The Effective Size of Populations Infected With Cytoplasmic Sex-Ratio Distorters

Abstract

Many arthropod species are infected with maternally inherited endosymbionts that induce a shift in the sex ratio of their hosts by feminizing or killing males (cytoplasmic sex-ratio distorters, or SRDs). These endosymbionts can have profound impacts on evolutionary processes of their hosts. Here, I derive analytical expressions for the coalescent effective size N e of populations that are infected with SRDs. Irrespective of the type of SRD, N e for mitochondrial genes is given by the number of infected females. For nuclear genes, the effective population size generally decreases with increasing prevalence of the SRD and can be considerably lower than the actual size of the population. For example, with male-killing bacteria that have near perfect maternal transmission, N e is reduced by a factor that is given to a good approximation by the proportion of uninfected individuals in the population. The formulae derived here also yield the effective size of populations infected with mutualistic endosymbionts or maternally inherited bacteria that induce cytoplasmic incompatibility, although in these cases, the reduction in N e is expected to be less severe than for cytoplasmic SRDs.

Effect of parasitic sex-ratio distorters on host gene frequencies in a mainland-island context

“…We also confirmed that this formula can be derived by substituting χ i = t (1 + B )/2 and χ u = χ m = (1 − t )(1 + B )/2 into Engelstädter's (2010) formula [Note that his original formula is incorrect and the correct one is given in a recently published errata note (Genetics 186, pp. 2503)].…”

“…First, let us consider the calculation of the effective population size of the island population. Recently, Engelstädter (2010) has obtained a formula of effective population size applicable to various kinds of cytoplasmic sex‐ratio distorters. Here, we instead use a more general formula for a class‐structured population, which was derived by previous authors (Nagylaki, 1980; Nordborg & Krone, 2002; Rousset, 2004; Sjödin et al.…”

Modification of Insect and Arachnid Behaviours by Vertically Transmitted Endosymbionts: Infections as Drivers of Behavioural Change and Evolutionary Novelty

“…Whilst the hypothesis that endosymbiont driven changes in behaviour can lead directly to reproductive isolation is difficult to test, it is certainly true that changes in both reproductive and non-reproductive behaviours such as mating preference or dispersal likely influence genetic diversity at a local scale. For instance restriction in the number of successfully reproducing individuals via male-killing or CI will lower the effective population size [ 25 ]. Such impacts on population size and the various impacts on reproductive traits will have further consequences for the local intensity and dynamics of sexual selection and conflict [ 26 , 27 , 28 ].…”

A neo-W chromosome in a tropical butterfly links colour pattern, male-killing, and speciation

“…Male-killing can increase infected female fitness at the expense of non-transmitting infected males by increasing the resources available to females, either by reducing inter-sibling competition or via cannibalism [ 1 ]. Endosymbionts that have near-perfect transmission may reduce the effective population size ( N e ) by a factor that approximates to the proportion of uninfected individuals in the population [ 2 ]. Theoretical studies of male-killing endosymbionts, where hybridizing subpopulations interbreed, have shown that local adaptation can be strongly impeded in the subpopulation with the more biased population sex ratio [ 3 ].…”