Studies of reproductive isolation have shown that the number of genes that are involved is often large. Between closely related species of Drosophila, the number of such genes might often be in the hundreds.
Speciation genes — that is, the genes that underlie reproductive isolation — can contribute to hybrid inviability, sterility or behavioural aberration, as well as ecological maladaptation.
In the genic view of speciation, loci that are not closely linked to speciation genes might continue to be shared between nascent species, as in parapatric speciation.
At least five cases of speciation genes have been confirmed so far. Most bear a strong signature of positive selection. Three of the five known speciation genes are related to transcriptional regulation, supporting the postulate that species divergence is often regulatory in nature.
For a gene to diverge in function, it needs to be released from the old functional niche (genetic 'niche-release'). So, speciation genes are often non-essential, with functions that are only loosely coupled to reproductive isolation. In a few cases, gene knock-out studies — the ultimate way to test the dispensability of a gene — have been able to show the non-essential nature of specific speciation genes.
Genomic data might help to resolve the long-running debate on the relative importance of allopatric and parapatric modes of speciation. Models of allopatric speciation make specific testable predictions about the level of divergence across the whole genome.
It is only in the past five years that studies of speciation have truly entered the molecular era. Recent molecular analyses of a handful of genes that are involved in maintaining reproductive isolation between species (speciation genes) have provided some striking insights. In particular, it seems that despite being strongly influenced by positive selection, speciation genes are often non-essential, having functions that are only loosely coupled to reproductive isolation. Molecular studies might also resolve the long-running debate on the relative importance of allopatric and parapatric modes of speciation.
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We thank S. Shi and N. Osada for technical help and two anonymous referees for critical comments. Grants from the National Institutes of Health (USA) to C.-I Wu and from the National Science Council (ROC) to C.-T. Ting are gratefully acknowledged.
The authors declare no competing financial interests.
- NEO-DARWINIAN (SYNTHESIS)
The modern theory of evolution that combines both natural selection and population genetics, in which the Darwinian concept of spontaneous variation is explained in terms of mutation and genetic recombination.
Selection by which harmoniously interacting genes accumulate in the gene pool of a population.
The integration of a genomic region from one species into the genome of another species. Even a few percent of the introgressed genome can lead to hybrid incompatibility.
- POSITIONAL CLONING
The procedure by which we identify and isolate genes on the basis of their location in the genome, involving detailed genetic and physical maps of chromosomes.
- DEFICIENCY MAPPING
Uses chromosomes that have different sections deleted to locate the position of a gene of interest. Without the deficiency, the normal functional gene usually masks the effect of (that is, complements) the defective or foreign copy that we wish to identify.
- ALLELE COMPLEMENTATION
A test of whether a wild-type phenotype can be restored with two given alleles in a diploid genome.
- RECEPTOR TYROSINE KINASE SUPERFAMILY
One of the important cell-surface receptors that interacts with water-soluble ligands.
A gene that induces uncontrolled cell proliferation.
- MATERNAL EFFECT
The effect of the maternal genotype on the phenotype of the offspring, or the zygotes, usually at the embryonic stage (see also zygotic effects).
- ZYGOTIC EFFECT
The effect of the zygotes' own genotype on their own phenotype.
- MCDONALD AND KREITMAN TEST
A test that contrasts interspecific divergence against intraspecific polymorphism. It is a powerful test to detect excess of non-synonymous substitutions between species.
- CONTACT PHEROMONES
Chemical signals that are transmitted through the direct physical contact of two individuals. Contact pheromones in Drosophila are often sexual signals.
- KA/KS RATIOS
Ratios of non-synonymous substitutions to synonymous substitutions per site.
- INCIPIENT SPECIATION
The initial stage of species formation during which reproductive isolation is only partial.
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