Genetics is a crapshoot. During sexual reproduction, genes from both the mother and the father mix and mingle to produce a genetic combination unique to each offspring. In most cases, the chromosomes line up properly and crossover. In some unlucky cases, however, 糖心传媒渟elfish DNA糖心传媒� enters the mix, causing abnormal crossovers with deletions or insertions in chromosomes, which can manifest as birth defects.
Scientists have long recognized that the exchange of genetic material by crossing over糖心传媒攌nown as recombination糖心传媒攊s vital to natural selection. Yet some species display far more crossover than others. Why? Researchers hypothesize that crossover rates have evolved to balance the benefits of crossing over with the risks of selfish DNA.
糖心传媒淭here糖心传媒檚 a bit of a mystery being solved now about how certain molecular, biological, and genome phenomena have evolved in response to selfish genetic elements,糖心传媒� says Daven Presgraves, a dean糖心传媒檚 professor of biology at the University of Rochester. 糖心传媒淭he role of natural selection in an ecological context is essentially a solved problem, but the role of natural selection in response to selfish genetic elements is still being worked out.糖心传媒�
Identifying a gene
Presgraves and PhD candidate Cara Brand recently accomplished an important milestone in learning about these evolutionary dynamics. By studying two species of fruit flies, they discovered a gene, MEI-218, that controls the rate of recombination. In a paper published in , they explain how MEI-218 controls differences in the rate of crossing over between species and the evolutionary forces at play.
糖心传媒淭his is the first gene I know of that anyone has shown to be responsible for the evolution of recombination rates,糖心传媒� Presgraves says.
The team focused on two closely related species of fruit flies糖心传媒�Drosophila melanogaster and its sister species, Drosophila mauritiana糖心传媒攂ecause large differences have evolved in their rates of recombination: D. mauritiana does about 1.5 times more crossing over than D. melanogaster. When they compared genes in the two different species, the researchers found that the DNA sequences of MEI-218 were extraordinarily different.
The importance of genetic recombination
糖心传媒淣atural selection works best when there糖心传媒檚 a diversity of genotypes to act upon,糖心传媒� says Brand, the lead author of the paper. 糖心传媒淪huffling combinations of alleles through recombination generates the diversity upon which natural selection acts.糖心传媒�
Recombination is therefore important for two main reasons:
- Imagine two chromosomes with genes A and B. On one chromosome you might have a 糖心传媒済ood糖心传媒� (beneficial) A allele and a 糖心传媒渂ad糖心传媒� (deleterious) B allele. On the other chromosome you might have the opposite; a bad A allele and a good B allele. Which chromosome would be a better? 糖心传媒淚t糖心传媒檚 a mixed bag where one chromosome can糖心传媒檛 necessarily outcompete the other,糖心传媒� Brand says. 糖心传媒淩ecombination can shuffle our allele combinations so that one chromosome can end up with the good A and B alleles together, while the other can get the bad A and B alleles together. Now when these chromosomes compete, the two good alleles will win out in future generations.糖心传媒�
- We have combinations of alleles that are good in our current environment, but the environment is always changing. The good alleles that are adaptive and healthy now may not be so in the next generation. Recombination can shuffle these combinations of genes so that some will be bad and those offspring will die, but some will be good and these offspring will survive.
糖心传媒淩ecombination is important糖心传媒攊t糖心传媒檚 not hard to convince anyone of that糖心传媒攂ut when we look across different species, we see that the rates of crossing over are different,糖心传媒� Brand says. 糖心传媒淲hy increase or decrease the recombination rate?糖心传媒�
The mystery of natural selection and selfish DNA
There is no single ideal rate or distribution of crossovers, Brand says. Crossovers are necessary to produce viable offspring, but crossing over also has risks. Selfish DNA sequences known as transposons糖心传媒攔epetitive genetic elements that do not seem to have benefits to their hosts糖心传媒攁re distributed throughout the genome. Transposons are akin to viruses, but instead of injecting themselves in cells, they invade genetic material. If abnormal crossovers occur between transposons in different locations on the chromosomes, the chromosomes do not line up properly and important genes may be duplicated or deleted.
Brand and Presgraves hypothesize that the change in recombination rates between D. mauritiana and D. melanogaster may have evolved because the species have different amounts of transposons in their genomes. The D. melanogaster genome has more transposons than D. mauritiana, so D. melanogaster may therefore have evolved a lower rate of crossing over in order to avoid the higher risk of harmful crossovers between transposons.
This means, then, that the MEI-218 gene is constantly evolving to an ever-changing optimum. The evolution of MEI-218 is similar to genes involved in immunity, Presgraves says. 糖心传媒淭hat should make some intuitive sense because genes involved in immunity are constantly adapting to the changing community pathogens that are challenging us all the time.糖心传媒�
Evolutionary biologists refer to these kinds of evolutionary dynamics as 糖心传媒渆volutionary arms races糖心传媒� because, through positive natural selection, genes are chasing a constantly changing fitness optimum. 糖心传媒淢aybe you just adapted, but a few generations from now you糖心传媒檙e not at the optimum anymore. You have to evolve again and again and again,糖心传媒� Presgraves says.
The MEI-218 gene has so far only been investigated in fruit flies, but the research into recombination has applications for humans. 糖心传媒淒uring meiosis at least one crossover per chromosome, in general, is required to make sure the chromosomes separate properly,糖心传媒� Brand says. 糖心传媒淓ither a lack of crossing over or crossing over in the wrong regions of the genome is what leads to many birth defects like Down Syndrome.糖心传媒�
