Contributed
by: Zhenguo Lin
Darwin
used the title "On the origin of species" for his most famous book
published in 1859. In this book he explained how a single species changes over
time, but did not provide a proper explanation about how a species split into
two or more different species. The problem of speciation has now become an
important subject in evolutionary biology. From Hugo de Vries, Theodosius
Dobzhansky, and Ernst Mayr to contemporary workers such as Jerry Coyne and
Allen Orr, this problem has been studied extensively.
In
this case it seems to be crucial to study speciation at the molecular level. In
their recent review article, Nei and Nozawa (1) emphasized the importance of
mutations in speciation by presenting many cases of molecular studies. One of the mechanisms they considered is hybrid
incapacity associated with heterochromatin. Specifically, they stated that hybrid
sterility or inviability may occur by changes in repeat DNA elements in
heterochromatin regions of the genome. Two representative examples were presented
in the review article. (1) The different numbers of 359 bp repeats (zygote
hybrid rescue locus, Zhr) caused
hybrid inviability between Drosophila
melanogaster males and D. simulans
females. (2) The localization of Odysseus
homeobox (OdsH) protein to heterochromatic Y chromosome causes hybrid male
sterility between D. mauritiana
females and D. simulans males. Recently conducting a comparative study of the
genomic sequences from two closely related flycatcher bird species, Ellegren et
al. (2) suggested that the divergence of complex genomic repeat structures
(centromere and telomeres) may have generated the two species.
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Figure
1 a,
Male collared flycatcher. b, Male pied flycatcher. (From Ellegren et al. (2)).
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The
collared flycatcher Ficedula albicollis
and the pied flycatcher Ficedula
hypoleuca diverged less than 2 million years ago. They look very similar except for the
presence of white collar in the former species (Figure 1). The authors from
Uppsala University in Sweden have sequenced the ~1.1Gb genomic regions for 10
unrelated males in each species. By comparing these genomic regions, the author
identified 50 "divergence islands", which show significantly high
levels of sequence divergence between the two species. The length of an
"island" ranges from 100 kb to 3 Mb, with a mean of 625kb. Interestingly,
these “islands” are over-represented in the telomere or centromere regions,
which are rich in repeat structures (Figure 2). After detailed analyses of various
evolutionary patterns of these "divergence islands" , such as local mutation rates, levels of nucleotide diversity, allele-frequency spectra, levels of linkage disequilibrium and shared
polymorphisms, the authors confirmed
that these islands have experienced parallel selection in each species.
Although no direct evidence was provided to support how these "divergence
islands" contributed to the speciation, the authors believed that these
observations "raise the possibility that centromeres or other
heterochromatic repeats themselves are the driver of speciation" (2).
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Figure
2.
Distribution of divergence measured as the density of fixed differences per bp
for 200-kb windows across the genome. Chromosomes are listed in numerical order
and are separated by gaps. Red horizontal bars show the approximate location of
centromeres in homologous chromosomes of zebra finch. Open read symbols are
used to indicate that avian microchromosomes are generally acro- or
telocentric. Both ends of these chromosomes are labeled as the orientation is
not known. For chromosomes 4, 6 and 8, there is a lack of an in situ mapped marker 5′ of the
centromere in zebra finch. (from Ellegren et al. (2)).
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References
1. Nei, M. and Nozawa, M.
(2011), 'Roles of mutation and selection in speciation: from Hugo de Vries to the modern genomic era', Genome Biol Evol, 3, 812-29.
2. Ellegren, H., et al.
(2012), 'The genomic landscape of species divergence in Ficedula flycatchers', Nature.
doi:10.1038/nature11584
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