Reciprocal silencing

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Reciprocal silencing, a genetic phenomenon that primarily occurs in plants, refers to the pattern of redundant genes being silenced following a polyploid event. Polyploidy (wholesale genome duplication) is common in plants and constitutes an important method of speciation. [1] When a polyploid species arises, its genome contains homoeologs, duplicated chromosomes with equivalent genetic information. However silencing of redundant genes occurs rapidly in new polyploids through genetic and epigenetic means. This primarily occurs because redundancy allows one of the two genes present for each locus to be silenced without affecting the phenotype of the organism, and thus mutations that eliminate gene expression are much less likely to be deleterious or lethal. [1] [2] This allows mutations that would be lethal in diploid populations to accumulate in polyploids. Reciprocal silencing refers to the specific pattern of silencing where equivalent loci in are both silenced and expressed in a reciprocal manner. This phenomenon is observed on two distinct scales.

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Reciprocal silencing between populations

Allopolyploids are species whose increased complement of genetic material is the result of hybridization of two closely related species. Thus homeologous chromosomes in allopolyploids are equivalent, but not identical. These differences mean that the precise pattern of silencing and expression can have important phenotypic effects. Reciprocal silencing on the population level refers to the case where two populations are each descended from the same allopolyploid. In one population, one of the two equivalent locci (A) is expressed while the other (B) has been silenced, while in the other population the reciprocal pattern occurs, with B being expressed and A silenced. It is important to note that this refers to equivalent loci, specific locations within the genome, rather than the entire homeologous chromosome.

Reciprocal silencing on the population level has been proposed as a means of allopatric speciation following a polyploid event. [1] Allopatric speciation occurs when two populations of the same species become spatially separated and accumulate enough genetic differences to lose the ability to interbreed. As redundant genes are silenced in allopolyploids there is the potential for rapid genetic differences to accumulate through reciprocal silencing. These differences can lead to the loss of ability to interbreed between separated populations at a faster rate than other methods of speciation, given the relative speed with which genes are silenced following a polyploid event. Faster still, redundant genes can be silenced through epigenetic means, although the importance of this phenomenon is not fully understood. [2]

Reciprocal silencing between tissues

Reciprocal silencing on the tissue level refers to the same pattern of silencing and expression of homeologous loci. However, in this case, the differences in silencing and expression occur between two types of tissue within the same individual, rather than in individuals of different populations. This is an example of neofunctionalization, a process where duplicated genes that were once at equivalent loci evolve to carry out two separate functions. [3] Since different tissues require different genes to be expressed, reciprocal silencing can occur between tissues. Importantly, while the pattern of gene expression is the same as in the population case, the genetic means by which this pattern is achieved are very different. While silencing mutations are thought to be the main source of reciprocal silencing at the population level, at the tissue level only epigenetic factors are in play, since expressible copies of both homeologous loci must exist in all cells in an individual if different tissues express different homeologs.

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Microevolution is the change in allele frequencies that occurs over time within a population. This change is due to four different processes: mutation, selection, gene flow and genetic drift. This change happens over a relatively short amount of time compared to the changes termed macroevolution.

Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book On the Origin of Species. He also identified sexual selection as a likely mechanism, but found it problematic.

<span class="mw-page-title-main">Polyploidy</span> Condition where cells of an organism have more than two paired sets of chromosomes

Polyploidy is a condition in which the cells of an organism have more than one pair of (homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one from each of two parents; each set contains the same number of chromosomes, and the chromosomes are joined in pairs of homologous chromosomes. However, some organisms are polyploid. Polyploidy is especially common in plants. Most eukaryotes have diploid somatic cells, but produce haploid gametes by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid. Males of bees and other Hymenoptera, for example, are monoploid. Unlike animals, plants and multicellular algae have life cycles with two alternating multicellular generations. The gametophyte generation is haploid, and produces gametes by mitosis; the sporophyte generation is diploid and produces spores by meiosis.

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<span class="mw-page-title-main">Paleopolyploidy</span> State of having undergone whole genome duplication in deep evolutionary time

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<span class="mw-page-title-main">Genome evolution</span> Process by which a genome changes in structure or size over time

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<span class="mw-page-title-main">Leslie D. Gottlieb</span> American biologist

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Diploidization is the process of converting a polyploid genome back into a diploid one. Polyploidy is a product of whole genome duplication (WGD) and is followed by diploidization as a result of genome shock. The plant kingdom has undergone multiple events of polyploidization followed by diploidization in both ancient and recent lineages. It has also been hypothesized that vertebrate genomes have gone through two rounds of paleopolyploidy. The mechanisms of diploidization are poorly understood but patterns of chromosomal loss and evolution of novel genes are observed in the process.

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<span class="mw-page-title-main">Sexual selection in Arabidopsis thaliana</span>

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Eukaryote hybrid genomes result from interspecific hybridization, where closely related species mate and produce offspring with admixed genomes. The advent of large-scale genomic sequencing has shown that hybridization is common, and that it may represent an important source of novel variation. Although most interspecific hybrids are sterile or less fit than their parents, some may survive and reproduce, enabling the transfer of adaptive variants across the species boundary, and even result in the formation of novel evolutionary lineages. There are two main variants of hybrid species genomes: allopolyploid, which have one full chromosome set from each parent species, and homoploid, which are a mosaic of the parent species genomes with no increase in chromosome number.

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This glossary of genetics is a list of definitions of terms and concepts commonly used in the study of genetics and related disciplines in biology, including molecular biology, cell biology, and evolutionary biology. It is split across two articles:

References

  1. 1 2 3 Werth, C. and M. Whindam (1991) A Model for Divergent, Allopatric Speciation of Polyploid Pteridophytes Resulting from Silencing of Duplicate Gene Expression. American Naturalist 137: 515-526
  2. 1 2 Pikaard, C. S (2001) Genomic change and gene silencing in polyploids. Trends in Genetics 12:675-677
  3. ↑ Chaudhary, B. Flagel, L. Stupar, R. Udal, J. Verma, N. Springer, N. and J. F. Wendel (2009) Reciprocal Silencing, Transcriptional Bias and Functional Divergens of Homeologs in Polyploid Cotton (Gossypium). Genetics 183: 503-517
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