Gene flow

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Gene flow (also known as gene migration) is the transfer of alleles of genes from one population to another.

Migration into or out of a population may be responsible for a marked change in allele frequencies (the number of individual members carrying a particular variant of a gene). Immigration may result in the addition of new genetic material to the established gene pool of a particular species or population, and conversely emigration may result in the removal of genetic material.

There are a number of factors that affect the rate of gene flow between different populations. One of the most significant factors is mobility, and animals tend to be more mobile than plants. Greater mobility of an individual tends to give it greater migratory potential.

1 Barriers to gene flow
2 Gene flow in humans
3 Gene flow between species
4 Models of gene flow
5 Gene flow mitigation
6 External links
7 References

[edit] Barriers to gene flow

Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges or vast deserts, or something so simple as the Great Wall of China, which has hindered the natural flow of plant genes [1]. Examples of the same species which grow on either side have been shown to be genetically different.

[edit] Gene flow in humans

Gene flow has been observed in humans, for example in the United States, where a white European population and a black West African population were recently brought together. The Duffy blood group gives carriers some resistance to malaria, and as a result in West Africa, where malaria is prevalent, the Fy^o allele is essentially one hundred percent. In Europe, which has much lower levels of malaria, have either allele Fy^a or Fy^b. By measuring the frequencies, the rate of gene flow between the two populations can be measured, showing that gene flow is greater in the Northern U.S. than in the South.

[edit] Gene flow between species

Genes can flow between species, as when bacterial DNA is transferred to animals or plants.

One source of genetic variation is gene transfer, the movement of genetic material across species boundaries, which includes horizontal gene transfer, antigenic shift, and reassortment. Viruses can transfer genes between species [2]. Bacteria can incorporate genes from other dead bacteria, exchange genes with living bacteria, and can have plasmids "set up residence separate from the host's genome" [3]. "Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes." [4]

Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research" therefore "biologists [should] use the metaphor of a mosaic to describe the different histories combined in individual genomes and use [the] metaphor of a net to visualize the rich exchange and cooperative effects of HGT among microbes." [5]

"Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT [horizontal gene transfer]. Combining the simple coalescence model of cladogenesis with rare HGT [horizontal gene transfer] events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times." [6]

[edit] Models of gene flow

Models of gene flow can be derived from population genetics, e.g. Sewall Wright's neighborhood model, Wright's island model and the stepping stone model.

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[edit] Gene flow mitigation

When cultivating genetically modified (GM) plants, it may be necessary sometimes to prevent their genetic modification from from reaching another plant population by using gene flow mitigation. Reasons to limit gene flow may include biosafety or agricultural co-existence, in which GM and non-GM cropping systems work side by side.

Scientists in several large research programmes are investigating methods of limiting gene flow in plants. Among these programmes are Transcontainer, which investigates methods for biocontainment, SIGMEA, which focuses on the biosafety of genetically modified plants, and Co-Extra, which studies the co-existence of GM and non-GM product chains.

Generally, there are three approaches to gene flow mitigation: keeping the genetic modification out of the pollen, preventing the formation of pollen, and keeping the pollen inside the flower.

The first approach requires transplastomic plants. In transplastomic plants, the modified DNA is not situated in the cell's nucleus but is present in plastids, which are cellular compartments outside the nucleus. An example for plastids are chloroplasts, in which photosynthesis occurs. In some plants, the pollen does not contain plastids and, consequently, any modification located in plastids cannot be transmitted by the pollen.
The second approach relies on male sterile plants. Male sterile plants are unable to produce functioning flowers and therefore cannot release viable pollen. Cytoplasmic male sterile plants are known to produce higher yields. Therefore, researchers are trying to introduce this trait to genetically modified crops.
The third approach works by preventing the flowers from opening. This trait is called cleistogamy and occurs naturally in some plants. Cleistogamous plants produce flowers which either open only partly or not at all. However, it remains unclear how reliable cleistogamy is for gene flow mitigation: a Co-Extra research project on rapeseed investigating the matter has published preliminary results which cast doubt on the attainment of a high degree of reliability.

[edit] External links

[edit] References

Su, H et al. (2003) "The Great Wall of China: a physical barrier to gene flow?." Heredity, Volume 9 Pages 212-219

Basic topics in evolutionary biology v • d • e</div>
Evidence of evolution
Processes of evolution: adaptation - macroevolution - microevolution - speciation
Population genetic mechanisms: selection - genetic drift - gene flow - mutation
Evo-devo concepts: phenotypic plasticity - canalisation - modularity
Modes of evolution: anagenesis - catagenesis - cladogenesis
History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis
Other subfields: ecological genetics - human evolution - molecular evolution - phylogenetics - systematics
List of evolutionary biology topics \| Timeline of evolution