Genome size increases?

[Danijel Gorupec]

I hope I don't ask something dumb...  Are there studies to check if, on average among species, genome size increased in last tens of millions of years? Would you expect that evolution continuously increases genome sizes, or did we reach some sort of equilibrium already?

[CharonY]

3 hours ago, Danijel Gorupec said:

I hope I don't ask something dumb...  Are there studies to check if, on average among species, genome size increased in last tens of millions of years? Would you expect that evolution continuously increases genome sizes, or did we reach some sort of equilibrium already?

 

There are multiple things here. Do you mean within or between species? But generally speaking there is no trend in evolution with regard to genome size. Rather, it depends a lot on the species and how they deal with additional genomic baggage. Many bacteria have a somewhat efficient genome size, as cell replication is slower with a larger one and any additional material would need to offset that cost. Eukaryotes tend to be somewhat less restricted and there is no correlation between genome size and e.g. overall complexity and there is often an excess of non-coding DNA. The fact that closely related eukaryotic species typically have very similar number of genes (i.e. coding DNA) but vastly different genome sizes is also referred to as the c-value paradox.

Humans are around a modest 3.1 Gbp, which and in most mammals the variation is not huge. Yet, among animals, the largest genome size belongs to a bone fish with 130 Gbp. And in a fern a genome size of 160 Gbp was found. In amoeba size variation have been huge, ranging from 23-ish Mbp to many Gbp (though it could be a bit overestimated, as the size estimates were not done by sequencing in many cases). 

But in short, no we do not expect any specific evolutionary trends in genome size. Gene numbers, on the other hand, are under more constraints.

 

 

[Danijel Gorupec]

Thanks CharonY, nice answer. One more question: you mentioned large differences in genome sizes even within related eukaryotic species - does it mean that genome size can change in large steps (I suppose this is more likely than series of many small unidirectional steps)?

[CharonY]

To some degree and depending species. But more commonly are incremental but stochastic increases e.g. due to duplications of repetitive regions. Polyploidy is another mechanism, though in that case genes are also duplicated. Others are related mechanisms of chromosomal structures that can lead to duplication of larger chunks.