Pre-copulatory mechanisms in animals The genetics of
ethological isolation barriers will be discussed first. Pre-copulatory isolation occurs when the genes necessary for the sexual reproduction of one species differ from the equivalent genes of another species, such that if a male of species A and a female of species B are placed together they are unable to copulate. Study of the genetics involved in this reproductive barrier tries to identify the genes that govern distinct sexual behaviors in the two species. The males of
Drosophila melanogaster and those of
D. simulans conduct an elaborate courtship with their respective females, which are different for each species, but the differences between the species are more quantitative than qualitative. In fact the
simulans males are able to hybridize with the
melanogaster females. Although there are lines of the latter species that can easily cross there are others that are hardly able to. Using this difference, it is possible to assess the minimum number of genes involved in pre-copulatory isolation between the
melanogaster and
simulans species and their chromosomal location.
Post-copulation or fertilization mechanisms in animals Reproductive isolation between species appears, in certain cases, a long time after fertilization and the formation of the zygote, as happens – for example – in the twin species
Drosophila pavani and
D. gaucha. The hybrids between both species are not sterile, in the sense that they produce viable gametes, ovules and spermatozoa. However, they cannot produce offspring as the sperm of the hybrid male do not survive in the semen receptors of the females, be they hybrids or from the parent lines. In the same way, the sperm of the males of the two parent species do not survive in the reproductive tract of the hybrid female. The development of a zygote into an adult is a complex and delicate process of interactions between genes and the environment that must be carried out precisely, and if there is any alteration in the usual process, caused by the absence of a necessary gene or the presence of a different one, it can arrest the normal development causing the non-viability of the hybrid or its sterility. It should be borne in mind that half of the chromosomes and genes of a hybrid are from one species and the other half come from the other. If the two species are genetically different, there is little possibility that the genes from both will act harmoniously in the hybrid. From this perspective, only a few genes would be required in order to bring about post copulatory isolation, as opposed to the situation described previously for pre-copulatory isolation. In many species where pre-copulatory reproductive isolation does not exist, hybrids are produced but they are of only one sex. This is the case for the hybridization between females of
Drosophila simulans and
Drosophila melanogaster males: the hybridized females die early in their development so that only males are seen among the offspring. However, populations of
D. simulans have been recorded with genes that permit the development of adult hybrid females, that is, the viability of the females is "rescued". It is assumed that the normal activity of these
speciation genes is to "inhibit" the expression of the genes that allow the growth of the hybrid. There will also be regulator genes. A different gene, also located on Chromosome II of
D. simulans is "Shfr" that also allows the development of female hybrids, its activity being dependent on the temperature at which
development occurs. Other similar genes have been located in distinct populations of species of this group. In short, only a few genes are needed for an effective post copulatory isolation barrier mediated through the non-viability of the hybrids. As important as identifying an isolation gene is knowing its function. The
Hmr gene, linked to the X chromosome and implicated in the viability of male hybrids between
D. melanogaster and
D. simulans, is a gene from the
proto-oncogene family
myb, that codes for a transcriptional regulator. Two variants of this gene function perfectly well in each separate species, but in the hybrid they do not function correctly, possibly due to the different genetic background of each species. Examination of the allele sequence of the two species shows that change of direction substitutions are more abundant than
synonymous substitutions, suggesting that this gene has been subject to intense natural selection. The
Dobzhansky–Muller model proposes that reproductive incompatibilities between species are caused by the interaction of the genes of the respective species. It has been demonstrated recently that
Lhr has functionally diverged in
D. simulans and will interact with
Hmr which, in turn, has functionally diverged in
D. melanogaster to cause the lethality of the male hybrids.
Lhr is located in a heterochromatic region of the genome and its sequence has diverged between these two species in a manner consistent with the mechanisms of positive selection. An important unanswered question is whether the genes detected correspond to old genes that initiated the speciation favoring hybrid non-viability, or are modern genes that have appeared post-speciation by mutation, that are not shared by the different populations and that suppress the effect of the primitive non-viability genes. The
OdsH (abbreviation of
Odysseus) gene causes partial sterility in the hybrid between
Drosophila simulans and a related species,
D. mauritiana, which is only encountered on
Mauritius, and is of recent origin. This gene shows
monophyly in both species and also has been subject to natural selection. It is thought that it is a gene that intervenes in the initial stages of speciation, while other genes that differentiate the two species show
polyphyly.
Odsh originated by duplication in the genome of
Drosophila and has evolved at very high rates in
D. mauritania, while its
paralogue,
unc-4, is nearly identical between the species of the group
melanogaster. Seemingly, all these cases illustrate the manner in which speciation mechanisms originated in nature, therefore they are collectively known as "speciation genes", or possibly, gene sequences with a normal function within the populations of a species that diverge rapidly in response to positive selection thereby forming reproductive isolation barriers with other species. In general, all these genes have functions in the transcriptional
regulation of other genes. The
Nup96 gene is another example of the evolution of the genes implicated in post-copulatory isolation. It regulates the production of one of the approximately 30 proteins required to form a
nuclear pore. In each of the
simulans groups of
Drosophila the protein from this gene interacts with the protein from another, as yet undiscovered, gene on the X chromosome in order to form a functioning pore. However, in a hybrid the pore that is formed is defective and causes sterility. The differences in the sequences of
Nup96 have been subject to adaptive selection, similar to the other examples of
speciation genes described above. Post-copulatory isolation can also arise between chromosomally differentiated populations due to
chromosomal translocations and
inversions. If, for example, a reciprocal translocation is fixed in a population, the hybrid produced between this population and one that does not carry the translocation will not have a complete
meiosis. This will result in the production of unequal gametes containing unequal numbers of chromosomes with a reduced fertility. In certain cases, complete translocations exist that involve more than two chromosomes, so that the meiosis of the hybrids is irregular and their fertility is zero or nearly zero. Inversions can also give rise to abnormal gametes in heterozygous individuals but this effect has little importance compared to translocations. In such cases, selection gives rise to population-specific isolating mechanisms to prevent either fertilization by interspecific gametes or the development of hybrid embryos. Because many sexually reproducing species of plants are exposed to a variety of interspecific
gametes, natural selection has given rise to a variety of mechanisms to prevent the production of hybrids. These mechanisms can act at different stages in the developmental process and are typically divided into two categories, pre-fertilization and post-fertilization, indicating at which point the barrier acts to prevent either zygote formation or development. In the case of
angiosperms and other pollinated species, pre-fertilization mechanisms can be further subdivided into two more categories, pre-pollination and post-pollination, the difference between the two being whether or not a
pollen tube is formed. (Typically when pollen encounters a receptive stigma, a series of changes occur which ultimately lead to the growth of a pollen tube down the style, allowing for the formation of the zygote.) Empirical investigation has demonstrated that these barriers act at many different developmental stages and species can have none, one, or many barriers to hybridization with interspecifics.
Examples of pre-fertilization mechanisms A well-documented example of a pre-fertilization isolating mechanism comes from study of Louisiana iris species. These iris species were fertilized with interspecific and conspecific pollen loads and it was demonstrated by measure of hybrid progeny success that differences in pollen-tube growth between interspecific and conspecific pollen led to a lower fertilization rate by interspecific pollen. This demonstrates how a specific point in the reproductive process is manipulated by a particular isolating mechanism to prevent hybrids. Another well-documented example of a pre-fertilization isolating mechanism in plants comes from study of the 2 wind-pollinated birch species. Study of these species led to the discovery that mixed conspecific and interspecific pollen loads still result in 98% conspecific fertilization rates, highlighting the effectiveness of such barriers. In this example, pollen tube incompatibility and slower generative mitosis have been implicated in the post-pollination isolation mechanism.
Examples of post-fertilization mechanisms Crosses between diploid and tetraploid species of Paspalum provide evidence of a post-fertilization mechanism preventing hybrid formation when pollen from tetraploid species was used to fertilize a female of a diploid species. There were signs of fertilization and even endosperm formation but subsequently this endosperm collapsed. This demonstrates evidence of an early post-fertilization isolating mechanism, in which the hybrid early embryo is detected and selectively aborted. This process can also occur later during development in which developed, hybrid seeds are selectively aborted.
Effects of hybrid necrosis Plant hybrids often suffer from an autoimmune syndrome known as hybrid necrosis. In the hybrids, specific gene products contributed by one of the parents may be inappropriately recognized as foreign and pathogenic, and thus trigger pervasive cell death throughout the plant. In at least one case, a pathogen receptor, encoded by the most variable gene family in plants, was identified as being responsible for hybrid necrosis.
Chromosomal rearrangements in yeast In brewers' yeast
Saccharomyces cerevisiae,
chromosomal rearrangements are a major mechanism to reproductively isolate different strains. Hou et al. showed that reproductive isolation acts postzygotically and could be attributed to chromosomal rearrangements. These authors crossed 60 natural isolates sampled from diverse niches with the reference strain S288c and identified 16 cases of reproductive isolation with reduced offspring viabilities, and identified reciprocal chromosomal translocations in a large fraction of isolates. in the cytoplasm which alters
spermatogenesis leading to sterility. It is interesting that incompatibility or isolation can also arise at an intraspecific level. Populations of
D. simulans have been studied that show hybrid sterility according to the direction of the cross. The factor determining sterility has been found to be the presence or absence of a microorganism
Wolbachia and the populations tolerance or susceptibility to these organisms. This inter population incompatibility can be eliminated in the laboratory through the administration of a specific
antibiotic to kill the microorganism. Similar situations are known in a number of insects, as around 15% of species show infections caused by this
symbiont. It has been suggested that, in some cases, the speciation process has taken place because of the incompatibility caused by this bacteria. Two
wasp species
Nasonia giraulti and
N. longicornis carry two different strains of
Wolbachia. Crosses between an infected population and one free from infection produces a nearly total reproductive isolation between the semi-species. However, if both species are free from the bacteria or both are treated with antibiotics there is no reproductive barrier.
Wolbachia also induces incompatibility due to the weakness of the hybrids in populations of spider mites (
Tetranychus urticae), between
Drosophila recens and
D. subquinaria and between species of
Diabrotica (beetle) and
Gryllus (cricket). == Selection ==