Parallelism and convergence in post-domestication adaptation in cereal grasses

Abstract

The selection of desirable traits in crops during domestication has been well studied. Many crops share a suite of modified phenotypic characteristics collectively known as the domestication syndrome. In this sense, crops have convergently evolved. Previous work has demonstrated that, at least in some instances, convergence for domestication traits has been achieved through parallel molecular means. However, both demography and selection during domestication may have placed limits on evolutionary potential and reduced opportunities for convergent adaptation during post-domestication migration to new environments. Here we review current knowledge regarding trait convergence in the cereal grasses and consider whether the complexity and dynamism of cereal genomes (e.g., transposable elements, polyploidy, genome size) helped these species overcome potential limitations owing to domestication and achieve broad subsequent adaptation, in many cases through parallel means. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Keywords: adaptation; convergence; domestication; evolution; plants.

Figure 1.

Parallelism in convergence. Convergence is the phenomenon whereby similar traits, such as purple pigmentation in potato and rice in this example, arise independently in different species. Parallelism is when convergent traits are caused by modification of the same molecular pathways, genes or nucleotides. (Online version in colour.)

Figure 2.

Simple cladogram of major cereal speciation. Numbers are in Ma (millions of years ago). Orange sun: grass speciation event 75 Ma. Blue stars: polyploidy events; the major grass polyploidy event immediately after the grass speciation event occurred approximately 70 Ma. The Ehrhartoideae clade, which includes rice, arose approximately 55 Ma. The Pooideae clade, which includes wheat and barley, arose around 44 Ma; Chloridoideae, which contains foxtail millet 28 Ma, and the Panicoids, which include maize and sorghum, arose approximately 24 Ma. The branch length is not proportional to the number of substitutions per site. (Online version in colour.)

Figure 3.

Likelihood that post-domestication adaptation would be parallel. (a,b) describes a simple representation of spike domestication in wheat (a) and maize (b). Alongside the domestication of each crop is the resulting bottleneck. After the domestication of both crops, any adaptation must result from a population with lower diversity in each crop species. The likelihood of parallel adaptation is compromised due to this lower diversity, but parallelism may still result if certain criteria are met (c). (Online version in colour.)