Outcrossing
Out-crossingorout-breedingis the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasinggenetic diversity.
Outcrossing in animals
[edit]Outcrossing can be a useful technique inanimal breeding.The outcrossing breeder intends to remove the traits by using "new blood." With dominant traits, one can still see the expression of the traits and can remove those traits, whether one outcrosses, line breeds or inbreeds. With recessive traits, outcrossing allows for the recessive traits to migrate across a population. Many traits are Mendelian and therefore exhibit a more complicated intermediate phenotype. The outcrossing breeder then may have individuals that have manydeleteriousgenes that may be expressed by subsequent inbreeding. There is now a gamut of deleterious genes within each individual in many dog breeds.[1]
Increasing the variation of genes or alleles within the gene pool may protect against extinction by stressors from the environment among inbred animal populations. For example, in this context, a 2009 veterinary medicine study tried to determine the genetic diversity within cat breeds.[2]
A degree of outcrossing to avoid mating between very close relatives is believed to happen in the wild.[1]
Outcrossing in plants and fungi
[edit]Outcrossing in plants is usually enforced byself-incompatibility.The primary adaptive function offlowersis the facilitation of outcrossing, a process that allows the masking of deleteriousmutationsin thegenomeof progeny. The masking effect of outcrossing is known asgenetic complementation,[3]an effect also recognized ashybrid vigororheterosis.Once outcrossing is established in a lineage of flowering plants due to the benefit of genetic complementation, subsequent switching to inbreeding becomes disadvantageous because it allows expression of the previously masked deleterious recessive mutations, i.e.inbreeding depression.
Outcrossing in fungi involvessyngamybetween haploid cells produced by separate diploid individuals.[4]
Life-history traits are said to increase the probability of outcrossing in fungi, such as long-distance dispersal and persistence of the haploid stage. Some studies even show that fungi favor outcrossing in comparison to other mating types. In a study performed with the commercial button mushroom,Agaricus bisporus,outcrossed populations of the fungi showed higher fitness than inbred ones in several fitness components.[5]
General practice
[edit]Breedersinbreedwithin their genetic pool, attempting to maintain desirable traits and tocullthose traits that are undesirable. When undesirable traits begin to appear, mates are selected to determine if a trait is recessive or dominant. Removal of the trait is accomplished by breeding two individuals known not to carry it.[6]
Gregor Mendelused outcrossing in his experiments with flowers. He then used the resulting offspring to chart inheritance patterns, using the crossing of siblings, andbackcrossingto parents to determine how inheritance functioned.[7]
Darwin's perspective
[edit]Charles Darwin,in his bookThe Effects of Cross and Self-Fertilization in the Vegetable Kingdom,.[8]: 462 stated regarding outcrossing that "the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigor and fertility over the self-fertilizing offspring from either one of the same parents". He thought that this observation was amply sufficient to account for outcrossingsexual reproduction.The disadvantages of self-fertilized offspring (inbreeding depression) are now thought to be largely due to the homozygous expression of deleterious recessive mutations;[9]and the fitness advantages of some outcrossed offspring are thought to be largely due to the heterozygous masking of such deleterious mutations except when such mutations lead tooutbreeding depression.
See also
[edit]References
[edit]- ^abSharp, C.A. (26 February 1999)."The Downside of Inbreeding: It's Time For a New Approach".Canine-Genetics.com.Double Helix Network News Vol. VII, No. 1 (Winter 1999). Archived fromthe originalon 26 January 2012.
- ^"Feline Genetics".UC Davis Veterinary Genetics Laboratory. Archived fromthe originalon 2009-03-12.
- ^Bernstein H, Byerly HC, Hopf FA, Michod RE. Genetic damage, mutation, and the evolution of sex. Science. 1985 Sep 20;229(4719):1277-81. doi: 10.1126/science.3898363. PMID 3898363
- ^Billiard, S.; López-Villavicencio, M.; Hood, M. E.; Giraud, T. (2012)."Sex, outcrossing and mating types: unsolved questions in fungi and beyond".Journal of Evolutionary Biology.25(6): 1020–1038.doi:10.1111/j.1420-9101.2012.02495.x.PMID22515640.S2CID25007801.
- ^Xu, J. (1995)."Analysis of inbreeding depression in Agaricus bisporus".Genetics.141(1): 137–145.doi:10.1093/genetics/141.1.137.PMC1206712.PMID8536962.
- ^David M. Hillis."Inbreeding, Linebreeding, and Outcrossing in Texas Longhorns".University of Texas at Austin.
- ^"Mendel's Paper (English - Annotated)".www.mendelweb.org.
- ^"Darwin, C. R. 1876. The effects of cross and self fertilisation in the vegetable kingdom. London: John Murray".darwin-online.org.uk.
- ^Bernstein H, Hopf FA, Michod RE (1987). "The Molecular Basis of the Evolution of Sex".Molecular Genetics of Development.Advances in Genetics. Vol. 24. pp. 323–70.doi:10.1016/s0065-2660(08)60012-7.ISBN9780120176243.PMID3324702.