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The evolutionary history of maize (Zea mays subsp, mays) is of general interest because of its economic and scientific importance. Here we show that many cellular traits described previously in developing caryopses of maize are also seen in its wild progenitor teosinte (Zea mays subsp. purviglumis). These features, each with a possible role in development, include (1) an early programmed cell death in the maternal placento-chalazal (P-C) layer that may lead to increased hydrolytic conductance to the developing seed; (2) accumulation of phenolics and flavonoids in the P-C layer that may be related to antimicrobial activity; (3) formation of wall ingrowths in the basal endosperm transfer layer (BETL); (4) localization of cell wall invertase in the BETL, which is attributed to the increased transport capacity of photosynthates to the sink; and (5) endoreduplication in endosperm nuclei suggested to contribute to increased gene expression and greater sink capacity of the developing seed. In maize caryopsis, these cellular traits have been previously attributed to domestication and selection for larger seed size and vigor. Given the conservation of the entire cellular program in developing teosinte caryopses described here, we suggest that these traits evolved independently of domestication and predate human selection pressure. Key words: cell wall invertase; endoreduplication; genome size; maize; Poaceae; programmed cell death; teosinte; Zea mays subsp, parviglumis. doi: 10.3732/ajb.0900059

Plant architecture is elaborated through the activity of shoot apical meristems (SAMs), which produce repeating units known as phytomers, that are comprised of leaf, node, internode, and axillary bud. Insight into how SAMs function and how individual phytomer components are related to each other can been obtained through characterization of recessive mutants with perturbed shoot development. In this study, we characterized a new mutant to further understand mechanisms underlying shoot development in maize. The filifolium 1-0 (ffm1-0) mutants develop narrow leaves on dwarfed shoots. Shoot growth often terminates at the seedling stage from depletion of the SAM, but if plants survive to maturity they are invariably bushy. KN1-like homeobox (KNOX) proteins are inappropriately regulated in mutant apices, adaxial identity is not specified in mutant leaves, and axillary meristems develop precociously. We propose that FFM1 acts to demarcate zones within the SAM so that appropriate fates can be conferred on cells within those zones by other factors. On the basis of the mutant phenotype, we also speculate about different relationships between phytomer components in maize and Arabidopsis. Key words: axillary; maize; meristems; leaves; phytomer; Poaceae; polarity; Zea mays. doi: 10.3732/ajb.0800388

Questions that still surround the origin and early dispersals of maize (Zea mays L.) result in large part from the absence of information on its early history from the Balsas River Valley of tropical southwestern Mexico, where its wild ancestor is native. We report starch grain and phytolith data from the Xihuatoxtla shelter, located in the Central Balsas Valley, that indicate that maize was present by 8,700 calendrical years ago (cal. B.P.). Phytolith data also indicate an early preceramic presence of a domesticated species of squash, possibly Cucurbita argyrosperma. The starch and phytolith data also allow an evaluation of current hypotheses about how early maize was used, and provide evidence as to the tempo and timing of human selection pressure on 2 major domestication genes in Zea and Cucurbita. Our data confirm an early Holocene chronology for maize domestication that has been previously indicated by archaeological and paleoecological phytolith, starch grain, and pollen data from south of Mexico, and reshift the focus back to an origin in the seasonal tropical forest rather than in the semiarid highlands. early Holocene | maize domestication | phytoliths | starch grains

Molecular evidence indicates that the wild ancestor of maize is presently native to the seasonally dry tropical forest of the Central Balsas watershed in southwestern Mexico. We report here on archaeological investigations in a region of the Central Balsas located near the Iguala Valley in Guerrero state that show for the first time a long sequence of human occupation and plant exploitation reaching back to the early Holocene. One of the sites excavated, the Xihuatoxtla Shelter, contains well-stratified deposits and a stone tool assemblage of bifacially flaked points, simple flake tools, and numerous handstones and milling stone bases radiocarbon dated to at least 8700 calendrical years B.P. As reported in a companion paper (Piperno DR, et al., in this issue of PNAS), starch grain and phytolith residues from the ground and chipped stone tools, plus phytoliths from directly associated sediments, provide evidence for maize (Zea mays L.) and domesticated squash (Cucurbita spp.) in contexts contemporaneous with and stratigraphically below the 8700 calendrical years B.P. date. The radiocarbon determinations, stratigraphic integrity of Xihuatoxtla's deposits, and characteristics of the stone tool assemblages associated with the maize and squash remains all indicate that these plants were early Holocene domesticates. Early agriculture in this region of Mexico appears to have involved small groups of cultivators who were shifting their settlements seasonally and engaging in a variety of subsistence pursuits. agricultural origins | Mesoamerican preceramic | maize domestication

We focused on a region encompassing a major maize domestication locus, Tb1, and a locus involved in the flowering time variation, Dwarf8 (D8), to investigate the consequences of two closely linked selective sweeps on nucleotide variation and gain some insights into maize geographical diffusion, through climate adaptation. First, we physically mapped D8 at ~300 kb 3' of Tb1. Second, we analyzed patterns of nucleotide variation at Tb1, D8, and seven short regions (400-700 bp) located in the Tb1-D8 region sequenced on a 40 maize inbred lines panel encompassing early-flowering temperate and late-flowering tropical lines. The pattern of polymorphism along the region is characterized by two valleys of depleted polymorphism while the region in between exhibits an appreciable amount of diversity. Our results reveal that a region ~100 kb upstream of the D8 gene exhibits hallmarks of divergent selection between temperate and tropical lines and is likely closer than the D8 gene to the target of selection for climate adaptation. Selection in the tropical lines appears more recent than in the temperate lines, suggesting an initial domestication of early-flowering maize. Simulation results indicate that the polymorphism pattern is consistent with two interfering selective sweeps at Tb1 and D8.

Los restos de maiz en sitios arqueologicos de la region central de Chile son notablemente escasos. Para estudiar la introduccion y adopcion de este cultivo en las poblaciones prehispanicas se [...]

Improvement of local germplasm through artificial selection is regarded as the main force behind maize evolution and diversity in Mexico, the crop's center of origin. This perspective neglects the larger social context of maize evolution. Using a theoretical approach and Mexico-wide data, we show that farmer-led evolution of maize is largely driven by a technological diffusion and appropriation process that selectively integrates nonlocal germplasm into local seed stocks. Our approach construes farmer practices as events in the life history of seed to build a demographic model. The model shows how random and systematic differences in management combine to structure maize seed populations into subpopulations that can spread or become extinct, in some cases independently of visible agronomic advantages. The process involves continuous population bottlenecks that can lead to diversity loss. Nonlocal germplasm thus might play a critical role in maintaining diversity in individual localities. Empirical estimates show that introduction of nonlocal seed in Central and Southeastern Mexico is rarer than previously thought; prompt replacement further prevents new seed from spreading. Yet introduced seed perceived as valuable diffuses rapidly, contributing variation in the form of type diversity or through introgression into local seed. Maize seed dynamics and evolution are thus part of a complex social process driven by farmers' desire to appropriate the value in maize farming, not always achieved by preserving or improving local seed stocks. crop diversity and evolution | metapopulations | technological diffusion

Maize was domesticated more than 6,000 years ago in central Mexico, and remains a vital staple food and cultural symbol in Mesoamerica. Maize yield in the central highlands is strongly dependant on adequate rainfall early in the growing season (April-June) because late maturation of the crop may result in damage from autumn frost. Climate-induced crop failures with profound socioeconomic impacts have punctuated Mexican history. However, reliable records of maize harvest have not been available until very recently, and historical records of crop yield and price are discontinuous and can be difficult to interpret. We have developed a continuous, exactly dated, tree-ring reconstruction of maize yield variability in central Mexico from 1474 to 2001 that provides new insight into the history of climate and food availability in the heartland of the Mesoamerican cultural province. The reconstruction indicates that seven of the most severe agricultural crises in Mexican history occurred during decadal-scale episodes of reconstructed maize shortfalls.

A sedimentary pollen sequence from the coastal plain of Veracruz, Mexico, demonstrates maize cultivation by 5,000 years ago, refining understanding of the geography of early maize cultivation. Methodological issues related to bioturbation involved in dating that record combine with its similarity to a pollen sequence from the coastal plain of Tabasco, Mexico, to suggest that the inception of maize cultivation in that record occurred as much as 1,000-2,000 years more recently than the previously accepted 7,000 years ago. Our analysis thereby has substantive, theoretical, and methodological implications for understanding the complex process of maize domestication. Substantively, it demonstrates that the earliest securely dated evidence of maize comes from macrofossils excavated near Oaxaca and Tehuacan, Mexico, and not from the coastal plain along the southern Gulf of Mexico. Theoretically, that evidence best supports the hypothesis that people in the Southern Highlands domesticated this important crop plant. Methodologically, sedimentary pollen and other microfossil sequences can make valuable contributions to reconstructing the geography of early maize cultivation, but we must acknowledge the limits to precision that bioturbation in coastal lagoons imposes on the dating of such records. Mesoamerica | Tabasco | Veracruz

Teosinte, the probable progenitor of maize, has kernels that are encased in hardened fruitcases, which interfere with the use of the kernels as food. Although the components of the fruitcase are present in maize, their development is disrupted so that the kernels are not encased as in teosinte but exposed on the ear. The change from encased to exposed kernels represents a key step in maize evolution. The locus that largely controls this morphological difference between maize and teosinte, teosinte glume architecture 1, is described and genetically mapped., Genetic and biosystematic research has provided substantial evidence that maize is a domesticated derivative of the wild Mexican grass teosinte (Zea mays ssp. parviglumis) [1-6]. Nevertheless, because of the profound [...]

The regional history in relation to the shaping of maize diversity in the western highlands of Guatemala is explored. A closer examination of regional history suggests which processes might be important for shaping the present geographical distribution of maize diversity.