Olivia Judson (aka, Dr. Tatiana) has a blog at the NYTimes website. It's usually a good read, but she has been known to go off the deep end. In this week's entry, Judson posts on how bones are not the only fossils. What other fossils does Judson write about? Genomes.
Judson's focus is on genome size. She's clear that differences in genome size are the result of stuff other than genes, but she doesn't want to call that extra stuff "junk". Here's why:
I don't like "junk," which suggests the DNA is useless: even if it doesn't hold the instructions for making proteins, it may still serve a valuable purpose. For instance, stretches of non-coding DNA between parts of a gene can regulate how fast the protein gets made.
Um, Olivia, most of that stuff is useless. It's there to be there. We know because of positive evidence that a lot of the extra stuff is well characterized junk. Don't believe me? Read the posts at Genomicron.
But that's not Judson's main focus, so I'll lay off. Instead, she focuses on the marks left by genome size in fossils. For example, we can study cell sizes in fossils and use those to infer the size of the genomes of those extinct organisms. That was done in a study of dino-genomics, which found that the small genome sizes of birds evolved tens of millions of years before birds took flight. That threw a wrench in the hypothesis that small genomes in birds were selected for flight.
I was surprised that, in a discussion of genomic fossils, that Judson chose to focus on the literal fossils of genomes. I expected a discussion of the fossils in genomes. You know, the transposable elements and such that make up all that junk in the genome.
First I've heard about estimating genome size from fossils! Does the tissue really preserve well enough to get a good estimate of cell size? And is cell size really a good measure of genome size? Considering the vast range of different cell sizes in the human body, I wouldn't have expected cell size to be a remotely useful indicator of genome size. Or can the fossils actually tell us the size of the nucleus?
Peter, I think they focus on the same cell type in all organisms. I'm hazy on the details, but you should check out the dino-genomics paper from Scott Edwards' lab that I linked above.