Hybrid inviability
Encyclopedia
Hybrid inviability is a post-zygotic barrier
, which reduces a hybrid's capacity to mature into a healthy, fit adult. The relatively low health of these hybrids relative to pure-breed individuals prevents gene flow
between species. Thus, hybrid inviability acts as an isolating mechanism
, limiting hybridization and allowing for the differentiation of species.
The barrier of hybrid inviability occurs after mating species overcome pre-zygotic barriers (behavioral, mechanical, etc.) to produce a zygote
. The barrier emerges from the cumulative effect of parental gene
s; these conflicting genes interfere with the embryo
's development and prevents its maturation
. Most often, the hybrid embryo dies before birth. However, sometimes, the offspring develops fully with mixed traits, forming a frail, often infertile
adult. This hybrid displays reduced fitness, marked by decreased rates of survival
and reproduction relative to the parent species. The offspring fails to compete with purebred individuals, limiting genes flow between species.
In the 1970s, Allan C. Wilson and his colleagues first investigated the evolution of hybrid inviability in tetrapod
s, specifically mammal
s, bird
s, and frog
s.
Recognizing that hybrid viability decreases with time, the researchers used molecular clock
s to quantify divergence
time. They identified how long ago the common ancestor of hybridizing species diverged into two lines, and found that bird and frog species can produce viable hybrids up to twenty million years after speciation
. In addition, the researchers showed that mammal species can only produce viable hybrids up to two or three million years after speciation.
Wilson et al. (1974) proposes two hypotheses
to explain the relatively faster evolution
of hybrid inviability in mammals: the Regulatory and the Immunological Hypotheses. Subsequent research finds support for these hypotheses.
The Regulatory Hypothesis accounts for two characteristics of mammals, and explains the general formation of hybrid inviability in mammals, birds, and frogs.
First, mammals display relatively lower protein diversity than frogs. As Wilson et al. (1974) suggests, “mammals that can hybridize with each other differ only slightly at the protein level, whereas frogs that differ substantially in protein sequence hybridize readily.” This analysis suggests that gene divergence is not the only determinate of hybridization in mammals, birds, or frogs.
Second, the evolution of anatomical diversity occurred far faster in mammals than in either birds or frogs. As Fitzpatrick (2004) indicates, “the morphological disparities among bat
s, mole rats, and whale
s are more dramatic than any disparities in birds and frogs.” This anatomical diversity is evidence for the diversification of regulatory system
s. This mammalian characteristic suggests that, although mammals are genetically similar, dramatic changes in regulatory genes caused distinct developmental differences.
The Regulatory Hypotheses specifically attributes hybrid inviability in mammals, birds, and frogs to differences in gene regulation. It proposes that hybrid inviability evolved faster in mammalian taxa
because mammals have accumulated significantly more changes in regulatory systems than birds or frogs, and it suggests that organisms with distinctly different systems of gene regulation may not produce viable hybrids.
Wilson et al. (1974) recognizes that the development of embryos in the mammalian placenta
requires regulatory compatibility. Both the regulatory genes of the sperm
and egg
contribute to the expression of other protein-coding genes in the zygote; if certain regulatory genes are not expressed or are expressed at the wrong time, the inter-specific zygote will abort or develop unhealthy traits. Moreover, because the development of the zygote depends on maternal characteristics, such as cytoplasmic determinants, the regulatory traits of the mother may not support the hybrid's developmental needs.
The Immunological Hypothesis proposes that the divergence of certain protein structures associated with mother and child causes hybrid inviability. The hypothesis applies only to mammals, where fertilization and development is internal. In birds and in frogs, fertilization is primarily external, and the mother’s immune system
does not interfere with fetal development
.
This hypothesis stems from the immunological characteristics of the placenta, where the growing fetus
is in constant contact with the fluids and tissues of the mother. Variation within species and variation between species may contribute to fetal-maternal incompatibility, and according to the hypothesis, if the proteins of the fetus varies significantly from the proteins of the placenta, the mother may produce anti-bodies
that will attack and abort the fetus. Therefore, if the fetal proteins of the father species are incompatible the mother's placental proteins, the mother's immune system may abort the embryo.
Evidence for the Immunological Hypothesis varies considerably. Wilson et al. (1974) recognizes studies that provide no support to the Immunological Hypotheses. In these experiments, the use of immunological suppressants provided no additional viability to inter-specific hybrids. In contrast, Elliot and Crespi (2006) documents the effects of placental immunology on hybrid inviability, showing that mammals with hemochorial placentas more readily hybridize than mammals with epitheliochorial or endotheliochorial placentas. These different placenta types possess divergent immunological systems, and consequently, they cause varying degrees of hybrid inviability.
Post-zygotic mutation
A post-zygotic mutation is a mutation that an organism acquires during its lifespan, rather than inheriting from its parent by the fusion of the haploid pronuclei in the sperm and egg....
, which reduces a hybrid's capacity to mature into a healthy, fit adult. The relatively low health of these hybrids relative to pure-breed individuals prevents gene flow
Gene flow
In population genetics, gene flow 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...
between species. Thus, hybrid inviability acts as an isolating mechanism
Isolating mechanisms
Isolating Mechanisms are features of behavior, morphology, or genetics which serve to prevent breeding between species. Reproductive isolation of populations is established. It is particularly important to the biological species concept, as species are defined by reproductive isolation...
, limiting hybridization and allowing for the differentiation of species.
The barrier of hybrid inviability occurs after mating species overcome pre-zygotic barriers (behavioral, mechanical, etc.) to produce a zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
. The barrier emerges from the cumulative effect of parental gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s; these conflicting genes interfere with the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
's development and prevents its maturation
Maturation
Maturation could refer to any of the following:* Fetal development* Developmental biology* Emotional development* Or physical maturation of any biological life form - see individual articles for maturation of different life forms....
. Most often, the hybrid embryo dies before birth. However, sometimes, the offspring develops fully with mixed traits, forming a frail, often infertile
Infertility
Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...
adult. This hybrid displays reduced fitness, marked by decreased rates of survival
Survival
Survival is the struggle to remain alive and living. The term may refer to:- Companies and organisations :* Survival International, a non-governmental human rights organization working for tribal peoples- Literature :...
and reproduction relative to the parent species. The offspring fails to compete with purebred individuals, limiting genes flow between species.
Evolution of Hybrid Inviability in Tetrapods
In the 1970s, Allan C. Wilson and his colleagues first investigated the evolution of hybrid inviability in tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s, specifically mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, and frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...
s.
Recognizing that hybrid viability decreases with time, the researchers used molecular clock
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
s to quantify divergence
Divergence
In vector calculus, divergence is a vector operator that measures the magnitude of a vector field's source or sink at a given point, in terms of a signed scalar. More technically, the divergence represents the volume density of the outward flux of a vector field from an infinitesimal volume around...
time. They identified how long ago the common ancestor of hybridizing species diverged into two lines, and found that bird and frog species can produce viable hybrids up to twenty million years after speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
. In addition, the researchers showed that mammal species can only produce viable hybrids up to two or three million years after speciation.
Wilson et al. (1974) proposes two hypotheses
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
to explain the relatively faster evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of hybrid inviability in mammals: the Regulatory and the Immunological Hypotheses. Subsequent research finds support for these hypotheses.
The Regulatory Hypothesis accounts for two characteristics of mammals, and explains the general formation of hybrid inviability in mammals, birds, and frogs.
First, mammals display relatively lower protein diversity than frogs. As Wilson et al. (1974) suggests, “mammals that can hybridize with each other differ only slightly at the protein level, whereas frogs that differ substantially in protein sequence hybridize readily.” This analysis suggests that gene divergence is not the only determinate of hybridization in mammals, birds, or frogs.
Second, the evolution of anatomical diversity occurred far faster in mammals than in either birds or frogs. As Fitzpatrick (2004) indicates, “the morphological disparities among bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s, mole rats, and whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s are more dramatic than any disparities in birds and frogs.” This anatomical diversity is evidence for the diversification of regulatory system
Regulation of gene expression
Gene modulation redirects here. For information on therapeutic regulation of gene expression, see therapeutic gene modulation.Regulation of gene expression includes the processes that cells and viruses use to regulate the way that the information in genes is turned into gene products...
s. This mammalian characteristic suggests that, although mammals are genetically similar, dramatic changes in regulatory genes caused distinct developmental differences.
The Regulatory Hypotheses specifically attributes hybrid inviability in mammals, birds, and frogs to differences in gene regulation. It proposes that hybrid inviability evolved faster in mammalian taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
because mammals have accumulated significantly more changes in regulatory systems than birds or frogs, and it suggests that organisms with distinctly different systems of gene regulation may not produce viable hybrids.
Wilson et al. (1974) recognizes that the development of embryos in the mammalian placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
requires regulatory compatibility. Both the regulatory genes of the sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
and egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
contribute to the expression of other protein-coding genes in the zygote; if certain regulatory genes are not expressed or are expressed at the wrong time, the inter-specific zygote will abort or develop unhealthy traits. Moreover, because the development of the zygote depends on maternal characteristics, such as cytoplasmic determinants, the regulatory traits of the mother may not support the hybrid's developmental needs.
The Immunological Hypothesis proposes that the divergence of certain protein structures associated with mother and child causes hybrid inviability. The hypothesis applies only to mammals, where fertilization and development is internal. In birds and in frogs, fertilization is primarily external, and the mother’s immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
does not interfere with fetal development
Fetal development
Prenatal or antenatal development is the process in which a human embryo or fetus gestates during pregnancy, from fertilization until birth. Often, the terms fetal development, foetal development, or embryology are used in a similar sense.After fertilization the embryogenesis starts...
.
This hypothesis stems from the immunological characteristics of the placenta, where the growing fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
is in constant contact with the fluids and tissues of the mother. Variation within species and variation between species may contribute to fetal-maternal incompatibility, and according to the hypothesis, if the proteins of the fetus varies significantly from the proteins of the placenta, the mother may produce anti-bodies
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
that will attack and abort the fetus. Therefore, if the fetal proteins of the father species are incompatible the mother's placental proteins, the mother's immune system may abort the embryo.
Evidence for the Immunological Hypothesis varies considerably. Wilson et al. (1974) recognizes studies that provide no support to the Immunological Hypotheses. In these experiments, the use of immunological suppressants provided no additional viability to inter-specific hybrids. In contrast, Elliot and Crespi (2006) documents the effects of placental immunology on hybrid inviability, showing that mammals with hemochorial placentas more readily hybridize than mammals with epitheliochorial or endotheliochorial placentas. These different placenta types possess divergent immunological systems, and consequently, they cause varying degrees of hybrid inviability.