Hybrid incompatibility

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Hybrid incompatibility is a phenomenon in plants and animals, wherein offspring produced by the mating of two different species or populations have reduced viability and/or are less able to reproduce. Examples of hybrids include mules and ligers from the animal world, and subspecies of the Asian rice crop Oryza sativa from the plant world. Multiple models have been developed to explain this phenomenon. Recent research suggests that the source of this incompatibility is largely genetic, as combinations of genes and alleles prove lethal to the hybrid organism. [1] Incompatibility is not solely influenced by genetics, however, and can be affected by environmental factors such as temperature. [2] The genetic underpinnings of hybrid incompatibility may provide insight into factors responsible for evolutionary divergence between species. [3]

Contents

Background

Hybrid incompatibility occurs when the offspring of two closely related species are not viable or suffer from infertility. Charles Darwin posited that hybrid incompatibility is not a product of natural selection, stating that the phenomenon is an outcome of the hybridizing species diverging, rather than something that is directly acted upon by selective pressures. [4] The underlying causes of the incompatibility can be varied: earlier research focused on things like changes in ploidy in plants. More recent research has taken advantage of improved molecular techniques and has focused on the effects of genes and alleles in the hybrid and its parents.

Dobzhansky-Muller model

The first major breakthrough in the genetic basis of hybrid incompatibility is the Dobzhansky-Muller model, a combination of findings by Theodosius Dobzhansky and Joseph Muller between 1937 and 1942. The model provides an explanation as to why a negative fitness effect like hybrid incompatibility is not selected against. By hypothesizing that the incompatibility arose from alterations at two or more loci, rather than one, the incompatible alleles are in one hybrid individual for the first time rather than throughout the population - thus, hybrids that are infertile can develop while the parent populations remain viable. The negative fitness effects of infertility are not present in the original population. [5] [6] In this way, hybrid infertility contributes in some part to speciation by ensuring that gene flow between diverging species remains limited. Further analysis of the issue has supported this model, although it does not include conspecific genic interactions, a potential factor that more recent research has begun to look in to. [4]

Gene identification

Decades after the research of Dobzhansky and Muller, the specifics of hybrid incompatibility were explored by Jerry Coyne and H. Allen Orr. Using introgression techniques to analyze the fertility in Drosophila hybrid and non-hybrid offspring, specific genes that contribute to sterility were identified; a study by Chung-I Wu which expanded on Coyne and Orr's work found that the hybrids of two Drosophila species were made sterile by the interaction of around 100 genes. [7] These studies widened the scope of the Dobzhansky-Muller model, who thought it likely that more than two genes would be responsible. [5] [6] The ubiquity of Drosophila as a model organism has allowed many of the sterility genes to be sequenced in the years since Wu's study.[ citation needed ]

Modern directions

With modern molecular techniques, researchers have been able to more accurately identify the underlying genetic causes of hybrid incompatibility. This has led to both the development of expansions to the Dobzhansky-Muller model. Recent research has also explored the possibility of external influences on sterility as well.

The "snowball effect"

An extension of the Dobzhansky-Muller model is the "snowball effect"; an accumulation of incompatible loci due to increased species divergence. Since the model posits that sterility is due to negative allelic interaction between the hybridizing species, as species become more diverged it follows that more negative interactions should develop. The snowball effect states that the number of these incompatibilities will increase exponentially over the time of divergence, particularly when more than two loci contribute to the incompatibility. This concept has been exhibited in tests with the flowering plant genus Solanum, with the findings supporting the genetic underpinnings of Dobzhansky-Muller:

"Overall, our results indicate that the accumulation of sterility loci follows a different trajectory from the accumulation of loci for other quantitative species differences, consistent with the unique genetic basis expected to underpin species reproductive isolating barriers. ...In doing so, we uncover direct empirical support for the Dobzhansky-Muller model of hybrid incompatibility, and the snowball prediction in particular." [8]

Environmental influences

Though the primary causes of hybrid incompatibility appear to be genetic, external factors may play a role as well. Studies focused primarily on model plants have found that the viability of hybrids can be dependent on environmental influence. Several studies on rice and Arabidopsis species identify temperature as an important factor in hybrid viability; generally, low temperatures seem to cause negative hybrid symptoms to be expressed while high temperatures suppress them, although one rice study found the opposite to be true. [9] [10] [11] There has also been evidence in an Arabidopsis species that in poor environmental conditions (in this case, high temperatures), hybrids did not express negative symptoms and are viable with other populations. When environmental conditions return to normal, however, the negative symptoms are expressed and the hybrids are once again incompatible with other populations. [12]

Lynch-Force model

Though a multitude of evidence supports the Dobzhansky-Muller model of hybrid sterility and speciation, this does not rule out the possibility that other situations besides the inviable combination of benign genes can lead to hybrid incompatibility. One such situation is incompatibility by way of gene duplication, or the Lynch and Force model (put forth by Michael Lynch and Allan Force in 2000). When gene duplication occurs, there is a possibility that a redundant gene can be rendered non-functional over time by mutations. From Lynch and Force's paper:

"The divergent resolution of genomic redundancies, such that one population loses function from one copy while the second population loses function from a second copy at a different chromosomal location, leads to chromosomal repatterning such that gametes produced by hybrid individuals can be completely lacking in functional genes for a duplicate pair." [12]

This hypothesis is relatively recent compared to Dobzhansky-Muller, but has support as well.

Epigenetic influences

A possible contributor to hybrid incompatibility that fits with the Lynch and Force model better than the Dobzhansky-Muller model is epigenetic inheritance. Epigenetics broadly refers to heritable elements that affect offspring phenotype without adjusting the DNA sequence of the offspring. When a particular allele has been epigenetically modified, it is referred to as an epialleleA study found that an Arabidopsis gene is not expressed because it is a silent epiallele, and when this epiallele is inherited by hybrids in combination with a mutant gene at the same locus, the hybrid is inviable. [1] This fits with the Lynch and Force model because the heritable epiallele, ordinarily not an issue in non-hybrid populations with non-epiallele copies of the gene, becomes problematic when it is the only copy of the gene in the hybrid population. [1]

See also

Related Research Articles

An allele, or allelomorph, is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.

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">Hybrid (biology)</span> Offspring of cross-species reproduction

In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different varieties, species or genera through sexual reproduction. Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from a different organism is called a chimera. Hybrids are not always intermediates between their parents, but can show hybrid vigor, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.

Selfish genetic elements are genetic segments that can enhance their own transmission at the expense of other genes in the genome, even if this has no positive or a net negative effect on organismal fitness. Genomes have traditionally been viewed as cohesive units, with genes acting together to improve the fitness of the organism. However, when genes have some control over their own transmission, the rules can change, and so just like all social groups, genomes are vulnerable to selfish behaviour by their parts.

Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and population structure.

<span class="mw-page-title-main">Haldane's rule</span> Observation in evolutionary biology

Haldane's rule is an observation about the early stage of speciation, formulated in 1922 by the British evolutionary biologist J. B. S. Haldane, that states that if — in a species hybrid — only one sex is inviable or sterile, that sex is more likely to be the heterogametic sex. The heterogametic sex is the one with two different sex chromosomes; in therian mammals, for example, this is the male.

Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a hybrid offspring. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions of its parents. The heterotic offspring often has traits that are more than the simple addition of the parents' traits, and can be explained by Mendelian or non-Mendelian inheritance. Typical heterotic/hybrid traits of interest in agriculture are higher yield, quicker maturity, stability, drought tolerance etc.

A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. Loci exhibiting heterozygote advantage are a small minority of loci. The specific case of heterozygote advantage due to a single locus is known as overdominance. Overdominance is a rare condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents, and heterozygous individuals have a higher fitness than homozygous individuals.

<span class="mw-page-title-main">Introgression</span> Transfer of genetic material from one species to another

Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Introgression is a long-term process, even when artificial; it may take many hybrid generations before significant backcrossing occurs. This process is distinct from most forms of gene flow in that it occurs between two populations of different species, rather than two populations of the same species.

<span class="mw-page-title-main">Hybrid zone</span>

A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize. Hybrid zones can form in situ due to the evolution of a new lineage but generally they result from secondary contact of the parental forms after a period of geographic isolation, which allowed their differentiation. Hybrid zones are useful in studying the genetics of speciation as they can provide natural examples of differentiation and (sometimes) gene flow between populations that are at some point between representing a single species and representing multiple species in reproductive isolation.

The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile. These barriers maintain the integrity of a species by reducing gene flow between related species.

The concept of a biological species as a group of organisms capable of interbreeding to produce viable offspring dates back to at least the 18th century, although it is often associated today with Ernst Mayr. Species of the fruit-fly Drosophila are one of the most commonly used organisms in evolutionary research, and have been used to test many theories related to the evolution of species. The genus Drosophila comprises numerous species that have varying degrees of premating and postmating isolation between them. These species are useful for testing hypotheses of the reproductive mechanisms underlying speciation.

<span class="mw-page-title-main">Plant genetics</span> Study of genes and heredity in plants

Plant genetics is the study of genes, genetic variation, and heredity specifically in plants. It is generally considered a field of biology and botany, but intersects frequently with many other life sciences and is strongly linked with the study of information systems. Plant genetics is similar in many ways to animal genetics but differs in a few key areas.

<span class="mw-page-title-main">Bateson–Dobzhansky–Muller model</span> Model of the evolution of genetic incompatibility

The Bateson–Dobzhansky–Muller model, also known as Dobzhansky–Muller model, is a model of the evolution of genetic incompatibility, important in understanding the evolution of reproductive isolation during speciation and the role of natural selection in bringing it about. The theory was first described by William Bateson in 1909, then independently described by Theodosius Dobzhansky in 1934, and later elaborated in different forms by Herman Muller, H. Allen Orr and Sergey Gavrilets.

Sterility is the physiological inability to effect sexual reproduction in a living thing, members of whose kind have been produced sexually. Sterility has a wide range of causes. It may be an inherited trait, as in the mule; or it may be acquired from the environment, for example through physical injury or disease, or by exposure to radiation.

Hybrizyme is a term coined to indicate novel or normally rare gene variants that are associated with hybrid zones, geographic areas where two related taxa meet, mate, and produce hybrid offspring. The hybrizyme phenomenon is widespread and these alleles occur commonly, if not in all hybrid zones. Initially considered to be caused by elevated rates of mutation in hybrids, the most probable hypothesis infers that they are the result of negative (purifying) selection. Namely, in the center of the hybrid zone, negative selection purges alleles against hybrid disadvantage. Stated differently, any allele that will decrease reproductive isolation is favored and any linked alleles also increase their frequency by genetic hitchhiking. If the linked alleles used to be rare variants in the parental taxa, they will become more common in the area where the hybrids are formed.

<span class="mw-page-title-main">Reinforcement (speciation)</span> Process of increasing reproductive isolation

Reinforcement is a process of speciation where natural selection increases the reproductive isolation between two populations of species. This occurs as a result of selection acting against the production of hybrid individuals of low fitness. The idea was originally developed by Alfred Russel Wallace and is sometimes referred to as the Wallace effect. The modern concept of reinforcement originates from Theodosius Dobzhansky. He envisioned a species separated allopatrically, where during secondary contact the two populations mate, producing hybrids with lower fitness. Natural selection results from the hybrid's inability to produce viable offspring; thus members of one species who do not mate with members of the other have greater reproductive success. This favors the evolution of greater prezygotic isolation. Reinforcement is one of the few cases in which selection can favor an increase in prezygotic isolation, influencing the process of speciation directly. This aspect has been particularly appealing among evolutionary biologists.

This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. Overlapping and related terms can be found in Glossary of cellular and molecular biology, Glossary of ecology, and Glossary of biology.

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.

Genetic incompatibility describes the process by which mating yields offspring that are nonviable, prone to disease, or genetically defective in some way. In nature, animals can ill afford to devote costly resources for little or no reward, ergo, mating strategies have evolved to allow females to choose or otherwise determine mates which are more likely to result in viable offspring.

References

  1. 1 2 3 Blevins, T.; Wang, J.; Pflieger, D.; Pontvianne, F.; Pikaard, C.S. (2017). "Hybrid incompatibility caused by an epiallele". Proceedings of the National Academy of Sciences. 114 (14): 3702–3707. doi: 10.1073/pnas.1700368114 . PMC   5389275 . PMID   28270603.
  2. ↑ Chen, C.; E, Z.; Lin, Hong-Xuan (2016). "Evolution and Molecular Control of Hybrid Incompatibility in Plants". Frontiers in Plant Science. 7: 1208. doi: 10.3389/fpls.2016.01208 . PMC   4980391 . PMID   27563306.
  3. ↑ Maheshwari, S.; Barbash, D. (2011). "The Genetics of Hybrid Incompatibilities". Annual Review of Genetics. 45: 331–355. doi:10.1146/annurev-genet-110410-132514. PMID   21910629.
  4. 1 2 Johnson, N. (2008). "Hybrid incompatibility and speciation". Nature Education. 1 (1): 20.
  5. 1 2 Dobzhansky, T. (1937). "Genetics and the Origin of Species".{{cite journal}}: Cite journal requires |journal= (help)
  6. 1 2 Muller, H. J. (1942). "Isolating mechanisms, evolution, and temperature". Biology Symposium. 6: 71–125.
  7. ↑ Wu, C. I.; et al. (1996). "Haldane's rule and its legacy: Why are there so many sterile males?". Trends in Ecology and Evolution. 11 (7): 281–284. doi:10.1016/0169-5347(96)10033-1. PMID   21237844.
  8. ↑ Moyle, L.; Nakazato, T. (2010). "Hybrid Incompatibility "Snowballs" between Solanum Species". Science. 329 (5998): 1521–1523. Bibcode:2010Sci...329.1521M. doi:10.1126/science.1193063. PMID   20847271. S2CID   206527969.
  9. ↑ Bomblies, K.; Lempe, J.; Epple, P.; Warthmann, N.; Lanz, C.; Dangl, J. L.; et al. (2007). "Autoimmune response as a mechanism for a Dobzhansky-Muller-type incompatibility syndrome in plants". PLOS Biol. 5:e236 (9): e236. doi: 10.1371/journal.pbio.0050236 . PMC   1964774 . PMID   17803357.
  10. ↑ Fu, C. Y.; Wang, F.; Sun, B. R.; Liu, W. G.; Li, J. H.; Deng, R. F.; et al. (2013). "Genetic and cytological analysis of a novel type of low temperature-dependent intrasubspecific hybrid weakness in rice". PLOS ONE. 8:e73886 (8): e73886. Bibcode:2013PLoSO...873886F. doi: 10.1371/journal.pone.0073886 . PMC   3758327 . PMID   24023693.
  11. ↑ Chen, C.; Chen, H.; Yen, Y. S.; Shen, J. B.; Shan, J. X.; Qi, P.; et al. (2014). "A two-locus interaction causes interspecific hybrid weakness in rice". Nat. Commun. 5:3357: 3357. Bibcode:2014NatCo...5.3357C. doi:10.1038/ncomms4357. PMC   3948059 . PMID   24556665.
  12. 1 2 Lynch, M.; Force, A. G. (2000). "The origin of interspecific genomic incompatibility via genome duplication". The American Naturalist. 156 (6): 590–605. doi:10.1086/316992. PMID   29592543. S2CID   4455679.
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