CARE FOR THE COMMUNITY: *
I've just read an interesting article in the journal Nature on the evolution of the adaptive immune system (1). Adaptive immunity differs from innate immunity in that the response of the immune system to encounters with foreign organisms or antigens depends on previous encounters. Adaptive immunity, in other words, "remembers" while innate immunity does not.
The article, 'Care for the Community' by Margaret McFall-Ngai claims that all vertebrates have an adaptive immune system along with the innate one while invertebrates rely entirely on the innate immune system (2). The presence of both immunological arms, according to the author,
"...has been thought to promote heightened resistance to microbial pathogens. But this assumption presents a conundrum-invertebrates are no less challenged by the microbial world, nor are they less able to remain healthy."
Why ? One answer is that invertebrates are small, short-lived and have many young at once ie they follow a "R-reproductive strategy" (3) to a greater extent than vertebrates do. In other words they have no need for the "long haul" system of adaptive immunity. The game will be long over before the first inning has barely begun. McFall-Nagai notes, however, that there are many invertebrates that are indeed long-lived, large and have only a limited number of offspring per year. She proposes a different explanation,
"...that adaptive immunity has evolved in part to recognize and manage complex communities of beneficial microbes living on or in vertebrates."
Complex microbial communities associated with vertebrates seem to be a shared feature of the subphylum while invertebrates have far fewer co-evolved partners (4). For instance there have been over 2,000 bacterial species found to exist as either commensal or mutualist partners of humans while fewer than 100 species of human pathogenic bacteria have been identified.
Invertebrates in contrast have far fewer "resident" bacteria. Their guts, for instance, have been shown to contain fewer than 8 residents to date. Most of the bacteria present are "tourists" from the outside world. Their innate immune system has evolved to limit the range of interactions to all but a few "welcome guests".
Three mechanisms have been suggested for how invertebrates "recognize and manage their resident microbial partners". One is intracellular maintenance where the bacteria are housed, hidden from the immune system, within cells. Another is a physical barrier such as the chitin layer of the hindguts of termites and their relatives. Thirdly, the ability of the innate immune system to recognize larger numbers of antigens than previously suspected has recently been demonstrated (5). The author concludes,
"The absence of these three strategies among vertebrates is consistent with the idea that the adaptive immune system has provided them with a different, more versatile, microbial-management strategy."
She suggests the following lines of inquiry to test her hypothesis:
a)Further research on comparative gut microbiota to test whether all vertebrates do indeed have a co-evolved microbial population while invertebrates rarely do.
b)Comparative physiology to see if these co-evolved populations do indeed always provide selective advantages to their hosts.
c)Finally, the price to be paid for this advantage. As she says,
"Is autoimmunity a collateral consequence of the bargain we have cut with microbes(6)."
Many other questions pop up in Molly's tiny feline mind. Like many other aspects of evolutionary biology, from the origins of organelles such as mitochondria and chloroplasts to the achievements of human societies, this is an example of the mutualist mechanism of evolution. Cooperation as opposed to competition will get you everywhere, even to multicellularity. How are interspecific partnerships first established ? What can be said to be evolving in these partnerships ? The vertebrate or the microbes ? It seems plain that the "unit of selection" is not just an animal but the animal along with its associated microflora. It gives a different twist to the idea of "individuality".
MOLLY NOTES:
*'Care for the Community' by Margaret McFall-Ngai, Nature, Vol. 445, 11 January 2007, p153.
1)For an interesting contrast between the innate and adaptive immune systems see Innate Vs Adaptive Immunity.
2)This is not entirely proven, as the author admits later in her article. A search can discover many hints of adaptive immunity in invertebrates. The argument backward from theory is from a lack of the molecular precursors of adaptive immunity in invertebrates. Their is no reason, however, that adaptive immunity couldn't evolve via separate pathways. Simple convergent evolution actually. Look up how many times eyes have evolved independently, with wholly different mechanisms, to see an example of such convergence. Also there is also some dispute about the immune system of "jawless fish".
3) For a brief description of r versus k reproductive strategies see Reproductive Strategies.
4)Once more this waits to be proven. Humans and domestic animals have, for obvious reasons, been the subject of greatest inquiry. To a large extent invertebrates are an unexplored continent.
5)Molly wonders if this isn't, in fact, a dawning realization that there may be an adaptive immune system in invertebrates albeit of a different type and with different mechanisms. It seems I repeat myself here.
6)And perhaps the mechanisms of cancer as well ? Not that invertebrates never develop cancer, but the rate may be significantly lower than amongst vertebrates. Why ?