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Spontaneous hybrid was used to be recognized as an important driving force of speciation as there are apparent evidence suggesting the case many plant species have hybrid ancestry. New species can be arisen by hybridization much more easier in plant since the hybrid plants can still reproduce even it is sterile by different mechanism of asexual reproduction such as vegetative propagation. However, a recent research found that the occurrence of spontaneous hybridization is not as ubiquitous as frequently believed. Instead, the occurrence of spontaneous hybridization concentrated in part of families and an even smaller proportion of genera which those may be regarded as full of potential of hybridization. In plant, the deleterious phenotype of hybrid is often due to an autoimmune syndrome called hybrid necrosis. There are several mechanisms underlying hybrid necrosis but apparently often involve the immune system. One example of hybrid necrosis is tapetal-specific that causing male sterility which usually found in hermaphroditic plants. It was found that the cytonuclear incompatibility plays a major role in this developmental aberration. A male-sterile phenotype is observed in the hybrid with tapetal deterioration, which tapetal tissue nurtures pollen mother cells. Mitochondrial genes triggers a standard pathway of programmed cell death(PCD) which destroy the tapetum, while nuclear genes suppress the male sterility and restore pollen fertility by a counteracting measure. There are different mitochondrial genotypes triggering cell death in different ways by altering the complex regulatory cascade leading to PCD, while each of them have a specific set of matching nuclear genes that block PCD and restore normal function. As a result, the tapetal development is regulated by the balance of disruptive effects of mitochondria and the defensive effect of the nuclear genes. However, this delicate mitochondrial-nuclear balance is disturbed in hybrid and thus upset the regulatory control of programmed cell death, causing tapetal abnormalities and male sterility. Moreover, Sometimes PCD further affect tissues throughout the plant as PCD is involved in part of the development of tissues like leave and xylem. Also, PCD is a defensive mechanism against pathogen in plant cells which imply that misregulation of PCD can lead to serious problem of cell death. This types of cytonuclear incompatibility may contribute to the investigation of evolution as rapid evolution of the mitochondrial and nuclear genes is expected. Hybrids from crosses between Nemophila menzesii and a diverged population show symptoms such as stunted growth, thickened and curled leaves, aberrant petals and anthers with little or no pollen, which match the characteristics of the PCD-induced aberrations.
Another case of hybrid necrosis was discovered when studying the specie Arabidopsis thaliana. Cross between A. thaliana strains show hybrid necrosis which was found to be a Dobzhansky-Muller-Type Incompatibility Syndrome, involving a negative epistatic interaction between 2 to 4 loci. A highly polymorphic NB-LRR genes was mapped and responsible for the improper regulation of immune system. Hybrid show lower threshold for activation of active immune response and some hybrid may have enhanced resistance against pathogen when comparing to their parents. Typically, mitochondrial genes are matrilineally transmitted only through seeds but not through pollen while nuclear gene are biparentally transmitted. In this case, hybrid survival is not dependent on the direction of the cross, that in other words which species act as the maternal parent, demonstrating it is not caused by aberrant nuclear-cytoplasmic interactions, such as those involved in cytoplasmic male sterility. This type of hybrid necrosis is experimentally found temperature sensitive which hybrid suffer lethality at 16Â°C but the autoimmune response is greatly suppressed at 23Â°C.
Hybrid incompatibility in animals:
Although a clear illustration regarding the hybrid sterility and inviability remains black boxes, a few hybrid incompatibility genes were identified with the powerful innovative genetic tools being invented such as lost of function muataion, chromosomal duplications and deletions and DNA engineering. A 'Dobzhansky-Muller" type incompatibility was identified in a platyfish species. Hybrid between platyfish Xiphophorus maculatus and a related species the swordtail Xiphophorus helleri show inviability. Also, backcross hybrid of those species often develop malignant tumor and eventually die. With the help of molecular genetic analysis, a X-linked hybrid incompatibility gene Xmrk-2 which code for a novel receptor tyrosine kinase was found misexpressed in hybrid, leading to cancer formation. Xmrk-2 gene functions as a spot producing gene in platyfish with a repressor gene locating at an autosome. The autosomal repressor is missing in hybrid resulting in improper regulation of the Xmrk2 gene expression. More HI genes were discovered during studies on a frequently used genetic tool -Drosophila. Those identified incompatibility genes include Odysseus-Homeobox (OdsH), Lethal hybrid rescue (Lhr), Hybrid male rescue(Hmr) and Nup96 genes. Research found loss of function mutation of Hmr gene in D. melanogaster and Lhr in D. simulans suppress hybrid male lethality. Both gene demonstrate asymmetry in causing hybrid lethality which is stated in Dobzhansky-Muller model by the result that deletion mutation of those alleles of another species cannot rescue the hybrid. Among those incompatibility genes,Nup96 seem to be more informative in nowadays study. Nup96 gene codes for a protein that stably bound to the nuclear pore complex which is the largest macromolecular complex in eukaryotes. The Nup96 nucleoporins play a structural roles in nuclear pore compex mediating nucleocytoplasmic trafficking of RNAs and proteins. In hybrid cross between D. melanogaster and D. simulans,a D. simulans, Nup96 allele cause lethality in hemizygous for the D. melanogaster X chromosome but not in hemizygous for the D. simulans X chromosome. The observation match the Dobzhansky-Muller model predication that a D. simulans allele of Nup96 would not be incompatible with loci on its own X chromosome.Nup96 nucleoporins is found interacting with others nucleoporins such as Nup
Interspecific hybrid incompatibility is one of the causes of reproductive isolation between species which is a defining feature of the biological concept of species. Knowing the genetic basis of hybrid incompatibility is therefore a crucial procedure of discovering the origin of speciation. As genetic drift and selection continues after speciation, the split species continue to diverge and the intensity of incompatibility will only keep on increasing. Therefore, finding a incompatibility gene not necessarily mean finding a speciation gene. As a result, speciation genes should be defined as evolved to cause incompatibility only at the time of speciation but it poses a great difficulty to isolate a speciation gene after speciation have taken place over millions years. However, hints about the phylogenetic relation among species may be obtained by observing the incompatibility pattern of hybrid crossed from different related species. For example, a Nup96-dependent lethality show in hybrid between D. melanogaster-D. simulans and D. melanogaster -D. sechellia, but not in D. melanogaster- D. mauritiana hybrid suggesting D. mauritiana may have speciated before D. simulans and D. sechellia. Apart from this, more evolutionary event could be predicted with the incidents that all non-synonymous substitution of Nup96 allele of D. simulans are also found in D. mauritiana ,which indicate Nup96 allele may had diverged prior to the divergence of D. simulans and D. mauritiana. Also, a gene locating on X chromosome incompatible with Nup96 alleles to cause lethality should have diverged on the D. melanogaster lineage due to the occurrence of lethality only on hybrids with the D. melanogaster X chromosome, neither with D. simulans nor D. sechellia .
Evolutionary forces that drive the divergence of speciation genes
Additional to the identity and the characteristic of the speciation genes, biologists are also interested in understanding the driving force to the divergence of speciation genes. Increasing evidence show that hybrid incompatibility gene evolves as a by-product of adaptive evolution and is rapidly evolving. For example, by gene mapping and DNA sequencing, Nup96 gene was found diverged among Drosophila species with significant excessive non-synonymous substitution relative to synonymous substitution, demonstrating Nup96 allele is under positive natural selection. In addition, a new hypothesis of the role of intragenomic conflict in driving the divergent of incompatibility suggest that selfish genetic factors