I was interested in what they had to say about junk DNA and the evolution of large complex genomes. The only article that directly addressed the topic was "Noncoding DNA Evolution: Junk DNA Revisited" by Michael Z. Ludwig of the Department of Ecology and Evolution at the University of Chicago. Ludwig is a Research Associate (Assistant Professor) who works with Martin Kreitman on "Developmental regulation of gene expression and the genetic basis for evolution of regulatory DNA."
As you could guess from the title of the article, Michael Ludwig divides the genome into two fractions; protein-coding genes and noncoding DNA. The fact that organismal complexity doesn't correlate with the number of genes (protein-coding) is a problem that requires an explanation, according to Ludwig. He assumes that the term "junk DNA" was used in the past to account for our lack of knowledge about noncoding DNA.
Eukaryotic genomes mostly consist of DNA that is not translated into protein sequence. However, noncoding DNA (ncDNA) has been little studied relative to proteins. The lack of knowledge about its functional significance has led to hypotheses that much nongenic DNA is useless "junk" (Ohno, 1972) or that it exists only to replicate itself (Doolittle and Sapienza, 1980; Orgel and Crick, 1980).Ludwig says that we now know some of the functions of non-coding DNA and one of them is regulation of gene expression.
These regulatory sequences are distributed among selfish transposons and middle or short repetitive DNAs. The genome is an extremely complex machine; functionally as well as structurally it is generally not possible to disentangle the regulatory function from the junk selfish activity. The idea of junk DNA needs to be revisited.Of course we all know about regulatory sequences. We've known about this function of non-coding DNA for half a century. The question that interests us is not whether non-coding DNA has a function but whether a large proportion of noncoding DNA is junk.
Ludwig seems to be arguing that a significant fraction of the mammalian genome is devoted to regulation. He doesn't ever specify what this fraction is but apparently it's large enough to "revisit" junk DNA.
The biggest obstacle to his thesis is the fact that only 8% of the human genome is conserved (Rands et al., 2014). Ludwig says that 1% of the genome is coding DNA and 7% "has a functional regulatory gene expression role" according to the Rands et al. study. This is somewhat misleading since Rands et al. specifically mention that not all of this conserved DNA will be regulatory.
All of this is consistent with a definition of function specifying that it must be under negative selection (i.e. conserved). It leads to the conclusion that about 90% of the human genome is junk. That doesn't require a re-evaluation of junk.
In order to "revisit" junk DNA, the proponents of the "complex machine" view of evolution must come up with plausible reasons why lack of sequence conservation does not rule out function. Ludwig offers up the standard rationales ...
- Some ultra-conserved sequences don't seem to have a function and this "shows that the extent of sequence conservation is not a good predictor of the functional importance of a sequence."
- The amount of conserved sequence depends on the alignment and alignment is difficult.
- About 40%-70% of the noncoding DNA in Drosophila melanogaster is under functional constraint within the species but not between D. melanogaster and D. simulans. Therefore, some large fraction of functional regulatory sequences might only be conserved in the human lineage and it won't show up in comparisons between species. (Does this explain onions?)
That's a ridiculous number, but so is 7%. Imagine that "only" 7% of the genome is functionally involved in regulating expression of the protein-coding genes. That's 224 million base pairs of DNA or approximately 10 thousand base pairs of cis-regulatory elements (CREs) for every protein-coding gene.
There is no evidence whatsoever that even this amount (7%) of DNA is required for regulation but Ludwig would like to think that the actual amount is much greater. The lack of conservation is dismissed by assuming rapid turnover while conserving function and/or stabilizing selection on polymorphic sequences.
The problem here is that Ludwig is constructing a just-so evolutionary story to explain something that doesn't require an explanation. If there's no evidence that a large fraction of the genome is required for regulation then there's no problem that needs explaining. Ludwig does not tell us why he believes that most of our genome is required for regulation. Maybe it's because of ENCODE?
Since this is published in the Encyclopedia of Evolutionary Biolgoy, I assume that this sort of evolutionary argument resonates with many evolutionary biologists. That's sad.
Rands, C. M., Meader, S., Ponting, C. P., and Lunter, G. (2014) 8.2% of the Human Genome Is Constrained: Variation in Rates of Turnover across Functional Element Classes in the Human Lineage. PLoS Genetics, 10(7), e1004525. [doi: 10.1371/journal.pgen.1004525]