Curiously, the entire avian

clade was shaped by the early loss of a gene

encoding a sweet receptor. How then, did

thousands of bird species that rely on nec-

tar and fruit evolve to perceive sugars? On

page 226 of this issue, Toda et al. ( 1 ) report a

shift from savory to sweet perception in the

early evolution of songbirds. The change

may have played a critical role in the radia-

tion of this diverse group.

Painstaking anatomical and physiological

studies have yielded important information

about sensory systems whose functions de-

pend on cellular- or organ-level adaption,

such as echolocation in cetaceans and bats

and specialized arthropod visual systems. By

contrast, genomic studies of animals have

rapidly yielded important insights into the

evolution of sensory traits in which func-

tion is defined at the molecular level, includ-

ing hearing ( 2 ), vision ( 3 ), olfaction ( 4 ), and

gustation ( 5 ). For instance, studies on the

evolution of opsins—light-sensitive proteins

found in photoreceptor cells—demonstrate

how gene duplication and loss can either ex-

pand or restrict the visual range of animals,

affecting their ability to identify resources

or to differentiate food resource quality ( 6 ).

Such sensory changes can have profound ef-

fects beyond food acquisition, also driving

coincident shifts in signaling modality and

information content ( 7 ).

Birds have provided classic examples of

adaptive radiation, such as the morphologi-

cally diverse Darwin’s finches and Hawaiian

honeycreepers. Broad comparative studies

of avian diversification have also identified

other rapidly speciating lineages, where the

imprint of adaptive radiation was not so clear

cut. One such lineage is the nectarivorous

hummingbirds ( 8 , 9 ). Recent molecular work

has shown that the insectivorous ancestor

of this lineage retooled its umami receptor

(sensitive to amino acids) for the detection of

sugars ( 5 ). This molecular shift—which could

PHOTO: GERALD ALLEN

Department of Ecology, Evolution and Behavior and Bell

Museum of Natural History, University of Minnesota, St.

Paul, MN, USA. Email: [email protected]

PERSPECTIVES

INSIGHTS

EVOLUTIONARY BIOLOGY

A shift in taste

The evolution of sugar perception in

songbirds began with a savory receptor

0709Perspectives.indd 154 7/1/21 6:34 PM