Speciation Genes

Abstract

Even though there is no universal criterion for species definition, the key event in speciation research is the advent of reproductive isolation or barriers that prevent gene flow between populations. The search for genetic factors underlying isolating barriers is the field of speciation genetics. The development of sophisticated methods of genetic analysis coupled with statistical advances allowed a finer dissection of the genetic factors underlying the origins of species down to the level of the individual loci and even at the deoxyribonucleic acid (DNA)‐sequence level. Traditionally, speciation geneticists concentrated, almost exclusively, on genes causing hybrid incompatibilities (especially intrinsic postzygotic barriers). However, barriers to gene flow caused by speciation genes can arise at multiple prezygotic and postzygotic life‐history stages. Thus, a general definition of speciation genes should refer to those genes that contributed to the splitting of two lineages by reducing the amount of gene flow between them.

Key Concepts:

  • The evolution of barriers to genetic exchange between groups is the crucial event for the origin of new sexually reproducing species.

  • Barriers to gene flow can be divided in two main categories: (i) prezygotic that preclude the formation of hybrid offspring and (ii) postzygotic that later prevent the success and propagation of hybrid offspring.

  • The identification and characterisation of genes that contribute to the interruption of gene flow between populations is currently one of the main focuses in the field of speciation.

  • Speciation genes are the genes that contributed to the splitting of two lineages by reducing the amount of gene flow between them.

  • The most solid evidence that a gene causes reproductive isolation combines fine mapping studies with genetic manipulation experiments (such as positional cloning, gene replacement or knockout, gene expression assays and transgenic manipulations).

  • Alleles underlying hybrid incompatibilities between species may arise by neutral or nearly neutral processes or may be a direct consequence of positive selection or a byproduct of adaptive evolution.

Keywords: biological species concept; barriers to gene flow; genetic mapping; hybrid incompatibility; pollinator isolation; Dobzhansky–Muller model

Figure 1.

Fertility assay of hybrid male genotypes produced in the backcross of female F1 hybrids between D. pseudoobscura and D. persimilis with parental males (redrawn from Dobzhansky, ). Testis length is a proxy for male hybrid fertility. Males from the top half of the plot carry a D. pseudoobscura X chromosome and are often fertile, whereas males from the bottom half carry a D. persimilis X chromosome and are almost complete sterile. Drosophila pseudoobscura chromosomes are shown in white, D. persimilis in black.

Figure 2.

Mapping hybrid incompatibility genes. (a) Recombination mapping based on introgressed chromosomes (blue horizontal bars). Sterile and fertile hybrids are genotyped for markers (M1–M8) on the introgressed chromosomes that are polymorphic between the two species. The pattern of marker distribution among different hybrids allows the location of the hybrid incompatibility gene to be mapped at a resolution that is dependent on the density of markers in the region. The vertical bar marks the region that is responsible for hybrid incompatibility. (b) Deficiency mapping. The phenotypes that are associated with overlapping chromosome deficiencies (indicated by the gaps in the bars) show the position of the hybrid incompatibility gene (vertical orange bar).

Modified from Wu and Ting 2004. Reproduced with permission from Nature Review Genetics. © Nature Publishing Group.
Figure 3.

The DM model of postzygotic isolation. Different colours represent independent allele fixations (a: allele in locus A; b: allele in locus B) in diverging populations. Crosses between individuals of these populations may lead to hybrid genetic incompatibilities due to negative epistasis between the new alleles of the A and B loci.

Figure 4.

Flowers of Mimulus lewisii (a), an F1 hybrid (b) and M. cardinalis (c). Reproduced with permission of Proceedings of the National Academy of Sciences from Schemske and Bradshaw . © National Academy of Sciences, USA.

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Further Reading

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Via S (2009) Natural selection in action during speciation. Proceedings of the National Academy of Science of the USA 106: 9939–9946.

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Mensch, Julián, Frankel, Nicolás, and Hasson, Esteban(Oct 2013) Speciation Genes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023640]