Some physical properties of teosinte (Zea mays subsp. parviglumis) pollen

In parts of the world where teosinte and maize are grown in close proximity, there is concern about gene flow between them. Pollen is the primary vehicle for gene flow. Quantifying the biophysical properties of pollen, such as its settling speed and dehydration rate, is important for evaluating outcrossing potential. These properties were measured for teosinte (Zea mays subsp. parviglumis) pollen. Pollen was found to have an average settling speed of 0.165 m s(-1), which agrees well with theoretical values based on the size of the pollen grains. The conductance of the pollen wall for water was derived from the time rate of change of pollen grain size and gave an average conductance of 3.42x10(-4) m s(-1). Water potential, psi, of teosinte pollen was determined at various values of relative water content (dry-weight basis), theta, by using a thermocouple psychrometer and by allowing samples of pollen to come to vapour equilibrium with various saturated salt solutions. Non-linear regression analysis of the data yielded psi (MPa) = -4.13 theta(-1.23) (r2=0.77). Results for conductance and psi were incorporated into a model equation for the rate of water loss from pollen grains, which yielded results that agreed well (r2=0.96) with observations of water loss from pollen grains in air. The data reported here are important building blocks in a model of teosinte pollen movement and should be helpful in establishing the main factors influencing the degree and the direction of pollination between teosinte populations and between maize and teosinte.     

A cellular study of teosinte Zea mays subsp. parviglumis (Poaceae) caryopsis development showing several processes conserved in maize

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. parviglumis). 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.     

Plant defense against fall armyworm in micro‐sympatric maize (Zea mays ssp. mays) and Balsas teosinte (Zea mays ssp. parviglumis)

Maize [Zea mays L. ssp. mays (Poaceae)] was domesticated from Balsas teosinte (Zea mays ssp. parviglumis Iltis & Doebley) in present‐day Mexico. Fall armyworm, Spodoptera frugiperda JE Smith (Lepidoptera: Noctuidae), is among the most important pests of maize in Mexico and Central America. We compared the strength of plant defenses against fall armyworm between micro‐sympatric landrace maize and Balsas teosinte in the field and laboratory. The field comparison, conducted in Mexico, consisted of comparing the frequency of fall armyworm infestation between young maize and Balsas teosinte plants in dryland agricultural fields in which Balsas teosinte grew as a weed. The laboratory comparison contrasted the performance of fall armyworm larvae provided a diet of leaf tissue excised from maize or Balsas teosinte plants that were intact or had been primed by larval feeding. In the field, maize plants were more frequently infested with fall armyworm than Balsas teosinte plants: over 3 years and three fields, maize was infested at a ca. 1.8‐fold greater rate than Balsas teosinte. In the laboratory, larval growth, but not survivorship, was differently affected by feeding on maize vs. Balsas teosinte, and on primed vs. intact plants. Specifically, survivorship was ca. 98%, and did not differ between maize and Balsas teosinte, nor between primed and intact plants. Larvae grew less on intact vs. primed maize, and similarly on intact vs. primed Balsas teosinte; overall, growth was 1.2‐fold greater on maize compared to Balsas teosinte, and on primed compared to intact plants. Parallel observations showed that the differences in growth could not be attributed to the amount of leaf tissue consumed by larvae. We discuss our results in relation to differences in the strength of plant defenses between crops and their ancestors, the relevance of unmanaged Balsas teosinte introgression in the context of fall armyworm defenses in maize, and whether greater growth of larvae on primed vs. intact plants signifies herbivore offense.     

Major regulatory genes in maize contribute to standing variation in teosinte (Zea mays ssp. parviglumis)

In plants, many major regulatory genes that control plant growth and development have been identified and characterized. Despite a detailed knowledge of the function of these genes little is known about how they contribute to the natural variation for complex traits. To determine whether major regulatory genes of maize contribute to standing variation in Balsas teosinte we conducted association mapping in 584 Balsas teosinte individuals. We tested 48 markers from nine candidate regulatory genes against 13 traits for plant and inflorescence architecture. We identified significant associations using a mixed linear model that controls for multiple levels of relatedness. Ten associations involving five candidate genes were significant after correction for multiple testing, and two survive the conservative Bonferroni correction. zfl2, the maize homolog of FLORICAULA of Antirrhinum, was associated with plant height. zap1, the maize homolog of APETALA1 of Arabidopsis, was associated with inflorescence branching. Five SNPs in the maize domestication gene, teosinte branched1, were significantly associated with either plant or inflorescence architecture. Our data suggest that major regulatory genes in maize do play a role in the natural variation for complex traits in teosinte and that some of the minor variants we identified may have been targets of selection during domestication.     

Population structure and its effects on patterns of nucleotide polymorphism in teosinte (Zea mays ssp. parviglumis)

Surveys of nucleotide diversity in the wild ancestor of maize, Zea mays ssp. parviglumis, have revealed genomewide departures from the standard neutral equilibrium (NE) model. Here we investigate the degree to which population structure may account for the excess of rare polymorphisms frequently observed in species-wide samples. On the basis of sequence data from five nuclear and two chloroplast loci, we found significant population genetic structure among seven subpopulations from two geographic regions. Comparisons of estimates of population genetic parameters from species-wide samples and subpopulation-specific samples showed that population genetic subdivision influenced observed patterns of nucleotide polymorphism. In particular, Tajima's D was significantly higher (closer to zero) in subpopulation-specific samples relative to species-wide samples, and therefore more closely corresponded to NE expectations. In spite of these overall patterns, the extent to which levels and patterns of polymorphism within subpopulations differed from species-wide samples and NE expectations depended strongly on the geographic region (Jalisco vs. Balsas) from which subpopulations were sampled. This may be due to the demographic history of subpopulations in those regions. Overall, these results suggest that explicitly accounting for population structure may be important for studies examining the genetic basis of ecologically and agronomically important traits as well as for identifying loci that have been the targets of selection.     

Phytoliths as indicators of prehistoric maize (Zea mays subsp.mays,Poaceae) cultivation

Maize (Zea mays L. subsp.mays) has been identified in archaeological contexts by a high proportion of large cross-shaped phytoliths. Given the numerous races of maize, this study was undertaken to determine if differences below the species level could be noted. It was also designed to see if phytoliths differed in various plant parts at various stages of growth. Several races were grown under experimental conditions. No significant differences were found. Furthermore, few phytoliths alleged to be diagnostic of maize were discovered. Systemic studies of maize and analyses of prehistoric cultivation by means of phytoliths seem not to be as promising as some researchers have argued.     

Inflorescence development in a perennial teosinte: Zea perennis (Poaceae)

The ontogeny of tassels and ears in a perennial Mexican teosinte, Zea perennis (Hitchc.) Reeves and Mangelsdorf, was examined using scanning electron microscopy and light microscopy. Ear development follows a pattern previously described for two annual teosintes, Z. mays subsp. mexicana and Z. mays subsp. parviglumis var. parviglumis (race Balsas), and for the bisexual mixed inflorescence in a diploperennial teosinte, Z. diploperennis; it differs from that described for the ear of Z. diploperennis plants grown at the latitudes of Iowa and Wisconsin. Common bud primordia of the ear are initiated in the axil of distichously arranged bracts along the ear axis. These common primordia bifurcate to form paired pedicellate and sessile spikelet primordia. Development of the pedicellate spikelets in the ear is arrested leaving the sessile spikelets, along with the adjoining rachis segment, to form solitary grains enclosed within cupulate fruitcases. The organogenesis of the central spike of the tassel is similar to that previously described in other Zea taxa. This developmental study supports the hypothesis that both femaleness and maleness are derived from and expressed on a common background; it is consistent with the view that the maize ear was derived from the central spike of a male inflorescence terminating a primary branch of the main axis of the inflorescence.     

Inflorescence development in a new teosinte: Zea nicaraguensis (Poaceae)

Inflorescence development in a newly discovered teosinte, Zea nicaraguensis (Poaceae), from Nicaragua has been investigated using scanning electron microscopy (SEM). The SEM examination revealed that the pattern of both male and female inflorescence development was similar to previously described inflorescence in other Zea taxa. Branch primordia were initiated acropetally in a distichous pattern along the rachis of male and female inflorescences. Spikelet pair primordia bifurcated into pedicellate and sessile spikelet primordia. Predictably, pedicellate spikelet development was arrested and aborted in the female teosinte inflorescence. Organogenesis of functional spikelets and florets was similar to that previously described in maize and teosintes. The results were consistent with our hypothesis that both femininity and masculinity share a common mechanism of inflorescence development in Zea and Tripsacum and are in accord with a putative common mechanism of sex determination in the Andropogoneae. Interestingly, this population of teosinte, unique in its ability to grow in water-logged soils, showed a stable pattern of early inflorescence development. Our results also revealed the uncharacteristic presence of inflorescence polystichy in this population of Zea nicaraguensis. We propose this novel phenotypic variation raises the possibility that a domestic evolution of polystichy in maize was enabled by an occasional polystichous phenotypic in teosinte.     

The genetic architecture of complex traits in teosinte (Zea mays ssp. parviglumis): new evidence from association mapping

Previous association analyses showed that variation at major regulatory genes contributes to standing variation for complex traits in Balsas teosinte, the progenitor of maize. This study expands our previous association mapping effort in teosinte by testing 123 markers in 52 candidate genes for association with 31 traits in a population of 817 individuals. Thirty-three significant associations for markers from 15 candidate genes and 10 traits survive correction for multiple testing. Our analyses suggest several new putative causative relationships between specific genes and trait variation in teosinte. For example, two ramosa genes (ra1 and ra2) associate with ear structure, and the MADS-box gene, zagl1, associates with ear shattering. Since zagl1 was previously shown to be a target of selection during maize domestication, we suggest that this gene was under selection for its effect on the loss of ear shattering, a key domestication trait. All observed effects were relatively small in terms of the percentage of phenotypic variation explained (<10%). We also detected several epistatic interactions between markers in the same gene that associate with the same trait. Candidate-gene-based association mapping appears to be a promising method for investigating the inheritance of complex traits in teosinte.     

Divergence with gene flow is driven by local adaptation to temperature and soil phosphorus concentration in teosinte subspecies (Zea mays parviglumis and Zea mays mexicana)

Patterns of genomic divergence between hybridizing taxa can be heterogeneous along the genome. Both differential introgression and local adaptation may contribute to this pattern. Here, we analysed two teosinte subspecies, Zea mays ssp. parviglumis and ssp. mexicana, to test whether their divergence has occurred in the face of gene flow and to infer which environmental variables have been important drivers of their ecological differentiation. We generated 9,780 DArTseqTM SNPs for 47 populations, and used an additional data set containing 33,454 MaizeSNP50 SNPs for 49 populations. With these data, we inferred features of demographic history and performed genome wide scans to determine the number of outlier SNPs associated with climate and soil variables. The two data sets indicate that divergence has occurred or been maintained despite continuous gene flow and/or secondary contact. Most of the significant SNP associations were to temperature and to phosphorus concentration in the soil. A large proportion of these candidate SNPs were located in regions of high differentiation that had been identified previously as putative inversions. We therefore propose that genomic differentiation in teosintes has occurred by a process of adaptive divergence, with putative inversions contributing to reduced gene flow between locally adapted populations.     

Early ontogenetic development of the pistillate inflorescence in a diploid perennial teosinte (Zea diploperennis, Poaceae)

CAMARA-HERNANDEZ J. & GAMBINO, S., 1991. Early ontogenetic development of the pistillate inflorescence in a diploid perennial teosinte (Zea diploperennis , Poaceae). The early ontogeny of pistillate inflorescences of %ea diploperennis in plants grown at the latitude of Buenos Aires, Argentina, is investigated using the scanning electron microscope. The pattern of development of the inflorescence is similar to that in staminate and mixed inflorescences, starting with the formation of a pair of spikelets from a common branch primordium initiated in the axil of a bract on the ear axis. This bract arrests its development and aborts early. After initiation of an outer glume on both spikelet primordia, the pedicellate spikelet arrests its growth and aborts resulting in the mature inflorescence having two rows of solitary spikelets arranged distichally. This is significantly different from the pattern observed by other authors in plants grown in different environments (such as in natural populations in Mexico).     

Early inflorescence and floral development in Zea mays land race Chapalote (Poaceae)

Tassel and ear primordia were collected from greenhouse-grown specimens of the Mexican maize landrace Chapalote and prepared for scanning electron microscopic (SEM) examination. Measurements of inflorescence apices and spikelet pair primordia (spp) were made from SEM micrographs. Correlation of inflorescence apex diameter with number of spikelet ranks showed no significant difference between tassels and ears, except at the two-rank level where the ear apical meristem had a significantly smaller diameter than corresponding two-ranked tassels. Within individual inflorescences, spp in different ranks enlarged at comparable rates, although the rates from one ear to the next along the stem differed. In both tassels and ears, spp divide to form paired sessile and pedicellate spikelet primordia when the spp is 150 μm wide; ear axes are significantly thicker than tassel axes at the time of bifurcation. The similarities in growth between ear and tassel primordia lend further support to the hypothesis that both the maize tassel and ear are derived from a common inflorescence pattern, a pattern shared with teosinte. Inflorescence primordial growth also suggests that a key character difference between teosinte and maize, distichous vs. polystichous arrangement of spikelets, may be related to size of the apical dome and/or rate of primordium production by the apical meristem. There appears to be more than a single morphological event in the shift from vegetative to reproductive growth. The evocation of axillary buds (ears) is independent of, and temporally separated from, the transition to flowering at the primary shoot apex (tassel).     

Population genetics of duplicated disease-defense genes,hm1 and hm2, in maize (Zea mays ssp. mays L.) and its wild ancestor (Zea mays ssp. parviglumis)

Plant defense genes are subject to nonneutral evolutionary dynamics. Here we investigate the evolutionary dynamics of the duplicated defense genes hm1 and hm2 in maize and its wild ancestor Zea mays ssp. parviglumis. Both genes have been shown to confer resistance to the fungal pathogen Cochliobolus carbonum race 1, but the effectiveness of resistance differs between loci. The genes also display different population histories. The hm1 locus has the highest nucleotide diversity of any gene yet sampled in the wild ancestor of maize, and it contains a large number of indel polymorphisms. There is no evidence, however, that high diversity in hm1 is a product of nonneutral evolution. In contrast, hm2 has very low nucleotide diversity in the wild ancestor of maize. The distribution of hm2 polymorphic sites is consistent with nonneutral evolution, as indicated by Tajima's D and other neutrality tests. In addition, one hm2 haplotype is more frequent than expected under the equilibrium neutral model, suggesting hitchhiking selection. Both defense genes retain >80% of the level of genetic variation in maize relative to the wild ancestor, and this level is similar to other maize genes that were not subject to artificial selection during domestication.     

The role of auxin transport during inflorescence development in maize (Zea mays, Poaceae)

Axillary meristems play a fundamental role in inflorescence architecture. Maize (Zea mays) inflorescences are highly branched panicles because of the production of multiple types of axillary meristems. We used auxin transport inhibitors to show that auxin transport is required for axillary meristem initiation in the maize inflorescence. The phenotype of plants treated with auxin transport inhibitors is very similar to that of barren inflorescence2 (bif2) and barren stalk1 (ba1) mutants, suggesting that these genes function in the same auxin transport pathway. To dissect this pathway, we performed RNA in situ hybridization on plants treated with auxin transport inhibitors. We determined that bif2 is expressed upstream and that ba1 is expressed downstream of auxin transport, enabling us to integrate the genetic and hormonal control of axillary meristem initiation. In addition, treatment of maize inflorescences with auxin transport inhibitors later in development results in the production of single instead of paired spikelets. Paired spikelets are a key feature of the Andropogoneae, a group of over 1000 grasses that includes maize, sorghum, and sugarcane. Because all other grasses bear spikelets singly, these results implicate auxin transport in the evolution of inflorescence architecture. Furthermore, our results provide insight into mechanisms of inflorescence branching that are relevant to all plants.     

Silicon amelioration of aluminium toxicity in teosinte (Zea mays L. ssp. mexicana)

The influence of different Al concentrations, (0, 60 and 120 M Al) on growth and internal concentrations of Al, Si and selected organic acids was analysed in plants of teosinte (Zea mays L. ssp. mexicana), a wild form of maize from acid soils from Mexico. The plants were grown in nutrient solutions (pH 4.0) with or without 4 M silicon. Analysis with the GEOCHEM speciation program did not reveal differences between free activities of Al³⁺ in solutions with and without 4 M Si, but solutions with Si yielded lower concentrations of monomeric Al species, [Al]_mono, when analysed by a modified aluminon method. Plants grown on solutions with similar [Al]_mono, but differing in silicon, showed highly significant differences in growth and tissue concentrations of Al and organic acids. Silicon prevented growth inhibition at [Al]_mono concentrations as high as 35 M, while plants grown without Si suffered severe growth reductions with 33 M [Al]_mono. In solutions with similar [Al]_mono concentrations plants with Si had lower tissue Al concentrations and higher concentrations of malic acid than plants without Si. In view of both the significant influence of Si on the response of plants to Al toxicity and the fact that some soluble Si is always present in soil solutions, the addition of low Si concentrations to nutrient solutions used for Al-tolerance screening is recommended.     

Genetic control of B chromosome transmission rate in Zea mays ssp. mays (Poaceae)

We selected genotypes of high and low B chromosome transmission rate (TR) in a native race of maize (Pisingallo) from northwest Argentina. We made 20 female OB male IB and 20 f.IB m.OB crosses. The former (GOm) showed a large variation of B TR, with a mean of TR ± SE = 0.52 ± 0.06, ranging from 0.17 to 0.98. In the latter (GOf) the mean was TR = 0.47 ± 0.02 ranging from 0.31 to 0.58. Plants showing the highest and the lowest TR were selected to constitute the progenitors of the G1 generations. We made 19 f.0B m.2B crosses, studying 24–30 plants per cross. The TR of the high (H) and low (L) lines in Glm (GlmH and GlmL) significantly differed (TRH = 0.65 ± 0.03, TRL = 0.40 ± 0.01), indicating that the H and L lines are different groups. The large variation in male TR suggests that preferential fertilization of gametes carrying B chromosomes does not always occur. We also selected plants showing high and low TR in the progenies of f.lB m.OB crosses (GOf), and made 24 f.1B m.0B crosses, studying 23–30 plants per cross. The TRs of the H and L lines in G1f (GlfH and GlfL) were significantly different (TRH = 0.48 ± 0.025, TRL = 0.40 ± 0.02). The TRs in G0f and G1fL were significantly different (TR = 0.47 ± 0.02 and 0.40 ± 0.02, respectively), while this was not the case between G0f and G1fH. Our results demonstrate the existence of genotypes controlling B TR in this native population of maize.     

Fine scale genetic structure in the wild ancestor of maize (Zea mays ssp. parviglumis)

Analysis of fine scale genetic structure in continuous populations of outcrossing plant species has traditionally been limited by the availability of sufficient markers. We used a set of 468 SNPs to characterize fine‐scale genetic structure within and between two dense stands of the wild ancestor of maize, teosinte (Zea mays ssp. parviglumis). Our analyses confirmed that teosinte is highly outcrossing and showed little population structure over short distances. We found that the two populations were clearly genetically differentiated, although the actual level of differentiation was low. Spatial autocorrelation of relatedness was observed within both sites but was somewhat stronger in one of the populations. Using principal component analysis, we found evidence for significant local differentiation in the population with stronger spatial autocorrelation. This differentiation was associated with pronounced shifts in the first two principal components along the field. These shifts corresponded to changes in allele frequencies, potentially due to local topographical features. There was little evidence for selection at individual loci as a contributing factor to differentiation. Our results demonstrate that significant local differentiation may, but need not, co‐occur with spatial autocorrelation of relatedness. The present study represents one of the most detailed analyses of local genetic structure to date and provides a benchmark for future studies dealing with fine scale patterns of genetic diversity in natural plant populations.     

Inflorescence development in a high-altitude annual Mexican teosinte (Poaceae)

Some have postulated that highland Mexican maize was derived from an ancient high-altitude teosinte and that later introgression between the two taxa occurred. We used scanning electron microscopy to examine the inflorescence development in both the tassel and ear of a high-altitude Toluca teosinte. One of the most interesting observations was the presence of atypical multiranked orthostiches in the central spike of some male Toluca teosinte inflorescences. Most tassels exhibited a central spike with a pure, four-ranked, tetrastichous phyllotaxy or an intermediate (distichous/tetrastichous) phyllotaxy. A few A(1) tassels had a more typical distichous (two-ranked) central spike. Most ears showed the two-rank condition expected for teosintes. However, three ears displayed an intermediate (distichous/tristichous or distichous/ tetrastichous) phyllotaxy and one ear was tetrastichous. Our analysis of spikelet and floret development in all Toluca inflorescences revealed a pattern similar to that in landrace and U.S. maize, as well as to their close relatives, the teosintes. We suggest that this investigation may reveal inflorescence development in a natural maize-teosinte hybrid. This study further supports our hypothesis that both maleness and femaleness in the Zea inflorescences are derived from a common developmental pathway and underpins a proposal that andropogonoid grasses share a common pattern of inflorescence development.     

GISHGenomic in situ hybridization reveals cryptic genetic differences between maize and its putative wild progenitor Zea mays subsp. parviglumis.

The aim of this paper is to test with genomic in situ hybridization the genomic affinities between maize and its putative progenitor Zea mays subsp. parviglumis. Blocking procedures were applied for the purpose of improving discrimination among chromosome regions. Unlabeled genomic DNA from Z. mays subsp. parviglumis as a blocking agent and labeled genomic DNA from maize were hybridized on maize chromosomes. On the other hand, mitotic metaphases from Z. mays subsp. parviglumis were blocked with unlabeled genomic DNA of maize and hybridized with labeled genomic DNA from Z. mays subsp. parviglumis. Both experiments showed that either maize or Z. mays subsp. parviglumis chromosomes have their own unique sequences. This means an unexpected degree of divergence if Z. mays subsp. parviglumis is the only progenitor of maize, a result that is discussed in relation to our previous genomic in situ hybridization observations and to the different scenarios proposed about the origin of maize.