Blackwell Publishing

Evolutionary genomics - How do genomes expand and contract during evolution?


Genomes, as a whole or in part, change size during evolution by means of duplications and deletions. Once a duplication or deletion has spread through the population, the genome size of the species will have gone up or down. The genome sequence of a modern species can be used to infer when duplications and deletions have occurred in the past, and we can ask what evolutionary events are associated with changes in genome site.

For example, the genome sequence of the weed Arabidopsis contains many blocks of genes that look like duplicates: that is, one stretch of DNA looks like a duplicate of another stretch of DNA in the same genome. The duplications are not for single genes, or the whole genome, but for intermediate lengths of DNA and contain several genes. For each apparently duplicated blocks, we can compare the sequence of the twocopies of each gene in the block. We can then use a molecular-clock inference to estimate the time when the duplication occurred.

The result shows that most of the duplications seem to date back to three periods, at 100, 140, and 170 million years ago. Thus, the modern Arabidopsis genome seems to reflect 3 major duplication periods in the past. These duplications mainly occurred in the Mesozoic geological era, 100-200 million years ago. The Mesozoic was a crucial time for the origin of the angiosperms. For instance, dicotyledons may have split from monocotyledons about 180-210 million. If so, the earliest (200 million year old) duplications now visible in the Arabidopsis genome may have originated at this time. One of the major breakthroughs of angiosperm evolution the origin of dicotyledons would have been associated with a round of increased gene numbers.

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