As I discussed in previous posts, ALU elements are unique to primates and the fact that their proliferation rates and transcription patterns are different in humans than other primates suggests that they may be responsible some uniquely human traits. In "Under The Genomic Radar: The stealth model of alu amplification" Han and Xing, put forward some ideas about why some subfamilies of ALUs suddenly proliferate after millions of years with no activity. They explain that the AluYa and AluYb alu subfamilies actually date back 18-26my. However, the proliferation of these sequences has only occurred in the human lineage. Since the parent sequences date much further back in the primate genome, it seems that the capacity for proliferation has been there for quite some time.
The traditional model has been that master ALU sequence continually spawns daughter sequences that proliferate throughout the genome. However, this simplistic model fails to account for the fact that proliferation rates vary widely over time and across taxa. Han and Xing explain their stealth driver model as an alternate explanation. They believe that a low activity sequence can lurk in the genome over long periods of time. Occasionally, this sequence may spawn a daughter sequence that is much more active, these daughter sequences are actually responsible for the vast majority of the activity.
These highly active daughter sequences are highly likely to be detrimental to the organism. Natural selection may weed these out, but the parent stealth driver would be invisible to natural selection so it would be allowed to persist to spawn another high activity daughter sequence in the future.
In this sense, ALU elements themselves have in fact evolved to evolve. However, there is no reason to think that they have our interests in mind. These stealth drivers lay low because they don't want to kill their hosts and end up dead themselves. But, if they lay low forever they will be drowned out by their competitors.
But when do they come out of their hiding place? According to Hedges in "Differential alu mobilization and polymorphism among human and chimp lineages" it may be that these sequences end up proliferating when natural selection is unable to weed them out. This would occur during a population bottleneck. I found this idea very intriguing because as Wray pointed out in my last post, some uniquely human characteristics may be regulated by ALU transcription in the human brain. For example, regulation of the prodynorphin gene which has roles in memory, emotional status and perception. If Hedges idea is true, then a population bottleneck 3 million years ago could have been responsible for the fact that certain subfamilies that have not expanded in other primate lineages ended up proliferating in the human lineage. Hedges explains that any deleterious ALU insertion would be selected out by natural selection. But, in a population bottleneck, natural selection may not be able to get rid of the high activity master ALU sequence itself. So, although deleterious changes would be weeded out as they appear, the sequences would continue to proliferate. The vast majority of those that persist would be either neutral or very slightly deleterious. The human lineage is known for its bushiness. This extra variation may have driven increased speciation events. Is it possible that these extra raw materials may have given rise to some unique human traits? I believe the answer might be yes.
I have a lot more on my list to read on this topic. My research has led to some papers coming from the U which is pretty cool. My goal is to shed some light on the unique path that human evolution has trod and perhaps show that whether or not it is always beneficial to the organism as a whole, selfish elements in our genome have evolved to evolve. Perhaps the friction among these rival elements are the spark that ignited human evolution.
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