I meant to post something about this a few weeks back, but with the current semester I never had the time.
You and I are different. I am guessing you had assumed this, but have you given thought to why? And I don't mean that in a philosophical/psychological sense. The concept of "self" and "other" is coded into us at a cellular level. This is why we have to check blood type before transfusions. This is why people have to be matched with specific organ doners. This is why we can't use the organs of other animals in humans. And there is a very
A vital part of the immune system are major histocompatability complexes. MHC genes generate MHC molecules. MHC molecules are what determine your cellular "self" from a cellular "other". Thus, should a virus infect your cells it is the MHC molecules that will take the viral proteins and present them to your immune system. Think of them as raising the alarm when invaders come to call. Now, because there are an awful lot of pathogens and those pathogens are constantly evolving, there must also be great variation in the MHC molecules to be able to keep up. In fact, there is so much variation that the MHC molecules from your body are so different from those in my body, that mine would identify yours as invaders. This establishing "self" and "other". We should be very thankful for that. Not everyone is so lucky.
The above is a Tasmanian devil (Sarcophilus harrisii)
. There are approximately 10,000 of them left. If estimates are accurate there will be 0 left by approximately 2035. All because of their MHC.
See, if your cells mix with mine, mine will identify yours as an invader. This isn't the case with Tasmanian devils. If cells from one devil mix with those from another, there is no immune response. So even though there are 10,000 of them left, there is no more diversity in their immune responses than if there were one.
Normally this would mean that the entire species would be at risk of being wiped out by an epidemic to which they could not generate an immune response. However, when that epidemic arrived its nature proved to be uniquely disturbing. They die of contagious cancer. Not an oncogenic virus, not a cancer generating from a common carcinogen in their environment. This is a cancer that can metastasize from individual to individual.
Because, at the cellular level, they aren't individuals. So when the cells of a healthy devil come into contact with malignant cells from an infected devil (commonly through fighting, copulating, or feeding/drinking from the same source), the malignancy spreads to the new devil in the same way it would spread between organs in a single organism.
Researchers are working to stem the tide of the epidemic, and there are reserve populations of healthy devils in captivity. However, even if this hurdle is overcome the underlying problem is still a quandary: what do you do when a species has lost too much diversity to be viable? Are other endangered species already past that point? Further, this is the third transmissible cancer that we have seen (there is a type which infects dogs and a type that infects hamsters). I doubt it will be the last.
I know that species rise and fall, but I really don't want to see these little guys go >_<
-Allograft theory: Transmission of devil facial-tumour disease. (Pearse & Swift 2006)
-Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial. (Siddle, et al. 2007)
-Tasmanian devil facial tumor disease: insights into reduced tumor surveillance from an unusual malignancy. (O'Neill 2010)