Avian speciation: Uniform vs. particolored plumage

Date:
July 14, 2020
Source:
Ludwig-Maximilians-Universita"t Mu"nchen
Summary:
Although carrion crows and hooded crows are almost indistinguishable
genetically, they avoid mating with each other. Researchers have now
identified a mutation that appears to contribute to this instance
of reproductive isolation.



FULL STORY ========================================================================== Although carrion crows and hooded crows are almost indistinguishable genetically, they avoid mating with each other. Researchers from Ludwig- Maximlian-Universitaet (LMU) in Munich have now identified a mutation
that appears to contribute to this instance of reproductive isolation.


==========================================================================
The carrion crow and the hooded crow are genetically closely related, but
they are distinguishable on the basis of the color of their plumage. The carrion crow's feathers are soot-black, while the hooded crow's plumage presents a particolored combination of black and light gray. Although
crosses between the two forms can produce fertile offspring, the region of overlap between their geographical distributions in Europe is strikingly narrow. For this reason, the two forms have become a popular model for
the elucidation of the processes that lead to species divergence. LMU evolutionary biologist Jochen Wolf and his team are studying the factors
that contribute to the divergence of the two populations at the molecular level. Genetic analyses have already suggested that differences in the
color of the plumage play an important role in limiting the frequency
of hybridization between carrion and hooded crows. The scientists have
now identified a crucial mutation that affects this character. Their
findings appear in the online journal Nature Communications, and imply
that all corvid species were originally uniformly black in color.

The ancestral population of crows in Europe began to diverge during the
Late Pleistocene, at a time when the onset of glaciation in Central
Europe forced the birds to retreat to refuge zones in Iberia and
the Balkans. When the climate improved at the end of the last glacial
maximum, they were able to recolonize their original habitats. However,
during the period of their isolation, the populations in Southwestern and Southeastern Europe had diverged from each other to such an extent that
they no longer interbred at the same rate, i.e. became reproductively
isolated. In evolutionary terms, the two populations thereafter
went their separate ways. The Western European population became the
carrion crow, while their counterparts in Eastern Europe gave rise to
the hooded crow. The zone in which the two now come into contact (the
'hybrid zone') is only 20 to 50 km wide, and in Germany it essentially
follows the course of the Elbe River. "Within this narrow zone, there
is a low incidence of interbreeding. The progeny of such crosses have
plumage of an intermediate color," Wolf explains. "The fact that this
zone is so clearly defined implies that hybrid progeny are subjected to negative selection." Wolf wants to understand the genetic basis of this instance of reproductive isolation. In previous work, he and his group had demonstrated that the two populations differ genetically from each other
only in segments of their genomes that determine plumage color. Moreover, population genetic studies have strongly suggested that mate selection
is indeed based on this very character - - the two forms preferentially
choose mating partners that closely resemble themselves. These earlier
studies were based on the investigation of single- base variation,
i.e. differences between individuals at single sites (base- pairs) within
the genomic DNA. "However, we have never been able to directly determine
the functional effects of such single-base variations on plumage color,"
says Matthias Weissensteiner, the lead author of the study. "Even when
we find an association between a single-base variant and plumage color,
the mutation actually responsible for the color change might be located thousands of base-pairs away." To tackle this problem, the researchers
have used a technically demanding method to search for interspecific differences that affect longer stretches of DNA. These 'structural'
variations include deletions, insertions or inversions of sequence
blocks. "Up until recently, high-throughput sequencing technologies could
only sequence segments of DNA on the order of 100 bp in length, which is
not long enough to capture large-scale structural mutations," says Wolf.

"Thanks to the new methods, we can now examine very long stretches of DNA comprising up to 150,000 base pairs." The team applied this technology
to DNA obtained from about two dozen birds, and searched for structural variations that differentiate carrion crows from hooded crows. The data
not only confirmed the results of the single-base analyses, they also
uncovered an insertion mutation in a gene which is known to determine
plumage color by interacting with a second gene elsewhere in the
genome. In addition, phylogenetic analysis of DNA from related species
revealed that their common ancestor carried the black variant of the
first of these genes. The variant found in the hooded crow represents a
new mutation, which first appeared about half a million years ago. "The
new color variant seems to be quite attractive, because it was able to establish itself very quickly, and therefore must have been positively selected," says Wolf. How the variant accomplished this feat is not yet
clear. The evidence suggests that it first appeared in the region which
now encompasses Iran and Iraq, and there are some indications that the
lighter plumage confers a selective advantage in hot regions, because it effectively reflects sunlight. This supports the idea that the mutation
might have initially been favored by natural selection. "Once it had
reached a certain frequency within the local population, it would have
been able to spread because parental imprinting, which enables nestlings
to recognize their parents, also causes mature birds to choose mates
that resemble their parents in appearance," Wolf explains. However, other possible scenarios, such as random genetic drift in small populations or
the involvement of selfish genes (which promote their own propagation),
are also conceivable and have yet to be ruled out.


========================================================================== Story Source: Materials provided by
Ludwig-Maximilians-Universita"t_Mu"nchen. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Matthias H. Weissensteiner, Ignas Bunikis, Ana Catala'n, Kees-Jan
Francoijs, Ulrich Knief, Wieland Heim, Valentina Peona, Saurabh D.

Pophaly, Fritz J. Sedlazeck, Alexander Suh, Vera M. Warmuth,
Jochen B. W.

Wolf. Discovery and population genomics of structural variation
in a songbird genus. Nature Communications, 2020; 11 (1) DOI:
10.1038/s41467- 020-17195-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200714101255.htm

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