Antagonistic pleiotropy hypothesis

From WikiProjectMed
Jump to navigation Jump to search
Strength of natural selection plot as a function of age

The antagonistic pleiotropy hypothesis was first proposed in a broad 1952 paper on the evolutionary theory of ageing by Peter Medawar and developed further in a landmark paper by George C. Williams in 1957.[1] Their original hypotheses have since spurred a huge and fruitful literature on the evolutionary explanation for senescence.[2] Pleiotropy is the phenomenon where a single gene influences more than one phenotypic trait in an organism.[3][4] It is one of the most commonly observed attributes of genes.[5] A gene is considered to exhibit antagonistic pleiotropy if it controls more than one phenotypic trait, where at least one of these traits is beneficial to the organism's fitness and at least one is detrimental to fitness.

This line of genetic research began as an attempt to answer the following question: if survival and reproduction should always be favoured by natural selection, why should ageing – which in evolutionary terms can be described as the age-related decline in survival rate and reproduction – nearly ubiquitous in the natural world?"[2] The antagonistic pleiotropy hypothesis provides a partial answer to this question. As an evolutionary explanation for ageing, the hypothesis relies on the fact that reproductive capacity declines with age in many species and, therefore, the strength of natural selection also declines with age (because there can be no natural selection without reproduction).[6][7] Since the strength of selection declines over the life cycles of human and some other organisms, natural selection in these species tends to favor "alleles that have early beneficial effects, but later deleterious effects".[8]

Antagonistic pleiotropy also provides a framework for understanding why many genetic disorders, even those causing life threatening health impacts (e.g. sickle cell anaemia), are found to be relatively prevalent in populations when, seen through the lens of simple evolutionary processes, they should be observed at very low frequencies due to the force of natural selection. Genetic models of populations show that that antagonistic pleiotropy allows genetic disorders to be maintained at reasonably high frequencies "even if the fitness benefits are subtle".[9] In this sense, antagonistic pleiotropy forms the basis of a "genetic trade-off between different fitness components."[10]

Trade-offs

In the theory of evolution, the concept of fitness has two components: mortality and reproduction. Antagonistic pleiotropy gets fixed in genomes by creating viable trade-offs between or within these two components. The existence of these trade-offs has been clearly demonstrated in human, botanical and insect species. For example, an analysis of global gene expression in the fruit fly, Drosophila melanogaster, revealed 34 genes whose expression coincided with the genetic trade-off between larval survival and adult size. The joint expression of these candidate 'trade-off' genes explained 86.3% of the trade-off. These tradeoffs can result from selection at the level of the organism or, more subtly, via mechanisms for the allocation of scarce resources in cellular metabolism.[11]

Another example is found in a study of the yellow monkey flower, an annual plant. The study documents a trade-off between days-to-flower and reproductive capacity. This genetic balancing act determines how many individuals survive to flower in a short growing season (viability) and also influences the seed set of survivors (fecundity). The authors find that tradeoffs between plant viability and fecundity can engender a stable polymorphism under surprisingly general conditions. Thus, for this annual flower, they reveal a tradeoff between mortality and fecundity and, according to the authors, this tradeoff is also relevant for other annual, flowering plants.[10]

Role in fecundity and senescence

Senescence refers to the process of physiological change in individual members of a species as they age.[9][12] An antagonistically pleiotropic gene can be selected for if it has beneficial effects in early life while having its negative effects in later life because genes tend to have larger impacts on fitness in an organism's prime than in their old age.[13] Williams's 1957 article has motivated many follow-up studies on the evolutionary causes of ageing.[14] These studies show clear trade-offs involving early increases in fecundity and later increases in mortality.

One such study tests the hypothesis that death due to cardiovascular disease in humans is linked to an antagonistic pleiotropy operating through inflammation. Because the human immune system evolved in an ancestral environment characterized by abundant pathogens, protective, pro-inflammatory responses were undoubtedly selected for in these environments. However, in terms of cardiovascular risk, these same inflammatory responses can be harmful – especially, more recently, in affluent countries where life expectancy is much longer. The study looks at mortality, over a period of 3 to 5 years, in a group of 311 85-year old Dutch women. Information on their reproductive history as well results of blood tests, genetic tests and physical examinations was recorded. The study found that individuals with a higher pro-inflammatory ratio TNFα/IL-10 had a significantly higher incidence of death due to cardiovascular disease in old age. This finding supports the hypothesis that this genotype was prevalent because higher ratios of TNFα/IL-10 allow individuals to more effectively combat infection during reproductive years.[15]

Role in disease

The survival of many serious genetic disorders in human evolutionary history has led researchers to reassess the role of antagonistic pleiotropy in disease. If genetic disorders are caused by mutations to a single deleterious alleles, then natural selection acting over evolutionary time should result in a lower frequency of mutations than are currently observed.[9] In a 2011 review article, Carter and Nguyen identify several genetic disorders, arguing that, far from being a rare phenomenon, antagonistic pleiotropy might be a fundamental mechanism by which "alleles with severe deleterious health effects can be maintained at medically relevant frequencies with only minor beneficial pleiotropic effects."[9]

An example of this is sickle cell anaemia, which results in an abnormality in the oxygen-carrying protein haemoglobin found in red blood cells.[16] Possessors of the deleterious allele have much lower life expectancies, with homozygotes rarely reaching 50 years of age. However, this allele also enhances resistance to malaria. Thus, in regions where malaria exerts or has exerted a strong selective pressure, sickle cell anemia has been selected for its conferred partial resistance to the disease. While homozygotes will have either no protection from malaria or a dramatic propensity to sickle cell anemia, heterozygotes have fewer physiological effects and a partial resistance to malaria.[17] Thus, the gene that is responsible for sickle cell disease has fixed itself with relatively high frequencies in populations threatened by malaria by engendering a viable tradeoff between death from this non-communicable disease and death from malaria.

In one of these studies, 99 individuals with Laron syndrome (a rare form of dwarfism) were monitored alongside their non-dwarf kin for a period of ten years. Patients with Laron syndrome possess one of three genotypes for the growth hormone receptor gene (GHR). Most patients have an A->G splice site mutation in position 180 in exon 6. Some others possess a nonsense mutation (R43X), while the rest are heterozygous for the two mutations. Laron syndrome patients experienced a lower incidence of cancer mortality and diabetes compared to their non-dwarf kin.[18] This suggests a role for antagonistic pleiotropy, whereby a deleterious mutation is preserved in a population because it still confers some survival benefit.[9]

Another instance of antagonistic pleiotropy is manifested in Huntington's disease, a rare neurodegenerative disorder characterized by a high number of CAG repeats within the Huntingtin gene. The onset of Huntington's is usually observed post-reproductive age and generally involves involuntary muscle spasms, cognitive difficulties and psychiatric problems. The high number of CAG repeats is associated with increased activity of p53, a tumor suppressing protein that participates in apoptosis. It has been hypothesized that this explains the lower rates of cancer among Huntington's patients. Huntington's disease is also correlated with high fecundity.[9]

Other pleiotropic diseases identified in the review article include: beta-thalassemia (also protects against malaria in the heterozygous state); cystic fibrosis (increased fertility); and osteoporosis in old age (reduced risk of osteoporosis in youth).[9]

Role in sexual selection

It is generally accepted that the evolution of secondary sexual characteristics persists until the relative costs of survival outweigh the benefits of reproductive success.[19] At the level of genes, this means a trade-off between variation and expression of selected traits. Strong, persistent sexual selection should result in decreased genetic variation for these traits. However, higher levels of variation have been reported in sexually-selected traits compared to non-sexually selected traits.[20] This phenomenon is especially clear in lek species, where males' courtship behavior confers no immediate advantage to the female. Female choice presumably depends on correlating male displays (secondary sexual characteristics) with overall genetic quality. If such directional sexual selection depletes variation in males, why would female choice continue to exist? Rowe and Houle answer this question (the lek paradox) using the notion of genetic capture, which couples the sexually-selected traits with the overall condition of the organism. They posit that the genes for secondary sexual characteristics must be pleiotropically linked to condition, a measure of the organism's fitness. In other words, the genetic variation in secondary sexual characteristics is maintained due to variation in the organism's condition.[21]

Ubiquity in population genetics

Advances in genome mappings have greatly facilitated research into antagonistic pleiotropy. Such research is now often carried out in laboratories, but also in wild populations. The latter context for testing has the advantage of introducing the full complexity of the selection experience – competitors, predators, and parasites – though it has the disadvantage of introducing idiosyncratic factors that are specific to given locations. In order to be able to assert with confidence that a given pleiotropy is, indeed, an antagonistic pleiotropy and not due to some other competing cause (e.g. the mutation accumulation hypothesis), one must have knowledge of the precise gene that is pleiotropic. This is now increasingly possible with organisms that have detailed genomic mappings (e.g. mice, fruit flies and humans). A 2018 review of this research finds that "antagonistic pleiotropy is somewhere between very common or ubiquitous in the animal world .... and potentially all living domains... ".[2]

See also

References

  1. ^ Williams GC (1957). "Pleiotropy, natural selection, and the evolution of senescence". Evolution. 11 (4): 398–411. doi:10.2307/2406060. JSTOR 2406060.
  2. ^ a b c Austad SN, Hoffman JM (2018). "Is antagonistic pleiotropy ubiquitous in aging biology?". academic.oup.com. Retrieved 19 April 2024.
  3. ^ Cheverud J (1996). "Developmental integration and the evolution of pleiotropy". American Zoology. 36: 44–50. CiteSeerX 10.1.1.526.8366. doi:10.1093/icb/36.1.44.
  4. ^ He X, Zhang J (August 2006). "Toward a molecular understanding of pleiotropy". Genetics. 173 (4): 1885–1891. doi:10.1534/genetics.106.060269. PMC 1569710. PMID 16702416.
  5. ^ Otto SP (April 2004). "Two steps forward, one step back: the pleiotropic effects of favoured alleles". Proceedings. Biological Sciences. 271 (1540): 705–714. doi:10.1098/rspb.2003.2635. PMC 1691650. PMID 15209104.
  6. ^ Elena SF, Sanjuán R (December 2003). "Evolution. Climb every mountain?". Science. 302 (5653): 2074–2075. doi:10.1126/science.1093165. PMID 14684807. S2CID 83853360.
  7. ^ Flatt T (May 2011). "Survival costs of reproduction in Drosophila" (PDF). Experimental Gerontology. 46 (5): 369–375. doi:10.1016/j.exger.2010.10.008. PMID 20970491. S2CID 107465469.
  8. ^ Rose MR, Rauser CL (2007). "Evolution and Comparative Biology". Encyclopedia of Gerontology. 2: 538–547. doi:10.1016/B0-12-370870-2/00068-8. ISBN 978-0-12-370870-0.
  9. ^ a b c d e f g Carter AJ, Nguyen AQ (December 2011). "Antagonistic pleiotropy as a widespread mechanism for the maintenance of polymorphic disease alleles". BMC Medical Genetics. 12: 160. doi:10.1186/1471-2350-12-160. PMC 3254080. PMID 22151998.
  10. ^ a b Brown KE, Kelly JK (January 2018). "Antagonistic pleiotropy can maintain fitness variation in annual plants". Journal of Evolutionary Biology. 31 (1): 46–56. doi:10.1111/jeb.13192. PMID 29030895.
  11. ^ Bochdanovits Z, de Jong G (February 2004). "Antagonistic pleiotropy for life-history traits at the gene expression level". Proceedings. Biological Sciences. 271 (Suppl 3): S75–S78. doi:10.1098/rsbl.2003.0091. PMC 1809996. PMID 15101424.
  12. ^ Promislow DE (June 2004). "Protein networks, pleiotropy and the evolution of senescence". Proceedings. Biological Sciences. 271 (1545): 1225–1234. doi:10.1098/rspb.2004.2732. PMC 1691725. PMID 15306346.
  13. ^ Wood JW, O'Connor KA, Holman DJ, Brindle E, Barsom SH, Grimes MA (2001). The evolution of menopause by antagonistic pleiotropy. Center for Demography and Ecology, Working Paper (Report).
  14. ^ Fox CW, Wolf JB, eds. (April 2006). Evolutionary genetics: concepts and case studies. Oxford University Press. ISBN 978-0-19-516817-4.
  15. ^ Van Den Biggelaar AH, De Craen AJ, Gussekloo J, Huizinga TW, Heijmans BT, Frölich M, et al. (June 2004). "Inflammation underlying cardiovascular mortality is a late consequence of evolutionary programming". FASEB Journal. 18 (9): 1022–1024. doi:10.1096/fj.03-1162fje. PMID 15084512. S2CID 1379140.
  16. ^ "Sickle Cell Disease - What Is Sickle Cell Disease? | NHLBI, NIH". www.nhlbi.nih.gov. 2023-08-30. Retrieved 2024-04-16.
  17. ^ Aidoo M, Terlouw DJ, Kolczak MS, McElroy PD, ter Kuile FO, Kariuki S, et al. (April 2002). "Protective effects of the sickle cell gene against malaria morbidity and mortality". Lancet. 359 (9314): 1311–1312. doi:10.1016/S0140-6736(02)08273-9. PMID 11965279.
  18. ^ Guevara-Aguirre J, Balasubramanian P, Guevara-Aguirre M, Wei M, Madia F, Cheng CW, et al. (February 2011). "Growth hormone receptor deficiency is associated with a major reduction in pro-aging signaling, cancer, and diabetes in humans". Science Translational Medicine. 3 (70): 70ra13. doi:10.1126/scitranslmed.3001845. PMC 3357623. PMID 21325617.
  19. ^ Bolstad GH, Pélabon C, Larsen LK, Fleming IA, Viken A, Rosenqvist G (June 2012). "The effect of purging on sexually selected traits through antagonistic pleiotropy with survival". Ecology and Evolution. 2 (6): 1181–1194. Bibcode:2012EcoEv...2.1181B. doi:10.1002/ece3.246. PMC 3402193. PMID 22833793.
  20. ^ Pomiankowski A, Moller AP (1995). "A Resolution of the Lek Paradox". Proceedings of the Royal Society of London. 260 (1357): 21–29. doi:10.1098/rspb.1995.0054. S2CID 43984154.
  21. ^ Rowe L, Houle D (1996). "The Lek Paradox and the Capture of Genetic Variance by Condition Dependent Traits". Proceedings of the Royal Society of London. 263 (1375): 1415–1421. Bibcode:1996RSPSB.263.1415R. doi:10.1098/rspb.1996.0207. S2CID 85631446.
Retrieved from "https://mdwiki.org/wiki/Antagonistic_pleiotropy_hypothesis"