Evolution Update

Evolution Update

The Vision System of Vertebrates Evolved by Whole Genome Duplications

Brandon Kieft March 29, 2015

When extra genetic material is generated, the resulting evolutionary trajectory of species can be eye opening.

Gene or whole genome duplication can lead to very dangerous birth defects in humans and other organisms. However, these phenomena are also important mechanisms for speciation and the evolution of complexity.

Interestingly, extant plants often have several copies of their genome in each cell, which is called polyploidy. In fact, wheat is a hexaploid, meaning each cell has six copies of its genome! But genome duplication also occurs in the animal kingdom as well. Notably, in mammals there have been two whole genome duplication events in the time between the dawn of vertebrates and now, each of which has played a part in our evolutionary history.

Genome duplication leads to speciation (the formation of new species) and evolution of complexity by two main mechanisms: neofunctionalization and subfunctionalization. These processes operate by either giving rise to a gene product with a new function (neofunctionalization) or a specialized function of the original(subfunctionalization).

A simplified example lends itself to understanding the evolutionary effects of genome duplication: Imagine an organism, say, a single-celled yeast, with just one chromosome; that is, just one set of genetic information (haploid). If this chromosome is duplicated, due to a reproduction error, then the next generation cell all of a sudden finds itself with two “blueprints” for all of the molecules it needs to survive and reproduce (diploid).

If this duplication of genetic information is not detrimental to the cell (not acted upon negatively by natural selection), the cell may go on living and reproducing and the error may be passed on to offspring. The significance of the duplication is that it creates redundancy. That is, this new diploid organism can survive with just one copy of all its genes (its ancestors all did), but now it has two. So the “extra” copy is not needed for survival, and therefore is not under selective pressure to retain its function. Over time, this may lead to random mutations in a gene on the second chromosome and, because of the mutations, a different gene product with a whole new function may arise (the neofunctionalization from above).

Now that you’re an expert in this complex evolutionary mechanism, we can take a look at recently discovered evidence for why genome duplication is important in human evolution.

A study published last week in PLOS ONE by researchers at Uppsala University has shown that a round of genome duplication over 350 million years ago contributed to development of vertebrate vision.

The research team, led by Professor David Lagman, studied a family of proteins in zebrafish called transducins, which are important in the molecular cascade that follows a light stimulus on the retina. Where humans have three types of transducins, zebrafish boast five. This is because humans have only experienced two genome duplications in the last 500 million years, where zebrafish have experienced three. The publication describes two important findings.

First, zebrafish share the same distinction between cone and rod (two cells types in the vertebrate eye that allow us to see colors and shapes, respectively) transducins that humans do. This shows that the genome duplication event that led to the transducins in these cells occurred at least 420 million years ago, before the fish lineage split from the mammals.

Second, they showed that the three transducin types in zebrafish have different levels of expression (the genes are “on” at different times and for different durations). They are also found each transducin protein in different parts of the retina, showing that they have specialized, possibly to different levels of light intensity, say researchers.

Their research concludes that the ancient genome duplications in the vertebrate lineage were crucial for producing our vision system as it is today. Without the duplication and subfunctionalization of genes like the transducin genes in this study, our vision may very well be much poorer than it is today.

The first sentence of the publication sums up this concept quite simply, saying “Gene duplications provide raw materials that can be selected for functional adaptations by evolutionary mechanisms”.

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