inbreeding depression - senecio7の日記

García-Dorado A 2008 A simple method to account for natural selection when predicting inbreeding depression. Genetics 180:1559-1566.

recently, a simpler method was proposed on the basis of results derived for the mutation–selection–drift balance (García-Dorado 2007)
based on predicting the increased efficiency of natural selection against recessive genes in small populations from the corresponding transient increase of the additive variance ascribed to dominance
the prediction of the transient increase in additive variance is relatively cumbersome
it involves assumptions leading to some underestimate of the purge
underestimation was at most 10%
this underestimate will become larger for very recessive deleterious alleles

under no management, our predictions from Equation 5 closely fit the corresponding simulation results
comparison to predictions from the classical neutral approach show that the effect of purging is generally very important
these predictions are more accurate than those based on the approach previously given by García-Dorado (2007)
averaging over all the cases considered in Table 1, that approach underestimates mean fitness by 21% at generation 50 (results not shown)
the average underestimate with the present approach is <5%

in a recent article (García-Dorado 2007), simple equations were proposed to compute the main fitness genetic properties at the MSD balance
a method was developed to predict the behavior of the fitness mean and additive variance through a period of reduced population size
during which purge was computed from the increase in this additive variance ascribed to dominance

although the inferences regarding the MSD balance were accurate, that for the increase in additive variance after population size reduction was downwardly biased for substantially recessive deleterious allele
resulting in a similar bias for the effect of purge
here I present a simple and intuitive prediction for inbreeding depression with purge that does not rely upon the prediction of the additive variance

the critical parameter determining the purge is not the homozygous deleterious effect but its excess above the corresponding additive expectation
the actual inbreeding depression can be computed using the purged inbreeding coefficient (g_t) for deleterious alleles
which accounts for the reduction of their probability of homozygosis by descent caused by the excess d of their homozygous deleterious effect

this approach relies on the assumption that the ancestral population was at equilibrium between mutation and selection or between mutation, selection, and drift
if the ancestral population contains an excess of copies of deleterious alleles compared to the equilibrium case, an increase in fitness might occur even in the absence of any reduction in effective population size
due to natural selection using up the corresponding excess of fitness additive variance
this excess in natural selection should also be added to predict the fitness changes after a reduction in population size
it cannot be considered part of the purge
a concept that has been generally used to describe the increase in selection efficiency against recessive alleles due to the increased probability of homozygous genotypes during bottlenecking or inbreeding

purge can become very relevant for the evolution of fitness even in quite small populations
although deliberate inbreeding is not advised, the present approach should allow one to include purging into the prospect of in situ scenarios for endangered populations or in the evaluation of breeding strategies in ex situ conservation programs, allowing one to devise conservation strategies in a much more realistic genetic context