Variance In Inbreeding Depression Research Articles

Little to no inbreeding depression in a tapeworm with mixed mating.

Meta-studies on hermaphrodites have found a negative relationship between primary selfing rates and levels of inbreeding depression (ID) and, thus, generally support purging in inbred systems. However, in plants, high among-taxa variance in ID results in no difference in the mean ID between outcrossing and mixed-mating taxa. Selective interference likely explains high ID among mixed-mating taxa, whereas low levels of ID among mixed-mating taxa are not as stressed. Among animal hermaphrodites, primarily molluscs, there are little data on mixed-mating systems. To fill a taxonomic and mating system gap, we tested for ID in a mixed-mating tapeworm, Oochoristica javaensis. We provide a direct estimate of ID across infection of an intermediate host by comparing selfing rates at two life history stages. We found little to no evidence for ID, and the level of ID falls in line with what is reported for highly selfing species even though O.javaensis has mixedmating. We discuss this result within the context of kin mating in O.javaensis. Our results emphasize that primary selfing rates alone may be insufficient to classify the inbreeding history in all species when testing for a relationship to ID. Mixed-mating taxa, and possibly some outcrossing taxa, may exhibit low levels of ID if biparental inbreeding is also driving purging. We advocate that ID studies report estimates of inbreeding history (e.g. FIS or identity disequilibrium) from nature-derived adult samples to provide context rather than relying on primary selfing rates alone.

Environmental conditions during early life determine the consequences of inbreeding in Agrostemma githago (Caryophyllaceae)

In an inbred population, selection may reduce the frequency of deleterious recessive alleles through a process known as purging. Empirical studies suggest, however, that the efficacy of purging in natural populations is highly variable. This variation may be due, in part, to variation in the expression of inbreeding depression available for selection to act on. This experiment investigates the roles of life stage and early-life environment in determining the expression of inbreeding depression in Agrostemma githago. Four population-level crosses ('self', 'within', 'near' and 'far') were conducted on 20 maternal plants from a focal population. Siblings were planted into one of three early environmental treatments with varying stress levels. Within the focal population, evidence for purging of deleterious recessive alleles, as well as for variation in the expression of inbreeding depression across the life cycle was examined. In addition, the effect of early environment on the expression of inbreeding depression and the interaction with cross-type was measured. We find that deleterious recessive alleles have not been effectively purged from our focal population, the expression of inbreeding depression decreases over the course of the life cycle, and a stressful early environment reduces the variance in inbreeding depression expressed later in life, but does not consistently influence the relative fitness of inbred versus outcrossed individuals.

Inbreeding depression in dioecious populations of the plant Mercurialis annua: comparisons between outcrossed progeny and the progeny of self-fertilized feminized males

Inbreeding depression is a key factor in the maintenance of separate sexes in plants through selection for the avoidance of self-fertilization. However, very little is known about the levels of inbreeding depression in dioecious species, obviously because it is difficult to self-fertilize males or females. We overcame this problem by clonally propagating males from lineages in a dioecious metapopulation of the European annual plant Mercurialis annua, feminizing some of them and crossing the feminized with the unfeminized clones. Using this method, we compared the fitness of selfed vs outcrossed progeny under field conditions in Spain, where this species grows naturally. Multiplicative inbreeding depression (based on seed germination, early and late survival, seed mass and pollen viability) ranged from -0.69 to 0.82, with a mean close to zero. We consider possible explanations for both the low mean and high variance in inbreeding depression in M. annua, and we discuss the implications of our results for the maintenance of dioecy over hermaphroditism.

The genetic consequences of fluctuating inbreeding depression and the evolution of plant selfing rates

The magnitude of inbreeding depression, a central parameter in the evolution of plant mating systems, can vary depending on environmental conditions. However, the underlying genetic mechanisms causing environmental fluctuations in inbreeding depression, and the consequences of this variation for the evolution of self-fertilization, have been little studied. Here, we consider temporal fluctuations of the selection coefficient in an explicit genetic model of inbreeding depression. We show that substantial variance in inbreeding depression can be generated at equilibrium by fluctuating selection, although the simulated variance tends to be lower than has been measured in experimental studies. Our simulations also reveal that purging of deleterious mutations does not depend on the variance in their selection coefficient. Finally, an evolutionary analysis shows that, in contrast to previous theoretical approaches, intermediate selfing rates are never evolutionarily stable when the variation in inbreeding depression is due to fluctuations in the selection coefficient on deleterious mutations.

NO CORRELATION BETWEEN INBREEDING DEPRESSION AND DELAYED SELFING IN THE FRESHWATER SNAIL PHYSA ACUTA

Inbreeding depression, one of the main factors driving mating system evolution, can itself evolve as a function of the mating system (the genetic purging hypothesis). Classical models of coevolution between mating system and inbreeding depression predict negative associations between inbreeding depression and selfing rate, but more recent approaches suggest that negative correlations should usually be too weak or transient to be detected within populations. Empirical results remain unclear and restricted to plants. Here, we evaluate, for the first time, the within-population genetic correlation between inbreeding depression and a trait that controls the amount of self-fertilization (the waiting time) in a self-fertile hermaphroditic animal, the freshwater snail Physa acuta. Using a large quantitative-genetic design (36 grand-families and 348 families), we observe abundant within-population family-level genetic variation for both inbreeding depression (estimated for survival, fecundity, and size) and the degree of behavioral selfing avoidance. However, we detected no correlation between waiting time and inbreeding depression across families. In agreement with recent models, this result shows that mutational variance rather than differential purging accounts for most of the genetic variance in inbreeding depression within a population.

Problems in measuring among‐family variation in inbreeding depression

Understanding the sources of variation in inbreeding depression within populations is important for understanding the evolution of selfing rates. At the population level, inbreeding depression is due to decreased heterozygosity caused by inbreeding, which decreases overdominance and increases the frequency of expression of recessive deleterious alleles. However, within individual families inbreeding has two distinct consequences: it reduces heterozygosity and it restricts the alleles present in offspring to those present in the parent. Outcrossing both increases heterozygosity and brings new alleles into a family (compared to the alleles present if the plant is self-pollinated). Both consequences of inbreeding affect offspring fitness, but the most common experimental design used to measure among-family variation in inbreeding depression cannot distinguish them. The result is that variance in inbreeding depression among families is confounded by genetic variation in the traits being measured. Also, correlations (among families) between measures of inbreeding depression or between inbreeding depression and mean trait values are confounded by genetic variation in the traits being measured. I conclude that more complex crossing designs that allow estimation of breeding values for individual families are required to accurately detect and measure among-family variation in inbreeding depression.

Environment-dependent inbreeding depression in a hermaphroditic freshwater snail.

Inbreeding depression was simultaneously studied under contrasted environments, laboratory and natural conditions, using individuals originating from 14 families of the freshwater snail Physa acuta. Both survival and growth of juveniles showed inbreeding depression under laboratory conditions. The same fitness components were monitored with mature snails either kept under laboratory conditions or released at a natural site and analysed using capture-mark-recapture models. Genetic composition of both samples was similar. Inbreeding depression on survival was highest in the laboratory while strong outbreeding depression was revealed in the field. Thus inbreeding depression may not be always higher under natural conditions, at the opposite of what is commonly assumed. We suggest that inbreeding depression is dependent on metabolic requirements imposed by the environment. Other evidences showing that inbreeding depression is environment-dependent are reviewed. We conclude that genetic models should include both genetic and environmental variance in inbreeding depression for studying mating system evolution.

The variance in inbreeding depression and the recovery of fitness in bottlenecked populations.

Theoretical analyses of inbreeding suggest that following an increased degree of inbreeding there may be a temporary recovery of fitness, because of selection either within or among inbred lineages. This is possible because selection can act more efficiently to remove deleterious alleles given the greater homozygosity of such populations. If common, recovery of fitness following inbreeding may be important for understanding some evolutionary processes and for management strategies of remnant populations, yet empirical evidence for such recovery in animals is scant. Here we describe the effects of single-pair population bottlenecks on a measure of fitness in Drosophila melanogaster. We compared a large number of families from each of 52 inbred lines with many families from the outbred population from which the inbred lineages were derived. Measures were made at the third and the 20th generations after the bottleneck. In both generations there was, on average, substantial inbreeding depression together with a highly significant variance among the inbred lines in the amount of fitness reduction. The average fitness of inbred lines was correlated across generations. Our data provide evidence for the possibility of recovery of fitness at two levels, because (i) the average fitness reduction in the F20 generation was significantly less than in the F3 generation, which implies that selection within lines has occurred, and (ii) the large variance in inbreeding depression among inbred lines implies that selection among them is possible. The high variance in inbreeding depression among replicate lines implies that modes of evolution which require a low level of inbreeding depression can function at least in a fraction of inbred populations within a species and that results from studies with low levels of replication should be treated with caution.

Individual variation in inbreeding depression: the roles of inbreeding history and mutation.

We use mutation-selection recursion models to evaluate the relative contributions of mutation and inbreeding history to variation among individuals in inbreeding depression and the ability of experiments to detect associations between individual inbreeding depression and mating system genotypes within populations. Poisson mutation to deleterious additive or recessive alleles generally produces far more variation among individuals in inbreeding depression than variation in history of inbreeding, regardless of selfing rate. Moreover, variation in inbreeding depression can be higher in a completely outcrossing or selfing population than in a mixed-mating population. In an initially random mating population, the spread of a dominant selfing modifier with no pleiotropic effects on male outcross success causes a measurable increase in inbreeding depression variation if its selfing rate is large and inbreeding depression is caused by recessive lethals. This increase is observable during a short period as the modifier spreads rapidly to fixation. If the modifier alters selfing rate only slightly, it fails to spread or causes no measurable increase in inbreeding depression variance. These results suggest that genetic associations between mating loci and inbreeding depression loci could be difficult to demonstrate within populations and observable only transiently during rapid evolution to a substantially new selfing rate.