Panmixia

Panmixia (or panmixis) means uniform random fertilization.^[1]^[2] A panmictic population is one where all potential parents may contribute equally to the gamete pool, and that these gametes are uniformly distributed within the gamete population (gamodeme). This assumes that there are no hybridising restrictions within the parental population: neither genetics, cytogenetics nor behavioural; and neither spatial nor temporal (see also Quantitative genetics for further discussion). Therefore, all gamete recombination (fertilization) is uniformly possible. Both the Wahlund effect and the Hardy Weinberg equilibrium assume that the overall population is panmictic.^[3]

In genetics and heredity, random mating^[4] usually implies the hybridising (mating) of individuals regardless of any spatial, physical, genetical, temporal or social preference. That is, the mating between two organisms is not influenced by any environmental, nor hereditary interaction. Hence, potential mates have an equal chance of being contributors to the fertilizing gamete pool. If there is no random sub-sampling of gametes involved in the fertilization cohort, panmixia has occurred. Such uniform random mating is distinct from lack of natural selection: in viability selection for instance, selection occurs before mating.

Description

In simple terms, panmixia (or panmicticism) is the ability of individuals in a population to interbreed without restrictions; individuals are able to move about freely within their habitat, possibly over a range of hundreds to thousands of miles, and thus breed with other members of the population.

To signify the importance of this, imagine several different finite populations of the same species (for example: a grazing herbivore), isolated from each other by some physical characteristic of the environment (dense forest areas separating grazing lands). As time progresses, natural selection and genetic drift will slowly move each population toward genetic differentiation that would make each population genetically unique (that could eventually lead to speciation events or extirpation).

However, if the separating factor is removed before this happens (e.g. a road is cut through the forest), and the individuals are allowed to move about freely, the individual populations will still be able to interbreed. As the species's populations interbreed over time, they become more genetically uniform, functioning again as a single panmictic population.

In attempting to describe the mathematical properties of structured populations, Sewall Wright proposed a "factor of Panmixia" (P) to include in the equations describing the gene frequencies in a population, and accounting for a population's tendency towards panmixia, while a "factor of Fixation" (F) would account for a population's departure from the Hardy–Weinberg expectation, due to less than panmictic mating. In this formulation, the two quantities are complementary, i.e. P = 1 − F. From this factor of fixation, he later developed the F statistics.

Background information

In a panmictic species, all of the individuals of a single species are potential partners, and the species gives no mating restrictions throughout the population.^[5] Panmixia can also be referred to as random mating, referring to a population that randomly chooses their mate, rather than sorting between the adults of the population.^[6]

Panmixia allows for species to reach genetic diversity through gene flow more efficiently than monandry species. However, outside population factors, like drought and limited food sources, can affect the way any species will mate.^[7] When scientists examine species mating to understand their mating style, they look at factors like genetic markers, genetic differentiation, and gene pool.^[8]

Panmictic species

A panmictic population of Monostroma latissimum, a marine green algae, shows sympatric speciation in southwest Japanese islands. Although panmictic, the population is diversifying.^[9] Dawson's burrowing bee, Amegilla dawsoni, may be forced to aggregate in common mating areas due to uneven resource distribution in its harsh desert environment.^[7] Pantala flavescens should be considered as a global panmictic population.^[10]

Related experiments and species

Anguilla rostrata, or the American eel, exhibits panmixia throughout the entire species. This allows the eel to have phenotypic variation in their offspring and survive in a wide range of environmental conditions^[11]^[8]
In 2016, BMC Evolutionary Biology conducted a study on Pachygrapsus marmoratus, the marbled crab, marking them as panmictic species. The study claimed that the crabs' mating behavior is characterized by genetic differentiation due to geographic breaks across its distribution range and not panmixia^[12]
In a heterogeneous environment such as the forests of Oregon, United States, Douglas squirrels (Tamiasciurus douglasii) exhibit local patterns of adaptation. In a study conducted by Chaves (2014) a population along an entire transect was found to be panmictic. Traits observed in this study included skull shape, fur color, etc.
Swordfish based in the Indian Ocean (Xiphias gladius) have been found to be a single panmictic population. Markers used in the study carried out by Muths et al. (2013) found large spatial and temporal homogeneity in genetic structure satisfactory in order to consider the swordfish a singular panmictic population.

References

^ King C and Stanfield W.D. (1997). Dictionary of genetics. Oxford University Press. ISBN 9780195143249. p. 262: "Panmixia (panmixis): random mating as contrasted with assortative mating."
^ Merriam-Webster Medical Dictionary. "Panmixia: Random mating within a breeding population."
^ Gayon, Jean; Cobb, Matthew (1998), Darwinism's Struggle for Survival: Heredity and the Hypothesis of Natural Selection, Cambridge University Press, p. 158, ISBN 978-0-521-56250-8
^ Choudhuri, Supratim (2014-05-09). Bioinformatics for Beginners: Genes, Genomes, Molecular Evolution, Databases and Analytical Tools. Elsevier. ISBN 978-0-12-410510-2.
^ "Of Terms in Biology: Panmictic".
^ "Random Mating". NOAA.
^ ^a ^b Beveridge, M.; Simmons, L. W. (2006). "Panmixia: An example from Dawson's burrowing bee (Amegilla dawsoni) (Hymenoptera: Anthophorini)". Molecular Ecology. 15 (4): 951–7. doi:10.1111/j.1365-294X.2006.02846.x. PMID 16599959. S2CID 22442167.
^ ^a ^b Pujolar, J. M. (2013). "Conclusive evidence for panmixia in the American eel". Molecular Ecology. 22 (7): 1761–2. doi:10.1111/mec.12143. PMID 23620904. S2CID 24345855.
^ Bast, Felix; Kubota, Satoshi; Okuda, Kazuo (11 November 2014). "Phylogeographic assessment of panmictic Monostroma species from Kuroshio Coast, Japan, reveals sympatric speciation". Journal of Applied Phycology. 27 (4): 1725–1735. doi:10.1007/s10811-014-0452-x. S2CID 17236629.
^ Daniel Troast; Frank Suhling; Hiroshi Jinguji; Göran Sahlén; Jessica Ware (2016). "A Global Population Genetic Study of Pantala flavescens". PLOS ONE. 11 (3): e0148949. Bibcode:2016PLoSO..1148949T. doi:10.1371/journal.pone.0148949. PMC 4775058. PMID 26934181.
^ Côté, Caroline L.; Castonguay, Martin; Kalujnaia, Mcwilliam; Cramb, Gordon; Bernatchez, Louis (2014). "In absence of local adaptation, plasticity and spatially varying selection rule: A view from genomic reaction norms in a panmictic species (Anguilla rostrata)". BMC Genomics. 15: 403. doi:10.1186/1471-2164-15-403. PMC 4229938. PMID 24884429.
^ Fratini, Sara; Ragionieri, Lapo; Deli, Temim; Harrer, Alexandra; Marino, Ilaria A. M.; Cannicci, Stefano; Zane, Lorenzo; Schubart, Christoph D. (2016). "Unravelling population genetic structure with mitochondrial DNA in a notional panmictic coastal crab species: Sample size makes the difference". BMC Evolutionary Biology. 16: 150. doi:10.1186/s12862-016-0720-2. PMC 4960869. PMID 27455997.