Mutation accumulation in populations of varying size: large effect mutations cause most mutational decline in the rotifer Brachionus calyciflorus under UV‐C radiation

Abstract

Theory predicts that fitness decline via mutation accumulation will depend on population size, but there are only a few direct tests of this key idea. To gain a qualitative understanding of the fitness effect of new mutations, we performed a mutation accumulation experiment with the facultative sexual rotifer Brachionus calyciflorus at six different population sizes under UV-C radiation. Lifetime reproduction assays conducted after ten and sixteen UV-C radiations showed that while small populations lost fitness, fitness losses diminished rapidly with increasing population size. Populations kept as low as 10 individuals were able to maintain fitness close to the nonmutagenized populations throughout the experiment indicating that selection was able to remove the majority of large effect mutations in small populations. Although our results also seem to imply that small populations are effectively immune to mutational decay, we caution against this interpretation. Given sufficient time, populations of moderate to large size can experience declines in fitness from accumulating weakly deleterious mutations as demonstrated by fitness estimates from simulations and, tentatively, from a long-term experiment with populations of moderate size. There is mounting evidence to suggest that mutational distributions contain a heavier tail of large effects. Our results suggest that this is also true when the mutational spectrum is altered by UV radiation.

Mutational Landscape of Spontaneous Base Substitutions and Small Indels in Experimental Caenorhabditis elegans Populations of Differing Size

“…(D) A lower base substitution rate in cores relative to arms and tips applies to exons, introns, and intergenic regions. Heilbron et al 2014;Luijckx et al 2018). Although the K a /K s ratio for different population sizes trends in the expected direction, this negative correlation was not significant.…”

WITHDRAWN: Responses of the Ecological Characteristics and Antioxidant Enzyme Activities in Rotaria Rotatoria to UV-B Radiation

“…Over the past few decades, it has been well demonstrated that nature ultraviolet-B (UV-B) radiation increased signi cantly with the continuing depletion of stratospherico zone, which has becoming one of the grave concerns and most striking global changes [10,11]. There is growing evidence that adverse biological effects of ambient ultraviolet radiation exposure occur in not only marine but also freshwater ecosystem at different trophic levels [11,12], involving algae [13], rotifers [14,15], copepods [16,17], cladocerans [18], shrimps [19,20] and shes [21]. In all cases, UVR was found to harm the organisms in question by damaging DNA directly at the molecular level and affect tness at the individual and population levels [22].…”

Causes of variability in estimates of mutational variance from mutation accumulation experiments

“…Similar results have been obtained for other studies in nematode (e.g., K atju et al . 2015) and rotifers (L uijckx et al . 2018), suggesting preponderance of mutations with large fitness effects.…”

“…The fate of a new mutation is determined by drift when its fitness effect is less than 1/2Ne (WRIGHT 1931;KIMURA 1983). Empirical evidence suggests that population sizes as low as ten may be sufficient to prevent fixation of mutations with deleterious fitness effects (ESTES et al 2004;KATJU et al 2015;LUIJCKX et al 2018). We imposed a minimum population size of 12 males and 12 females (Figure 2A).…”

“…found that deleterious mutations accumulated similarly in C. elegans when N=1 or N=5 (s > 0.2), whereas there was no evidence of mutation accumulation when N = 25 (s ~ 0.02). Similar results have been obtained for other studies in nematode (e.g.,KATJU et al 2015) and rotifers(LUIJCKX et al 2018), suggesting preponderance of mutations with large fitness effects. explanations of estimate heterogeneity in our data, we conclude that the observed variability is largely the consequence of sampling (measurement) error.…”