Friday, 28 June 2013

Biological Systems: Is Redundancy Redundant?

The term redundancy is often used to describe two or more processes in biological systems that appear to do the same job. The term has been borrowed from Information Theory. The definition in Wikipedia is as useful as any:
Redundancy in information theory is the number of bits used to transmit a message minus the number of bits of actual information in the message. Informally, it is the amount of wasted "space" used to transmit certain data.
I have never been comfortable with the term in biological systems except in discussions on signalling systems (endocrine, autocrine, paracrine, intracrine) where it can be used in its proper, Information Theory, sense. My qualms on using it for biochemical pathways and membrane transport systems, for example, are that it has implications of a mechanism being present but not needed. I have argued in the past that parallel pathways and different transport mechanisms carrying the same substrate are better described as safety mechanisms (belt-and-braces adaptations) or as systems that may be used in some circumstances and not others.

The other problem in using the term is that it can be confused with redundancy in the evolutionary sense, a vestigial character for example. I sometimes think we should leave the term in its more usual English meaning of superfluous in the hands of human resource departments, those parasitic forms of life, to apply to themselves. Auto-redundancy in these anti-personnel departments would do more than a little to aid economic recovery — and benefit science, technology and engineering.

The twitching of my ears and muscular contractions in other parts of my anatomy at hearing the term (along with ‘model organism’, ‘model system’ and the like) appear more and more to be justified as mechanisms are investigated in greater depth than the appearance of blots on gels.

A recent paper in Proceedings of the Royal Society* by Zoe Dumas, Adin Ross-Gillespie and Rolf K├╝mmerli sums up in its title the dangers of using the term willy-nilly: Switching between apparently redundant iron-uptake mechanisms benefit bacteria in changeable environments. I can do no better than to quote from the abstract:
Bacteria often possess multiple siderophore-based iron uptake systems for scavenging this vital resource from their environment. However, some siderophores seem redundant, because they have limited iron-binding efficiency and are seldom expressed under iron limitation. Here, we investigate the conundrum of why selection does not eliminate this apparent redundancy. We focus on Pseudomonas aeruginosa, a bacterium that can produce two siderophores—the highly efficient but metabolically expensive pyoverdine, and the inefficient but metabolically cheap pyochelin. We found that the bacteria possess molecular mechanisms to phenotypically switch from mainly producing pyoverdine under severe iron limitation to mainly producing pyochelin when iron is only moderately limited. We further show that strains exclusively producing pyochelin grew significantly better than strains exclusively producing pyoverdine under moderate iron limitation, whereas the inverse was seen under severe iron limitation. This suggests that pyochelin is not redundant, but that switching between siderophore strategies might be beneficial to trade off efficiencies versus costs of siderophores…
So, how many apparently redundant mechanisms can really be described as such? Perhaps we really should confine it to its proper use in signalling systems where redundancy in the message decreases the error in transmission.

Peaker, M. 1992. Chemical signalling systems: the rules of the game. Journal of Endocrinology 135 1-4
Proceedings of the Royal Society B 7 August 2013 vol. 280 no. 1764 20131055
Sir Barry Cross’s loudly whispered description in the early 1980s of the personnel department of the old Agricultural Research Council (later AFRC, now BBSRC)