The Origins of Triticum Domestication

Ben Grady Dept. of Botany UW-Madison

The genus Triticum

• Poaceae• Highly inconsistent taxonomy• Triticum and Aegilops separate genera• Triticum sensu lato

– Includes Aegilops

• Triticum sensu stricto ~ 10 species• Native to Mediterranean area• Base n = 7

Domesticated Wheats

• Triticum monococcum L. (cultivated einkorn)– T. boeoticum (wild einkorn)

– Diploid - AA

• Triticum dicoccum Schübl. (emmer wheat)– T. dicoccoides (wild emmer)

– Tetraploid – AABB

• Triticum aestivum L. (bread or common wheat)– Hexaploid - AABBDD

– T. dicoccoides (AABB) + Aegilops tauschii (DD)

Domestication Traits

• “As soon as one begins to plant seed in a seed-bed on a yearly basis and save seed for the following season, selection pressures are automatically set in motion, leading toward domestication” –J.R. Harlan

• Non-shattering rachis (two independent recessive loci) (Davies and Hillman, 1992)

• Seed dormancy periods• Uniform ripening of seeds• Seed size

Traits of Domestication

Salamini et al., 2002

Glutenin Loci Study

• HMW glutenin loci: Glu-1-1 & Glu-1-2

• 9 alleles of the B chromosome Glu- B1-1 (Glu-D1-1 outgroup)

• Wild and domesticated emmers

• Two well-supported clades a & b; divergence dated to 1.4-2.0 MYA

• Multiple domestication events?

• Domestication of mixed populations?

• Introgression?

NJ tree from Glu-B1-1 alleles in T. aestivum

a (black)

b (white)

from Brown et al., 2006

a & b Glu-B1-1 frequencies in wild emmers

• a – black

• b - white

From Brown et al., 2006

Glu-B1-1 subclades in cult. emmers

Figure 9.2 (from Brown et al., 2006) Proportion of α (black) & β (white) alleles

Implications of Glu-B1-1 allele distributions

• a alleles more common than b alleles• a alleles present in all accessions sampled• b alleles present in Turkey and NW, through

Europe• Do a alleles confer a selective advantage? –

probably not• Independent introductions of emmer into

Europe? – probably

Allele expansion in Europe

Higher % of A alleles

Higher % of B alleles

Origins of Glu-B1-1 allele subclades

• Multiple domestications of emmer wheat?– probably not

• Single domestication of diverse wild population? – not bloody likely

• New alleles introduced after domestication via introgression with wild relatives?– Yeah (supported by rDNA evidence)

Phylogenetic Relationships…Peterson et al., 2006

• Hybridization history of T. aestivum (allohexaploid)– DD (A. tauschii) + AABB (T. dicoccoides)– T. dicoccoides = AuAu (T. urartu) + BB (A.

speltoides

Wild Triticum species

www.icarda.org

• Strict consensus tree from 8 equally parsimonious trees, sequences from two nuclear genes, DMC1 & EF-G, and plastid gene ndhF (Peterson et al., 2006)

• Triticum polyphyletic

• Aegilops polyphyletic

Selected References

• Harlan, J.R. 1992. “Origins and Processes of Domestication”. In Grass Evolution and Domestication. Ed: G.P. Chapman. Cambridge U. Press, pgs: 159-166.

• Brown, T.A., S. Lindsay, & R.G. Allaby. 2006. “Using Modern Landraces of Wheat to study the Origins of European Agriculture”. In Darwin’s Harvest. Eds: T.J. Motley, N. Zerega, & H. Cross. Columbia U. Press, pgs: 197-212.

• Peterson, G., O. Seberg, M. Yde, & K. Berthelsen. 2006. Phylogenetic relationships of Triticum and Aegilops and evidence for the origin of the A, B, and D genomes of common wheat (Triticum aestivum). Molecular Phylogenetics and Evolution 39: 70-82

• Salamini, F., H. Ozkan, A. Brandolini, R. Schafer-Pregl, & W. Martin. 2002. Genetics and geography of wild cereal domestication in the Near East. Nature Reviews Genetics Vol. 3 June 2002 429-441.