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2016 ; 8
(9
): 2856-2869
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Inevitability of Genetic Parasites
#MMPMID27503291
Iranzo J
; Puigbò P
; Lobkovsky AE
; Wolf YI
; Koonin EV
Genome Biol Evol
2016[Sep]; 8
(9
): 2856-2869
PMID27503291
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Almost all cellular life forms are hosts to diverse genetic parasites with
various levels of autonomy including plasmids, transposons and viruses.
Theoretical modeling of the evolution of primordial replicators indicates that
parasites (cheaters) necessarily evolve in such systems and can be kept at bay
primarily via compartmentalization. Given the (near) ubiquity, abundance and
diversity of genetic parasites, the question becomes pertinent: are such
parasites intrinsic to life? At least in prokaryotes, the persistence of
parasites is linked to the rate of horizontal gene transfer (HGT). We
mathematically derive the threshold value of the minimal transfer rate required
for selfish element persistence, depending on the element duplication and loss
rates as well as the cost to the host. Estimation of the characteristic gene
duplication, loss and transfer rates for transposons, plasmids and virus-related
elements in multiple groups of diverse bacteria and archaea indicates that most
of these rates are compatible with the long term persistence of parasites.
Notably, a small but non-zero rate of HGT is also required for the persistence of
non-parasitic genes. We hypothesize that cells cannot tune their horizontal
transfer rates to be below the threshold required for parasite persistence
without experiencing highly detrimental side-effects. As a lower boundary to the
minimum DNA transfer rate that a cell can withstand, we consider the process of
genome degradation and mutational meltdown of populations through Muller's
ratchet. A numerical assessment of this hypothesis suggests that microbial
populations cannot purge parasites while escaping Muller's ratchet. Thus, genetic
parasites appear to be virtually inevitable in cellular organisms.
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