Inbreeding depression in Delphinium nelsonii

Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression, though inbreeding and outbreeding depression can simultaneously occur. Inbreeding depression seems to be present in most groups of organisms, but varies across mating systems.

Hermaphroditic In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have s ...

species often exhibit lower degrees of inbreeding depression than outcrossing species, as repeated generations of

selfing Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species ...

is thought to

purge In history, religion and political science, a purge is a position removal or execution of people who are considered undesirable by those in power from a government, another organization, their team leaders, or society as a whole. A group undertak ...

deleterious

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

s from populations. For example, the outcrossing nematode (roundworm) ''

Caenorhabditis remanei ''Caenorhabditis remanei'' is a species of nematode found in North America and Europe, and likely lives throughout the temperate world. Several strains have been developed in the laboratory.C. elegans'', which experiences outbreeding depression.

Mechanisms

Inbreeding (i.e., breeding between closely related individuals) results in more recessive traits manifesting themselves, as the

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

s of pair-mates are more similar. Recessive traits can only occur in an offspring if present in both parents' genomes. The more genetically similar the parents are, the more often recessive traits appear in their offspring. This normally has a positive effect, as most genes are undergoing purifying selection (the homozygous state is favored). However, for very closely related individuals, there is an increased likelihood of

homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

deleterious

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s in the offspring which can result in unfit individuals. For the

s that confer an advantage in the

heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

and/or homozygous-dominant state, the fitness of the homozygous-recessive state may even be zero (meaning sterile or unviable offspring). An example of inbreeding depression is shown to the right. In this case, ''a'' is the recessive allele which has negative effects. In order for the ''a'' phenotype to become active, the gene must end up as homozygous ''aa'' because in the geneotype A''a'', the A takes dominance over the ''a'' and the ''a'' does not have any effect. Some recessive genes result in detrimental phenotypes by causing the organism to be less fit to its natural environment. Another mechanism responsible for inbreeding depression is the fitness advantage of heterozygosity, which is known as overdominance. This can lead to reduced fitness of a population with many homozygous genotypes, even if they are not deleterious or recessive. Here, even the dominant alleles result in reduced fitness if present homozygously (see also hybrid vigour). Overdominance is rare in nature. For practical applications, e.g. in

livestock Livestock are the domesticated animals raised in an agricultural setting to provide labor and produce diversified products for consumption such as meat, eggs, milk, fur, leather, and wool. The term is sometimes used to refer solely to animal ...

breeding, the former is thought to be more significant – it may yield completely unviable offspring (meaning outright failure of a pedigree), while the latter can only result in relatively reduced fitness.

Natural selection

Natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...

cannot effectively remove all deleterious recessive genes from a population for several reasons. First, deleterious genes arise constantly through de novo mutation within a population. Second, most offspring will have some deleterious traits, so few will be more fit for survival than the others. Different deleterious traits are extremely unlikely to equally affect reproduction – an especially disadvantageous recessive trait expressed in a homozygous recessive individual is likely to eliminate itself, naturally limiting the expression of its

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...

. Third, recessive deleterious alleles will be "masked" by heterozygosity, and so in a dominant-recessive trait, heterozygotes will not be selected against. When recessive deleterious alleles occur in the heterozygous state, where their potentially deleterious expression is masked by the corresponding wild-type allele, this masking phenomenon is referred to as complementation (see complementation (genetics)). In general, sexual reproduction in eukaryotes has two fundamental aspects: genetic recombination during

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...

, and outcrossing. It has been proposed that these two aspects have two natural selective advantages respectively. A proposed adaptive advantage of meiosis is that it facilitates recombinational repair of DNA damages that are otherwise difficult to repair (see DNA repair as the adaptive advantage of meiosis). A proposed adaptive advantage of outcrossing is complementation, which is the masking of deleterious recessive allelesMichod, R.E. Eros and Evolution: A Natural Philosophy of Sex. (1996) Perseus Books (see hybrid vigor or heterosis). The selective advantage of complementation may largely account for avoidance of inbreeding (see

kin recognition Kin recognition, also called kin detection, is an organism's ability to distinguish between close genetic kin and non-kin. In evolutionary biology and psychology, such an ability is presumed to have evolved for inbreeding avoidance, though animals d ...

), though it is unlikely that animals avoid inbreeding.

Management

Hybridization as a conservation effort is be appropriate if the population has lost "substantial genetic variation through genetic drift and the detrimental effects of inbreeding depression are apparent" and a similar population should be used. Different populations of the same species have different deleterious traits, and therefore their cross breeding is less likely to result in

homozygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. ...

at most loci in the offspring. This is known as outbreeding enhancement, which can be performed in extreme cases of severe inbreeding by conservation managers and zoo captive breeders to prevent inbreeding depression. However, intermixing two different populations can give rise to unfit polygenic traits in outbreeding depression (i.e. yielding offspring which lack the genetic adaptations to specific environmental conditions). These, then, will have a lowered fitness than pure-bred individuals of a particular subspecies that has adapted to its local environment.

In humans

Inbreeding may have both detrimental and beneficial effects. The biological effects of inbreeding depression in humans can on occasion be confounded by socioeconomic and cultural influences on reproductive behavior. Studies in human populations have shown that age at

marriage Marriage, also called matrimony or wedlock, is a culturally and often legally recognized union between people called spouses. It establishes rights and obligations between them, as well as between them and their children, and between ...

, duration of marriage,

illiteracy Literacy in its broadest sense describes "particular ways of thinking about and doing reading and writing" with the purpose of understanding or expressing thoughts or ideas in Writing, written form in some specific context of use. In other wo ...

, contraceptive use, and reproductive compensation are the major determinants of apparent fertility, even amongst populations with a high proportion of consanguinous unions. However, several small effects on increased mortality, longer inter-birth intervals and reduced overall productivity have been noted in certain isolated populations, though other studies show increased fitness of offspring and no effect on lifespan past the 2nd cousin level.

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...

was one of the first scientists to demonstrate the effects of inbreeding depression, through numerous experiments on plants. Darwin's wife, Emma, was his first cousin, and he was concerned about the impact of inbreeding on his ten children, three of whom died at age ten or younger; three others had childless long-term marriages. Humans do not seek to completely minimize inbreeding, but rather to maintain an optimal amount of inbreeding vs. outbreeding. Close inbreeding reduces fitness through inbreeding depression, but some inbreeding brings benefits. Indeed, inbreeding "increases the speed of selection of beneficial recessive and co-dominant alleles, e.g. those that protect against diseases."

Factors reducing inbreeding depression

Whilst inbreeding depression has been found to occur in almost all sufficiently studied species, some taxa, most notably some angiosperms, appear to suffer lower fitness costs than others in inbred populations. Three mechanisms appear to be responsible for this: purging, differences in ploidy, and selection for heterozygosity. It must be cautioned that some studies failing to show an absence of inbreeding depression in certain species can arise from small sample sizes or where the supposedly outbred control group is already suffering inbreeding depression, which frequently occurs in populations that have undergone a recent bottleneck, such as those of the

naked mole rat The naked mole-rat (''Heterocephalus glaber''), also known as the sand puppy, is a burrowing rodent native to the Horn of Africa and parts of Kenya, notably in Somali regions. It is closely related to the blesmols and is the only species in the ...

Purging selection

Purging selection occurs where the phenotypes of deleterious recessive

alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...

are exposed through inbreeding, and thus can be selected against. This can lead to such detrimental mutations being removed from the population, and has been demonstrated to occur rapidly where the recessive alleles have a lethal effect. The efficiency of purging will depend on the relationship between the magnitude of the deleterious effect that is unmasked in the homozygotes and the importance of genetic drift, so that purging is weaker for non-lethal than for recessive lethal alleles. For very small populations, drift has a strong influence, which can cause the fixation of sublethal alleles under weak selection. The fixation of a single allele for a specific gene can also reduce fitness where

heterozygote advantage A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. Loci exhibiting heterozygote advantage are a small minority of ...

was previously present (i.e., where heterozygous individuals have higher fitness than homozygotes of either allele), although this phenomenon seems to make a usually small contribution to inbreeding depression. Although naturally occurring, purging can be important for population survival, deliberately attempting to purge deleterious mutations from a population is not generally recommended as a technique to improve the fitness of captive bred animals. In plants, genetic load can be assessed through a test analogous to an inbreeding depression test called an Autogamy depression test.

Polyploidy

Many

angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...

(flowering plants) can self-fertilise for several generations and suffer little from inbreeding depression. This is very useful for species which disperse widely and can therefore find themselves growing in a novel environment with no conspecifics present.

Polyploidy Polyploidy is a condition in which the biological cell, cells of an organism have more than one pair of (Homologous chromosome, homologous) chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning they ha ...

(having more than two paired sets of each chromosome), which is prevalent in angiosperms, ferns and a select few animal taxa, accounts for this. By having several copies of a chromosome, as opposed to two, homozygosity is less likely to occur in inbred offspring. This means that recessive deleterious alleles are not expressed as frequently as with many copies of a chromosome; it is more likely that at least one will contain a functional allele.

Selection for heterozygosity

Selection for heterozygosity is rare, as lost loci undergo purifying selection for homozygous loci. Inbreeding depression has also been found to occur more gradually than predicted in some wild populations, such as in the highly inbred population of Scandinavian wolves. This appears to be due to a selection pressure for more

individuals, which generally are in better condition and so are more likely to become one of the few animals to breed and produce offspring.

References

External links

Genetic Effects of Straying of Non-Native Hatchery Fish into Natural Populations
{{Incest * Breeding Incest Plant sexuality