such as "Introduction", "Conclusion"..etc
Aug. 16, 2007) — An international team of
scientists from Canada (Université Laval), the U.K. (University of
Hull, Cardiff University) and Spain (Doòana Biological Station), have
discovered that a pair of closely related species of East African
cichlid fishes -- a group of fish whose diversity comprising hundreds
of species has puzzled evolutionary biologists for decades -- evolved
divergent immune gene adaptations which might explain why they do not
interbreed, despite living side by side.
The two species ( Pseudotropheus emmiltos and Pseudotropheus
fainzilberi ) are found in the north western part of Lake Malawi. Until
now, the only known difference between them was the color of their
dorsal fin. Many researchers believe that African cichlids recognize
conspecifics from these kinds of colour differences, which are thought
to result from sexual selection. However, recent mate choice
experiments have shown that female P. emmiltos recognize males of their
own species from P. fainzilberi males based on olfactory communication
rather than color.
Some of the genes known to influence mating behavior through
olfaction in other vertebrate species are genes of the major
histocompatibility complex (MHC). These genes code for receptor that
bound molecules produced by infectious agents and present them to
specialized cells of the immune system which then launch an immune
attack on the microbes.
They are the most diverse genes found in vertebrate genomes and
individuals of some species, including humans, are able to "smell"
other individuals' variability at these genes and adjust their mate
choice in order to optimize the effectiveness of their offspring's
immune system. Analysis of MHC genes between P. emmiltos and P.
fainzilberi revealed that the two species were genetically more
different at these sites involved in contacting and presenting
molecules to immune cells than at other sites of the gene's DNA
sequence that do not play functional roles.
These results show that natural selection has driven the evolution
of these genes in different direction between the two species.
Furthermore, the researchers showed that infecting parasites found on
the two species were significantly different, as predicted based on the
known immune function of MHC genes. "The mechanisms having produced the
hundreds of species of East African cichlid fishes in a relatively
short period of time are unclear", says Jonatan Blais, the senior
author of the paper. "This is one of the first genetic adaptive
differences between closely related East African cichlid species
identified. As such, it improves our understanding of the recent
evolution of this incredibly diverse group of fish by pointing to a
trait that not only diverged for adaptive reasons but may also be
involved in mating behavior."
"The precise role that this divergence played in the evolution of
reproductive isolation has yet to be studied", comments Louis
Bernatchez, co-author of the study." But it offers an exciting new
perspective in the study of African cichlids speciation. "
Citation: Blais J, Rico C, van Oosterhout C, Cable J, Turner GF, et
al (2007) MHC Adaptive Divergence between Closely Related and Sympatric
African Cichlids. PLoS One 2(8): e734. doi:10.1371/journal.pone.0000734
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