November 2019, ScientificAmerican.com 61bird evolution. “The avian genome in general is very compact
and conserved compared with, for example, the mammalian
genome,” Larkin explains. The genomes of today’s mammals
range in size from less than two picograms to more than eight
picograms and are packaged into anywhere from six chromo-
somes to 102. In the tens of millions of years over which they
have been evolving, their chromosomes have been sliced and
diced and reshuffled and rejoined many times. These rear-
rangements have altered gene expression in ways that have pro-
duced diverse traits. Birds, in contrast, have genomes ranging
from just under one picogram to just over two. And they usual-
ly have right around 80 chromosomes, with comparatively little
of the “junk” DNA found in most mammals.
The reason bird genomes are small and streamlined, some
experts surmise, has to do with flight. Flying is an energetically
expensive activity. Larger genomes require larger cells, and both
are metabolically costlier than their smaller counterparts. Theintense metabolic demands of flying may have therefore limited
bird genome size. Because the GRC occurs only in germ-line
cells and not the far more numerous somatic cells, it could have
provided songbirds with a rare chunk of extra DNA—fodder for
the evolution of new traits—without the metabolic costs associ-
ated with having a larger somatic genome.
“Birds need additional copies of germ-cell-specific genes for a
very short breeding period only to produce a lot of sperm and load
[egg cells] with large amounts of proteins. They have no reason to
carry these genes throughout the year and in [the rest of the
body’s] cells when and where they are of no use,” Borodin says. If
songbirds found a way to obtain additional genes on a temporary
basis that could work during early stages of development while
keeping their basic genome intact, Larkin adds, such an arrange-
ment would be tremendously advantageous and could lead to the
huge variety seen in songbirds compared with other bird groups.
In theory, the GRC could have created the reproductive iso-
lation needed for new species to evolve by rendering those indi-
viduals that carried the extra chromosome unable to interbreed
and produce fertile offspring with those that did not. Once the
GRC originated in the last common ancestor of songbirds,
members of that ancestral species that carried the GRC could
produce fertile offspring only with mates that also had the GRC.
As the GRC evolved, acquiring new genes, songbirds with a par-
ticular variant of GRC could produce fertile offspring only with
mates that carried that same GRC variant.
ENGINE OF CHANGE?
according To Borodin and larkin , the discovery that GRC is
widespread among songbirds and absent in other birds dove-
tails with the results of another recent study. In April, Carl Olive-
ros of Louisiana State University and his colleagues reported on
the results of their analysis of DNA from dozens of members of
the passerine order of birds, which comprises the songbirds and
some other, far less speciose groups. Based on the DNA sequenc-
es and a handful of fossils of known age, the team reconstructed
how the various passerine families were related and when they
branched off. It then compared this time line of diversification
against climate and geologic records to see if the passerine diver-
sification trends correlated with events in Earth history, as pre-
dicted by some hypotheses. On the whole, fluctuations in the
diversification rates of these birds did not track changes in glob-
al temperature or dispersals of the birds into new continents.
The findings prompted the authors to suggest that more complex
mechanisms than temperature or ecological opportunity were
the main drivers of passerine speciation. “These conclusions are
very much in line with our hypothesis of GRC contribution to
songbird diversification,” Larkin asserts.
Not everyone is ready to embrace the sug-
gestion that GRC drove songbird diversifica-
tion, however. “In general, it is hard to estab-
lish causation between any one given trait,
like the presence of GRCs, and the evolution-
ary success of a particular group,” Oliveros
says. “The presence of the trait could by
chance have coincided with another trait—
nesting behavior, for example—that may
have played a larger role in a group’s evolu-
tionary success.”
But other researchers not involved in the new studies find the
notion intriguing. “The fact that [GRCs] have been maintained
over long evolutionary periods and also contain putatively func-
tional genes... suggests that they could play a role in reproduc-
tive isolation in birds,” observes David Toews of Pennsylvania
State University. If the sky-high diversification rate of songbirds
compared with that of other birds was promoted by a genomic
mechanism such as GRCs, “it would definitely be exciting and
not something that I would have predicted,” Toews says. He cau-
tions, though, that “we need to know more about what they are
actually doing to make that link with confidence.”
The work could have implications for understanding organ-
isms beyond birds. “We thought we knew a lot about how bird
genomes are organized,” Suh reflects, “but the GRC has been
right before our eyes yet has been overlooked.” Scientists have
found similar extra chromosomes in hagfishes and some insects.
What if GRCs are more widespread in the tree of life, he won-
ders: “The findings in songbirds open up a bunch of new direc-
tions for thinking about evolution and development.”MORE TO EXPLORE
Programmed DNA Elimination of Germline Development Genes in Songbirds.
Cormac M. Kinsella et al. Posted to Biorxiv preprint server December 22, 2018.
http://www.biorxiv.org/content/10.1101/444364v2
Germline-Restricted Chromosome Is Widespread among Songbirds.
Anna Torgasheva et al. in Proceedings of the National Academy of Sciences USA,
Vol. 116, No. 24, pages 11,845–11,850; June 11, 2019.
FROM OUR ARCHIVES
Taking Wing. Stephen Brusatte; January 2017.
scientificamerican.com/magazine/saThe GRC could have provided
songbirds with a rare chunk
of extra DNA—fodder for the
evolution of new traits.
© 2019 Scientific American