Review. The strength and genetic basis of reproductive isolating barriers in flowering plants

Abstract

Speciation is characterized by the evolution of reproductive isolation between two groups of organisms. Understanding the process of speciation requires the quantification of barriers to reproductive isolation, dissection of the genetic mechanisms that contribute to those barriers and determination of the forces driving the evolution of those barriers. Through a comprehensive analysis involving 19 pairs of plant taxa, we assessed the strength and patterns of asymmetry of multiple prezygotic and postzygotic reproductive isolating barriers. We then reviewed contemporary knowledge of the genetic architecture of reproductive isolation and the relative role of chromosomal and genic factors in intrinsic postzygotic isolation. On average, we found that prezygotic isolation is approximately twice as strong as postzygotic isolation, and that postmating barriers are approximately three times more asymmetrical in their action than premating barriers. Barriers involve a variable number of loci, and chromosomal rearrangements may have a limited direct role in reproductive isolation in plants. Future research should aim to understand the relationship between particular genetic loci and the magnitude of their effect on reproductive isolation in nature, the geographical scale at which plant speciation occurs, and the role of different evolutionary forces in the speciation process.

Figure 1

The individual strengths of prezygotic (dark grey) and postzygotic (light grey) reproductive isolating mechanisms compiled from 19 flowering plant systems in which at least two barriers were assayed. Values range from 0.0 for no isolation to 1.0 for complete isolation. Each bar indicates the mean±s.e. and numbers above each bar indicate the sample size for the corresponding barrier.

Figure 2

Comparison of total prezygotic isolation versus postzygotic isolation for 19 pairs of taxa. Note that the scale of the two axes is not identical. Negative values for total postzygotic isolation reflect hybrid performance that is greater than that of the parental taxa.

Figure 3

Asymmetry of reproductive isolating barriers, calculated for individual barriers as the absolute value of the difference in strength between the direction of potential matings or crosses, for premating (dark grey) and postmating (light grey) barriers. Values of zero indicate that the barrier acts symmetrically regardless of the direction of the cross or potential mating, and the upper bound is unlimited. Bars indicate mean±s.e., and numbers above each bar indicate the sample size for each barrier.