The origin of the naked grains of maize

Abstract

The most critical step in maize domestication (Zea mays ssp. mays) was the liberation of the kernel from the hardened, protective casing that envelops the kernel in the maize progenitor, teosinte 1 . This evolutionary step exposed the kernel on the surface of the ear such that it could be readily utilized as a food source by humans. Here, we show that this key event in maize domestication is controlled by a single gene (teosinte glume architecture; tga1) belonging to the SBP-domain family 2 of transcriptional regulators. The factor controlling the phenotypic difference between maize and teosinte maps to a 1 kilobase region within which maize and teosinte show only six fixed differences in their DNA sequences. One of these differences encodes a non-conservative amino acid substitution and may affect protein function, while the other five differences potentially affect gene regulation. Molecular evolution analyses show that this region was the target of selection during maize domestication. Our results demonstrate that modest genetic changes in single genes can induce dramatic changes in phenotype during domestication and evolution.The origin of the maize ear has been considered one of the greatest mysteries in both crop domestication 3 and plant evolution 4 . While a wealth of botanical and genetic information has identified the wild Mexican grass, teosinte (Zea mays ssp. parviglumis), as the direct progenitor of maize, the profound differences in the structure of the maize and teosinte female inflorescences (ears) has posed a challenge to formulating a compelling model for the developmental and genetic steps involved in this evolutionary transition 3 . At the heart of the problem is the fact that teosinte kernels are tightly encased in structures called cupulate fruitcases, while maize kernels are borne uncovered on the surface of the ear (Figure 1a, b). The strength with which the fruitcase envelops the teosinte kernel and the stony nature of this casing far exceed the relatively flimsy and loosely bound chaff that surrounds the kernels of the ancestors of the other domesticated cereals. Indeed, the stony fruitcase of teosinte had been considered such an obstacle to the use of teosinte as a grain that teosinte was dismissed by some as a possible progenitor of maize 5 . It was argued that the genetic steps to free the grain from this casing and thereby convert teosinte into a useful crop were too complex to have arisen under domestication.Each of the 5 to 12 cupulate fruitcases in a teosinte ear is formed from an invaginated internode (cupule) within which the kernel sits, and a glume that covers the opening of the cupule such that the kernel is completely hidden from view (Figure 1b, d). When mature, the teosinte ear

Rapid selective sweep of pre-existing polymorphisms and slow fixation of new mutations in experimental evolution of Desulfovibrio vulgaris

“…Association between a few mutations/genes and dramatic phenotypic changes or fitness improvement have been reported in prokaryotes, eukaryotes and viruses (Taubenberger et al, 2005;Wang et al, 2005;Zeyl 2005;Herring et al, 2006;Schoustra et al, 2009). Results from this study indicated the linkage between significantly increased salt tolerance and a few mutations.…”

Evaluating the role of natural selection in the evolution of gene regulation

“…Evidence of directional selection has been found in the upstream cis-regulatory regions of noncoding region of tb1, suggesting adaptive changes in expression of this gene (Clark et al, 2006). Regulatory changes have also been implicated in the evolution of naked grains in maize (Wang et al, 2005) and in the loss of seed shattering in rice (Konishi et al, 2006).…”

Identification of a Consensus DNA-Binding Site for the Arabidopsis thaliana SBP Domain Transcription Factor, AtSPL14, and Binding Kinetics by Surface Plasmon Resonance

“…We found significant variation with flow rate, suggesting a mass transfer limitation (data not shown). Varying concentrations of recombinant protein (5,10,15,20,25,30, and 50 nM) were injected at the higher flow rate (75 µL/min). The SPR response data were then fit to various models using BIAevaluation 3.0 software (Biacore).…”

“…To date, only a few functions for SBP domain proteins have been reported, perhaps due to genetic redundancy of closely related family members. In all cases, SBP domain proteins have been implicated in various aspects of plant growth and development, including metal sensing in algae and directing development of leaves, embryos, and floral organs in higher plants (9,(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). Some of these SBP do-main proteins may also be posttranscriptionally regulated by noncoding microRNAs to control their spatial and temporal expression (23)(24)(25).…”