Skal vi lave en ny art?

I artiklen Regulating Evolution: How Gene Switches Make Life (som i øvrigt indeholder en rig mængde henvisninger til bedre forståelse og mere information om forskellige områder) fra “Scientific American” fortælles om de genetiske koders, DNA, bevisførelse for evolution. Og at der ligefrem er blevet skabt en digital molekulær zoologisk have over de forskellige dyrs DNA-sekvenser. Et bibliotek der skal hjælpe forskere i at forstå hvordan vi forskellige dyr har udviklet os som vi har. Tidligere havde forskerne en ide om at der indenfor de forskellige arter måtte være en betydelig forskel i genomet (eller arvemassen), men resultater viser at ligheden mellem de forskellige arter er mindre end man havde regnet med (ex her). Variationerne skulle således forstås ved mere specifikke områder af genomet, fx i små mutationer for hvordan generne bliver brugt og hvornår.

Genetiske styreknapper

The most excited visitors to this new molecular zoo are evolutionary biologists, because within it lies a massive and detailed record of evolution. For many decades, scientists have longed to understand how the great diversity of species has arisen. We have known for half a century that changes in physical traits, from body color to brain size, stem from changes in DNA. Determining precisely what changes to the vast expanse of DNA sequences are responsible for giving animals their unique appearance was out of reach until recently, however.

Biologists are now deciphering the DNA record to locate the instructions that make assorted species of flies, fish or finches look different from one another and that make us humans different from chimpanzees. This quest has led to a profound change in our perspective. For most of the past 40 years or so, researchers have focused most of their attention on genes—the nucleotide sequences in DNA that encode the amino acid chains that form proteins. But to our surprise, it has turned out that differences in appearance are deceiving: very different animals have very similar sets of genes. By following the trail of evolution, devices are being found within DNA—genetic “switches”—that do not encode any proteins but that regulate when and where genes are used. Changes in these switches are crucial to the evolution of anatomy and provide new insights into how the seemingly endless forms of the animal kingdom have evolved.

Det som forskerne har fundet ud af er at det er små elementer i de forskellige DNA’en der har stor betydning for hvordan de forskellige har udviklet sig som de har gjort. De snakker således om nogle genetiske kontakter (Kontakterne i aktion), der er med til at bestemme om hvordan og hvornår de forskellige celler “opfører” sig på forskellige tidspunkter

For a long time, scientists certainly expected the anatomical differences among animals to be reflected in clear differences among the contents of their genomes. When we compare mammalian genomes such as those of the mouse, rat, dog, human and chimpanzee, however, we see that their respective gene catalogues are remarkably similar. The approximate number of genes in each animal’s genome (about 20,000 or so) and the relative positions of many genes have been fairly well maintained over 100 million years of evolution. That is not to say there are no differences in gene number and location. But at first glance, nothing in these gene inventories shouts out “mouse” or “dog” or “human.” When comparing mouse and human genomes, for example, biologists are able to identify a mouse counterpart for at least 99 percent of all our genes.

Only a small fraction of all genes—fewer than 10 percent—are devoted to the construction and patterning of animal bodies during their development from fertilized egg to adult. The rest are involved in the everyday tasks of cells within various organs and tissues.

[T]he discovery that body-building proteins are even more alike on average than other proteins was especially intriguing because of the paradox it seemed to pose: animals as different as a mouse and an elephant are shaped by a common set of very similar, functionally indistinguishable body-building proteins. The same applies to humans and our closest living relatives.

Både små og store forskelle

Forskerne har b.la. søgt i forskellene mellem pigmenteringen i tæt relaterede insekter, hvor mønstret og farverne i fx vinger divergerer fra art til art. Her har de søgt at finde ud af præcist hvad det er der giver de små forskelle, som illustreret på billedet (Klik her for større version)

A gene involved in coloring the body parts of the fruit fly illustrates the modular logic of this gene regulation system. The somewhat confusingly named Yellow gene encodes a protein that promotes the formation of black pigmentation (mutant flies without this protein are Yellow). The Yellow gene has separate enhancers that activate it during the development of a variety of fly body parts, including the wings and abdomen.

To figure out how Yellow is produced in a wing spot in some species and not others, we searched the DNA sequences around the Yellow gene for the enhancers that control its expression in various body parts. In unspotted species, there is an enhancer that drives Yellow expression in a low uniform pattern in the wing. This wing-enhancer activity generates the fly wings’ light-gray color. When the corresponding piece of DNA was analyzed from a spotted species, we found that it drives both this low-level pattern and the intense spot pattern of gene expression. What has happened in the course of evolution of spotted species is that new binding sites for transcription factors made in the wing evolved in the Yellow wing-enhancer DNA sequence. These changes created an expression pattern—wing spots—without altering where the Yellow protein is made or how it functions elsewhere in the body

Artiklen giver flere eksempler på hvorledes små mutationer i fx enhancerne har betydet store forskelle (Eks. udviklingen for bækkenpartiet hos fisk). Fx gør netop en mutation i enhancere at en stor gruppe mennesker der har levet i områder med malaria-epidemier i Vest-Afrika har udviklet en resistens overfor infektioner (scroll lidt ned og klik på linket “RBC Duffy Negativity”, da linket åbenbart ikke giver den direkte rute) af malaria. Og forskere mener også at disse små mutationer forklarer såvel små som større forskelle mellem mennesker internt, og også mellem mennesker og andre menneskeaber. Og således også har del i forskellen mellem de mange forskellige “dyr på Savannen”. Men tilbage til den nye “zoologiske have” giver den et fantastisk redskab for biologer i den videre forskning i området.

These are still early days for research into the evolution of gene-regulating DNA sequences. And hundreds of thousands of genetic switches in the virtual zoo of genomes have yet to be discovered or investigated. Biologists are already learning new principles, however, that have predictive value for future studies: evolutionary changes to anatomy, particularly those involving pleiotropic genes, are more likely to happen via changes to gene enhancers than to the genes themselves.

This phenomenon also reveals how very diverse groups of animals can share most, if not all, the genes involved in body building and body patterning—contrary to scientists’ early expectations, it is mostly a matter of how and when those genes are used that shapes the different forms of the animal kingdom. If we really want to understand what makes the human form different from that of other apes or what makes an elephant distinct from a mouse, for that matter, much of that information lies not in our respective genes and proteins but in an entirely different realm of our genomes that remains to be explored.

~ af sorensvendsen på maj 6, 2008.

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