60 Scientific American, November 2019Birds got their start more than 150 million years ago, evolving
from meat-eating dinosaurs called theropods, and they attained
an impressive degree of diversity in the first 85 million years or
so of their existence. But the ancestors of today’s birds—members
of the neornithine lineage—were mere bit players compared with
archaic birds such as the enantiornithines, which ruled the roost.
When the asteroid struck, however, neornithine fortunes shifted.
The impact extinguished all of the nonbird dinosaurs and most
birds. Only the neornithines made it through that apocalyptic
event. This clutch of survivors would give rise to one of the great-
est evolutionary radiations of all time.
Today there are more than 10,000 bird species, making them
the second most speciose class of vertebrate creatures alive,
outnumbered only by the bony fish. They come in every shape
and size—the land-bound ostrich tips the scales at more than
136 kilograms; the ever whirring bee hummingbird, less than
two grams. They have colonized virtually every major body of
land and water on the planet, from the sweltering tropics to the
frozen poles. And they have diversified to fill a vast array of
dietary niches, evolving adaptations to eating everything from
microscopic algae to large mammals.
Incredibly, roughly half of these species are songbirds, which
are characterized by a special voice box. The group includes the
warblers, canaries, larks and other mellifluous singers but also
the strident (to human ears, anyway) crows and their kin. To put
that number in perspective, there are approximately as many liv-
ing species of songbirds as there are of mammals.
How did this particular group of birds come to be so extraor-
dinarily diverse? Biologists have long sought to answer this ques-
tion, scouring the fossil record and DNA sequences of modern
birds for clues. But apart from pinpointing where songbirds orig-
inated (Australia), many of these studies produced inconclusive
or conflicting results. A detailed picture of where and when the
lineages leading to modern songbirds split off from one another—
and thus the factors driving this radiation—remained elusive.
In the absence of conclusive evidence to show how it all
transpired, researchers have advanced a number of competing
theories for songbird diversification that center variously on cli-
mate change, plate tectonics and sexual selection, in which
mate preferences spur evolution.
Now a new finding has set the field atwitter. All songbirds, it
seems, have a weird extra chromosome that does not appear to
exist in other birds. The discovery suggests a genetic mecha-
nism for creating barriers to reproduction between populations
of a species, which promotes speciation. Much remains to be
learned about this auxiliary package of DNA, but already some
researchers are wondering whether it just might be the secret of
the songbirds’ dazzling evolutionary success.
BACK POCKET GENES
The chromosome in question is called the germ-line-restricted
chromosome (GRC), so named for its presence in reproductivecells—eggs, sperm and their precursors—but not the rest of the
body’s cells, called somatic cells. Progenitors of both eggs and
sperm contain GRC, but by the time a sperm cell has developed
fully, the GRC has been eliminated from it. The chromosome is
thus transmitted to offspring via the mother exclusively.
Until recently the GRC was known only from two songbirds:
the zebra finch and its close relative the Bengalese finch. It
seemed to be an oddity of these two species, nothing more. But
when researchers decided to look for it in other lineages of
birds, a striking pattern emerged. In a paper published in the
June 11 Proceedings of the National Academy of Sciences USA ,
Anna Torgasheva and Pavel Borodin of the Russian Academy of
Sciences, Denis Larkin of the University of London and their
colleagues report that all 16 of the songbird species they exam-
ined—a sample that included representatives from across the
family tree of songbirds—had the GRC; none of the eight spe-
cies representing other major bird groups did. What is more,
the GRCs they found differed considerably from species to spe-
cies—even between closely related ones—suggesting that the
chromosome has evolved quickly in these different songbird
lineages since it first appeared in their common ancestor an
estimated 35 million years ago.
Cells of other organisms have previously been found to carry
extra chromosomes called B chromosomes. But their occur-
rence is erratic, varying between members of the same species
or even between different cells in the same individual. GRC, in
contrast, is “an obligatory element in the germ line of song
birds,” Larkin says. This ubiquity suggests that GRC is more
influential than B chromosomes.
Exactly what GRC is influencing is largely a mystery, howev-
er—researchers know very little about what its genes actually
do. But some hints have come to light. In another recent GRC
study, which has been posted to the bioRxiv preprint server but
not yet published in a peer-reviewed scientific journal, Cormac
M. Kinsella and Alexander Suh of Uppsala University in Sweden
and their colleagues found that the zebra finch GRC contains at
least 115 genes, including some that have been shown to make
proteins and RNA in the ovaries and testes of adult birds. This
expression pattern hints that these genes may help guide the
development of sperm and eggs. Other genes on the zebra finch
GRC are comparable to genes that are known from mouse stud-
ies to be involved in early embryonic development.
To Borodin and Larkin, these findings suggest that the GRC
may have allowed songbirds to circumvent key constraints onKate Wong is a senior editor for evolution
and ecology at Scientific American.CYRIL LAUBSCHERGetty Images(^1 ); OLEG MINITSKIYGetty Images(^2 ); LES STOCKERGetty Images(^3 ); REINHARD HOLZLGetty Images(^4 ); KIM TAYLORGetty Images(^5 );ALAMY (6 ); FERNANDO SANCHEZ DE CASTROGetty Images(^7 ); HANNE AND JENS ERIKSENNature Picture Library(^8 ); DP WILDLIFE VERTEBRATESAlamy(^9 )IN BRIEFSongbirds are the most species-rich bird group, ac -
counting for roughly half of the more than 10,000
bird species alive today.Biologists have long wondered how songbirds
came to be so diverse. Traditional explanations
have focused on factors such as climate change.Recent studies show that songbirds have an extra
chromosome not found in other birds, suggesting
that it might have been the key to their diversification.© 2019 Scientific American