What are the differences in convergent evolution



25.10.2018 20:00

And evolution repeats itself: How evolution makes stripes come and go

Julia Wandt Communication and Marketing Unit
University of Konstanz

Evolutionary biologists from Constance around Prof. Dr. Axel Meyer discover the genetic basis of the evolution of color patterns. The new findings about the stripes of the particularly species-rich East African cichlids explain how evolution can repeat itself at a world record pace. The study will be published in Science magazine on October 26th.

Why does evolution repeat itself? And what happens genetically when evolution repeats itself? Are the same or different genes and mechanisms responsible for producing similar looking organisms? Biologists from Konstanz have come a little closer to the answer to this question, which is as old as it is important, in evolutionary biology, and the answer is astonishing. They investigated a special color pattern that occurs often in the animal world and in the most varied of species: horizontal stripes. They were able to identify the basis of the repeated evolution of these strips using modern genomic and molecular biological methods such as CRISPR-Cas.

Cichlids, of which over 1200 species occur in the great African lakes, Malawi, Victoria and Tanganyika, are, as their name suggests, particularly bright and colorful. But, they are not only diverse in terms of color, they also have numerous patterns such as longitudinal or transverse stripes. "But not only that" explains Prof. Dr. Axel Meyer, “Cichlids are prime examples of evolution. They are extremely diverse in terms of social behavior, body shapes, color patterns and many other biological aspects, but at the same time certain topics repeat themselves independently of one another in different lakes ”. This principle of repetitive evolution - the biologist speaks of 'convergence' - gives the cichlids a key role in researching the genetic basis of such phenomena. Because if similar color and body shapes have arisen independently of one another in several evolutionary lines, this means that evolution has found the same answer to similar environmental conditions. The question that then arises: if evolution repeats itself, how does it do it genetically?

How exactly the stripes of the cichlids come and go during evolution, and which gene and which genetic mechanism is responsible for this, the scientists have now been able to understand in detail through genome analyzes, breeding and experiments, also with the CRISPR-Cas gene scissors in the laboratory . "It can be determined exactly through breeding experiments," explains Dr. Claudius Kratochwil, young scientist in Professor Meyer's working group and first author of the study in Science, "on which of the 22 chromosomes of the fish, more precisely on which area of ​​this chromosome, the genetic region causing the stripes is located." The responsible gene on this piece of chromosome is called agrp2 . This "stripe gene" and its origin and occurrence in other African lakes has been described through comparative molecular work. From an evolutionary point of view, the stripes of the cichlids are quite unstable. Over the course of a few million years, they have often been lost and re-created in the African lakes. Since these species (with and without stripes) are so young, they can also be crossed with each other. The possible crossing and simultaneous examination of cichlids with and without stripes in the laboratory shows that all cichlids have the "stripe gene". However, the switches (regulatory elements) of the stripe gene differ in cichlid species with and without stripes. “The decisive trick of this genetic switch is that it causes the gene to be switched on more strongly in species without stripes, so that a lot of protein is produced. The stripe gene agrp2 therefore functions as a “stripe inhibitor”: when the gene production is higher, the stripes are suppressed, and when the gene production is low, they remain. This could be shown by modern genetic methods. "If the gene in the genome of a species without stripes is removed with the help of the genetic severity CRISPR-Cas", explains Kratochwil further, "then even a" stripe-free "fish suddenly develops stripes, which shows that the stripe gene is the decisive genetic factor".

The new findings on this genetic mechanism, the switching on and off of strips by the "strip gene", were published in the current issue of "Science". Interestingly, the agrp2 gene of the cichlid is a copy of the agouti gene in mammals, which is also responsible for color variations in cats, dogs, horses and striped chicks. "Perhaps the animal world would be much less colorful without the existence of the agouti gene family," speculates Dr. Claudius Kratochwil. What the mechanism of the “stripe gene” clearly enables in cichlids is the option of repeated evolution in a comparatively short time. If characteristics are lost in evolution, this loss is usually permanent, as the Belgian paleontologist Louis Dollo recognized exactly 125 years ago. In 1893 he formulated his "law" named after him. A special feature of the stripe gene agrp2 is that it allows the evolutionary repeated evolution of a trait in a relatively simple manner. If a cichlid loses its stripes, it does not mean that they cannot return - and vice versa. This modern molecular biological work also shows that paleontological rules and evolutionary laws have to be questioned anew.

Fact overview:
• Evolutionary biologists from Constance are discovering the genetic basis of stripes in various species of cichlid in African lakes.
• The new knowledge about the appearance of stripes can be explained by the genetic control of evolutionary convergence, the independent evolution of the same characteristics. As in this case, streaks were created and lost several dozen times in just a few thousand years.
• The agrp2 gene is responsible for the comings and goings of stripes in cichlids. In the laboratory it could be shown that the strong production of the gene inhibits the banding.
• This mechanism of switching strips on and off works in the same way for different, distantly related species of cichlid and thus explains “convergence”, the parallel evolution of similarities between evolutionarily distantly related species.
• The new findings about the meaning of absent stripes and the agrp2 gene have recently been published in the renowned “Science” magazine.

Original publication: Agouti-related peptide 2 facilitates convergent evolution of stripe patterns across cichlid fish radiations
Manuscript #: science.aao6809
Further links: https://www.evolutionsbiologie-uni-konstanz.com

Note to the editors:
Photos can be downloaded below:
https: //cms.uni-konstanz.de/fileadmin/pi/fileserver/2018/Bilder/Buntbarsch_Evolu ...
Caption: African cichlids (from Lake Tanganyika, Lake Malawi and Lake Victoria) with horizontal stripe patterns that illustrate the repetitive (convergent) evolution.
Copyright: Claudius Kratochwil

https: //cms.uni-konstanz.de/fileadmin/pi/fileserver/2018/Bilder/Buntbarsch_Evolu ...
Caption: A fish of the species Pundamilia nyererei from Lake Victoria, modified with the Crispr-Cas9 gene scissors. Due to the genetic modification, the fish shows horizontal stripes on the flank.
Copyright: Claudius Kratochwil

https: //cms.uni-konstanz.de/fileadmin/pi/fileserver/2018/Bilder/Buntbarsch_Evolu ...
Caption: The particularly colorful species Haplochromis chilotes from Lake Victoria, which shows both horizontal and vertical stripes.
Copyright: Claudius Kratochwil

https: //cms.uni-konstanz.de/fileadmin/pi/fileserver/2018/Bilder/Buntbarsch_Evolu ...
Caption: Two amazingly similar fish species with their characteristic horizontal stripes, which, however, are only very distantly related and are therefore a beautiful example of the repetitive (convergent) evolution: Julidochromis ornatus (Lake Tanganyika, left) and Melanochromis auratus (Malawis, right) .
Copyright: Claudius Kratochwil

Contact:
University of Konstanz
Communication and marketing
Phone: + 49 7531 88-3603
Email: [email protected]

Professor Axel Meyer, Ph.D.
Department of Zoology / Evolutionary Biology
Faculty of Biology
Phone: + 49 7531 88-3069
Email: [email protected]

Dr. Claudius Kratochwil
Phone: +49 7531 88-4583
E-mail: [email protected]

- uni.kn


Scientific contact:

Professor Axel Meyer, Ph.D.
Department of Zoology / Evolutionary Biology
Faculty of Biology
Phone: + 49 7531 88-3069
Email: [email protected]

Dr. Claudius Kratochwil
Phone: +49 7531 88-4583
E-mail: [email protected]


Original publication:

Original publication: Agouti-related peptide 2 facilitates convergent evolution of stripe patterns across cichlid fish radiations
Manuscript #: science.aao6809


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biology
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Research results, scientific publications
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