Why are the genomes of some trees so large?

Question

For example, the current largest known genome belongs to a tree: http://motherboard.vice.com/read/the-largest-genome-ever-sequenced-belongs-to-a-treeI have heard that this could potentially be because trees require many duplicates of certain genes because they live their entire life exposed to direct sunlight.Does anyone know of some specific scientific hypotheses for why this might be the case?

Answer 1

There are many ways to interpret your question. I'm going to try my best because, even though it is an old question, it's an interesting idea.

As of about two years ago, the largest genome is found in the fern species Tmesipteris oblanceolata. Essentially, some of the largest genomes in the world are found in plants due to the fact that plants are really good at surviving polyploidy, or the duplication of chromosomes. If a human has a three of a specific chromosome instead of the normal two, this tends to be incompatible with life (with the exception of very small chromosomes such as number 21. This is how we get Down Syndrome).

However plants regularly survive having multiple copies of each chromosome, sometimes passing this mutation down to the next generation. Fast forward a few million years, and you get many plant species with much higher than the usual two copies of each chromosome.

Back to your original question about a hypothesis, I'm going to re-word it a little. You are asking for a hypothesis to prove that these trees evolved more genetic material in order to survive sunlight. You are looking for the cause of the polyploidy. While I could be misinterpreting, what I think you really mean is "Does having polyploidy provide an advantage to organisms exposed to more UV radiation?". These are two different questions.

Since polyploidy is primarily survivable in plants, you cannot compare plants to other types of organisms. I would take a bunch of plants and plot them into a graph based on their UV exposure in their natural habitat and compare that data to the number of chromosome copies each species has. Is there a pattern?

A follow up question might be "Does polyploidy confer any disadvantages?".

If you found many sun-dwelling species both with and without large genomes, but few shade-dwelling organisms, this may answer your question.

If both shade and sun-dwelling organisms both have similar instances of polyploidy, then it could mean several things. Some examples:

1. Polyploidy doesn't protect against damage from the sun
2. Polyploidy has no negative impact on the fitness of a plant, so any plant that mutated kept those mutations
3. Polyploidy provides an advantage that doesn't have to do with the sun.