Genetic evidence for differential selection of grain and embryo weight during wheat evolution under domestication

Abstract

Wheat is one of the Neolithic founder crops domesticated ~10 500 years ago. Following the domestication episode, its evolution under domestication has resulted in various genetic modifications. Grain weight, embryo weight, and the interaction between those factors were examined among domesticated durum wheat and its direct progenitor, wild emmer wheat. Experimental data show that grain weight has increased over the course of wheat evolution without any parallel change in embryo weight, resulting in a significantly reduced (30%) embryo weight/grain weight ratio in domesticated wheat. The genetic factors associated with these modifications were further investigated using a population of recombinant inbred substitution lines that segregated for chromosome 2A. A cluster of loci affecting grain weight and shape was identified on the long arm of chromosome 2AL. Interestingly, a novel locus controlling embryo weight was mapped on chromosome 2AS, on which the wild emmer allele promotes heavier embryos and greater seedling vigour. To the best of our knowledge, this is the first report of a QTL for embryo weight in wheat. The results suggest a differential selection of grain and embryo weight during the evolution of domesticated wheat. It is argued that conscious selection by early farmers favouring larger grains and smaller embryos appears to have resulted in a significant change in endosperm weight/embryo weight ratio in the domesticated wheat. Exposing the genetic factors associated with endosperm and embryo size improves our understanding of the evolutionary dynamics of wheat under domestication and is likely to be useful for future wheat-breeding efforts.

Keywords: Early vigour; Triticum turgidum ssp. dicoccoides; embryo; embryo/endosperm ratio; endosperm; grain size; tetraploid wheat; wheat domestication..

Fig. 1.

A comparison of grain and embryo weights between wild emmer wheat and durum wheat. Boxplot of (A) grain weight, (B) embryo weight, and (C) embryo weight/grain weight ratio among 12 wild emmer accessions and 12 cultivars of durum wheat. (D) A representative photo demonstrating the variation in grain and embryo size. Grains and embryos were arranged in order of their grain weight, from left to right. (E) Correlation analysis between grain weight and embryo weight. *** indicates a significant difference (P <0.0001). This figure is available in colour at JXB online.)

Fig. 2.

Grain characteristics of the durum wheat parental line Langdon (LDN), the substitution line DIC-2A, and the wild emmer chromosome donor Israel-A (Isr-A). (A) Representative photo showing grain width (n=20) and (B) grain length (n=10). This figure is available in colour at JXB online.)

Fig. 3.

Boxplot of the grain weight, embryo weight, and shape of the two parental lines, Langdon (LDN) and DIC-2A, and the wild emmer chromosome donor, Israel-A (Isr-A). (A) Grain width, (B) grain length, (C) grain weight, (D) embryo width, (E) embryo length, and (F) embryo weight. Values are means ±SD (n=3). Different letters indicate significant differences, as determined by Tukey LSD test (P ≤0.05).

Fig. 4.

Correlation (r) between grain weight and embryo weight and frequency distribution of the traits among the 94 recombinant inbred substitution lines of Langdon×DIC-2A. Data are means of three replicated plants. Arrows indicate the values of the parental lines Langdon (LDN) and DIC-2A (DIC).

Fig. 5.

Likelihood intervals for quantitative loci (QTL) associated with grain weight (GW), embryo weight (EmW), grain length (GL), and grain width (Gwid) in recombinant inbred substitution lines of Langdon×DIC-2A.

Fig. 6.

Effects of GW and EmW loci on grain weight and embryo weight. Graphical genotyping of six recombinant inbred substitution lines (RISL) and the two parental lines. Langdon (LDN) and DIC-2A. The effects of each genotype on GW and EmW are shown as the percentage difference relative to LDN. *, **, *** and **** indicates a significant difference at P ≤0.05, 0.01, 0.001 and 0.0001.

Fig. 7.

A boxplot demonstrating the effects of different grain-weight (GW) and embryo-weight (EmW) alleles on seedling weight. Domesticated and wild alleles are marked by LDN and DIC, respectively. * indicates a significant difference (P ≤0.05) from the group carrying both domesticated alleles.