"The main function of the vacuole is to contain and degrade cellular wastes and toxins," Zhang said. "In humans, the absence of vacuole proteins causes those wastes and toxins to accumulate, often leading to fatal neurological diseases."
The same thing happens in the mouse, but at a much later stage of life, often past reproductive age. As a result, "many of these vacuole proteins are not so 'essential' to the mouse," Zhang said. "Even without the proteins, the mouse can survive long enough to reproduce."
The researchers speculated that in the course of primate evolution, as life span increased and reproductive age was delayed, efficient waste management became increasingly important.
Additional results of their analysis support the idea. By developing an index that incorporated metabolic rate (a measure of how fast cellular waste products are generated) and reproductive age, and then using that index to compare human and mouse, the researchers determined that the total amount of waste produced per gram of body mass from birth to reproductive age is about 18 times higher for humans than for the mouse.
"Hence, waste management is much more important in humans than in the mouse for maintaining proper cellular functions until the time of reproduction," Zhang said. "And when a biological process becomes more important to a species, the genes involved in that process tend to become essential."
Zhang acknowledges that the study involved a relatively small number of genes, and he hopes that other researchers will be able to confirm the results as more information on human and mouse genes becomes available.
"If our sample is unbiased, o
|Contact: Nancy Ross-Flanigan|
University of Michigan