Cytogenetic studies in the genus Zea

Summary New cytological evidence supporting x = 5 as the basic chromosome number of the genus Zea has been obtained as a consequence of our analysis of the meiotic configurations of Zea mays ssp. mays, Z. diploperennis, Z. perennis and of four F₁ artificial interspecific hybrids. Z. mays ssp. mays (2n = 20) presents regular meiosis with 10 bivalents (II) and is considered here as a typical allotetraploid (A₂A₂B₂B₂). In Z. diploperennis (2n = 20) 10II are formed in the majority of the cells, but the formation of 1III + 8II + 1I or 1III + 711 + 3I in 4% of the cells would indicate its segmental allotetraploid nature (A₁A₁B₁B₁). Z. perennis (2n = 40) had 5IV + 10II in 55% of the cells and would be considered as an auto-allooctoploid (A₁A₁A'₁A'₁C₁C₁C₂C₂). Z. diploperennis x Z. mays ssp. mays (2n = 20) presents 10II in ca. 70% of the cells and no multivalents are formed. In the two 2n = 30 hybrids (Z. mays ssp. mays x Z. perennis and Z. diploperennis x Z. perennis) the most frequent meiotic configuration was 5III + 5II + 5I and in 2n = 40 hybrid (Z. diploperennis x Z. perennis) was 5IV + 10II. Moreover, secondary association was observed in the three abovementioned tetraploid taxa (2n = 20) where one to five groups of two bivalents each at diakinesis-metaphase I was formed showing the affinities between homoeologous genomes. The results, as a whole, can be interpreed by assuming a basic x = 5 in this polyploid complex. The main previous contributions that support this working hypothesis are reviewed and its phylogenetic implications studied are discussed.     

TAXONOMY OF ZEA (GRAMINEAE). I. A SUBGENERIC CLASSIFICATION WITH KEY TO TAXA

The genus Zea is here divided into the Sect. Luxuriantes Doebley & litis sect. n., including the perennials Z. diploperennis (2n = 20) and Z. perennis (2n = 40) and the annual Z. luxurians (2n = 20); and Sect. Zea , including the wild Z. mays ssp.parviglumis and Z. mays ssp. mexicana (both 2n = 20), and Z. mays ssp. mays (2n = 20), the highly domesticated and tremendously variable derivate of the latter. This division is verified by a multivariate analysis of a large number of morphological characters of the male inflorescence. Cytogenetic and chemotaxonomic evidence supports the morphological conclusions. A consideration of the phylogeny of Zea within the conceptual framework offered by this new sectioning of the genus points convincingly to annual teosinte (Z. mays ssp. mexicana) as the ancestor of cultivated maize.     

Chromosome localization and characterization of a family of long interspersed repetitive DNA elements from the genus Zea

This paper describes the characterization and chromosomal distribution of new long repetitive sequences present in all species of the genus Zea. These sequences constitute a family of moderately repetitive elements ranging approximately from 1350 to 1700 copies per haploid genome in modern maize (Zea mays ssp. mays) and teosinte (Zea diploperennis), respectively. The elements are long, probably larger than 9 kb, and they show a highly conserved internal organization among Zea subspecies and species. The elements are present in all maize chromosomes in an interspersed pattern of distribution, are absent from centromeric and pericentric heterochromatin, and with some clustering in the distal regions of chromosome arms.     

A Scanning Electron Microscopy Survey of Some Maydeae Pollen

Scanning electron microscopy was used to examine the wall sculpturing of pollen from Zea mays L. ssp. mays (maize), Zea mays ssp. mexicana (Schrad.) Iltis (teosinte), Zea perennis (Hitchc.) Reeves and Mangelsdorf (perennial teosinte), and two species of Tripsacum L. The Zea taxa are shown to possess similar pollen types, with spinules scattered regularly over the exine surface. Tripsacum exhibits a distinctly reticuloid pattern, with spinules clumped into isolated lacunae. Hybrids between Zea and Tripsacum are either intermediate in exine pattern or similar to Tripsacum, depending on the genome combination.     

Cryptic homoeology analysis in species and hybrids of genus Zea

Cryptic intergenomic pairing of genus Zea was induced by the use of a diluted colchicine solution in order to elucidate the phylogenetic relations and differentiation of the homoeologous genomes. Results indicate that in species and hybrids with 2n = 20, there was chromosome pairing between the homoeologous A and B genomes with a maximum of 5IV, with the exception of Zea diploperennis and their interspecific hybrids where cryptic homoeologous chromosome pairing was not induced. In almost all 2n = 30 hybrids, observed cryptic pairing increased to a maximum of 10III although Z. mays × Z. mays with 2n = 30 did not show significant differences between treated and untreated materials. Pairing was also observed in species and hybrids with 2n = 40, in which a maximum of 10IV was observed, with the exception of Z. mays with 2n = 40 where treated and untreated cells did not differ significantly.     

Chilling stress and oxygen metabolizing enzymes in Zea mays and Zea diploperennis*

Abstract. The activities of five active-oxygen scavenging enzymes were compared for cold-lability and three were compared for chilling induction in two Zea genotypes of contrasting susceptibility to photoinhibition during chilling. Activities of superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and glutathione reductase (GTR, EC 1.6.4.2) in leaf extracts from plants grown without chilling stress were assayed at 19°C and 5°C. Enzymes from the chilling-susceptible Z. Mays cv. LG11 had lower specific activities at 5°C than did enzymes from the chilling-tolerant Z. diploperennis, except for MDHAR where no significant differences were observed. The activities of SOD and APX from Z. diploperennis were double those of Z. mays at both assay temperatures. Monodehydroa-scrobate reductase and glutathione reductase activities in both species were reduced by 63–78% at a 5°C assay temperature. The dehydroascorbate reductase (DHAR) showed the greatest low-temperature lability losing 96% (Z. diploperennis) and 100% (Z. mays) of its activity at 5°C. To examine possible chilling-induced changes in levels of enzyme activity, plants of both Zea genotypes were transferred to growth chambers at 10°C at moderate light intensities. Glutathione reductase activity was found to increase within 24h in Z. diploperennis, but it decreased slightly in Z. mays. MDHAR activity decreased by 50% in Z. diploperennis but showed only a transient increase in activity in Z. mays.     

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.     

Cytogenetic studies in the genus Zea

Summary Premeiotic colchicine treatment brings about the production of one to five quadrivalents in Zea mays ssp. mays (maize, 2n=20) and an increase in the number of quadrivalents from five to ten in Zea perennis (2n=40). The results confirm the allotetraploid nature of maize and suggest that the species possesses two homoeologous genomes (A2A2 B2B2) that fail to pair, probably due to the presence of Ph-like genes. Moreover, the autoallooctoploid nature of Zea perennis, with a genome formula A1A1 A1A1 C1C1 C2C2, is supported by the present results.     

Quantitative trait loci controlling phenotypes related to the perennial versus annual habit in wild relatives of maize

We used quantitative trait locus/loci (QTL) mapping to study the inheritance of traits associated with perennialism in a cross between an annual (Zea mays ssp. parviglumis) and a perennial (Z. diploperennis) species of teosinte. The most striking difference between these species is that Z. diploperennis forms rhizomes, whereas Z. mays ssp. parviglumis lacks these over-wintering underground stems. An F₂ population of 425 individuals was genotyped at 95 restriction fragment length polymorphism marker loci and the association between phenotype and genotype was analyzed by composite interval mapping. We detected a total of 38 QTL for eight traits. The number of QTL found for each trait ranged from two for rhizome formation to nine for tillering. QTL for six of the traits mapped near each other on chromosome 2, and QTL for four traits mapped near each other on chromosome 6, suggesting that these regions play an important role in the evolution of the perennial habit in teosinte. Most of the 38 QTL had small effects, and no single QTL showed a strikingly large effect. The map positions that we determined for rhizome formation and other traits in teosinte may help to locate corresponding QTL in pasture and turf grasses used as forage for cattle and for erosion control in agro-ecosystems.     

High transmission of paternal plastid DNA in alfalfa plants demonstrated by restriction fragment polymorphic analysis

Summary A high frequency of paternal plastid transmission occurred in progeny from crosses among normal green alfalfa plants. Plastid transmission was analyzed by hybridization of radiolabeled alfalfa plastid DNA (cpDNA) probes to Southern blots of restriction digests of the progeny DNA. Each probe revealed a specific polymorphism differentiating the parental plastid genomes. Of 212 progeny, 34 were heteroplastidic, with their cpDNAs ranging from predominantly paternal to predominantly maternal. Regrowth of shoots from heteroplasmic plants following removal of top growth revealed the persistence of mixed plastids in a given plant. However, different shoots within a green heteroplasmic plant exhibited paternal, maternal, or mixed cpDNAs. Evidence of maternal nuclear genomic influence on the frequency of paternal plastid transmission was observed in some reciprocal crosses. A few tetraploid F₁ progeny were obtained from tetraploid (2n=4x=32) Medicago sativa ssp. sativa x diploid (2n=2x=16) M. sativa ssp. falcata crosses, and resulted from unreduced gametes. Here more than the maternal genome alone apparently functioned in controlling plastid transmission. Considering all crosses, only 5 of 212 progeny cpDNAs lacked evidence of a definitive paternal plastid fragment.Contribution No. 89-524-J from the Kansas Agricultural Experiment Station, Kansas State University, Manhattan     

Meiotic pairing behaviour reveals differences in genomic constitution between Hystrix patula and other species of the genus Hystrix Moench (Poaceae, Triticeae)

Interspecific and intergeneric hybridizations were carried out to evaluate the genomic relationships among species of Hystrix Moench and to study the relationships between Hystrix species and Psathyrostachys huashanica Keng (2n=2x=14, Ns^h). Meiotic pairing in hybrids of Hystrix duthiei ssp. duthiei × P. huashanica (2n=3x=21), Hystrix duthiei ssp. longearistata × P. huashanica (2n=3x=21) and H. patula × P. huashanica (2n=3x=21) averaged 5.18, 5.11 and 0.29 bivalents per cell, while H. patula × H. duthiei ssp. longearistata (2n=4x=28) averaged 25.36 univalents and 1.32 bivalents per cell, respectively. The results indicate that (i) H. duthiei ssp. duthiei and H. duthiei ssp. longearistata have one set of Ns genome from Psathyrostachys; (ii) H. patula has no Ns genome; (iii) genomes of H. duthiei ssp. duthiei and H. duthiei ssp. longearistata are non-homologous to those of H. patula. The genomic relationships between H. patula and other Hystrix species are also discussed.     

ORIGIN OF TRIPSACUM ANDERSONII (GRAMINEAE)

Tripsacum andersonii Gray (Gramineae) is a species with 2n = 64 chromosomes. Chromosome behaviour during meiosis of microsporogenesis suggests that the species combines three homologous haploid Tripsacum genomes of x = 18 (54 chromosomes), and an alien haploid genome of x = 10 chromosomes. Cytogenetic studies indicate that T. andersonii originated as a hybrid between a species of Tripsacum (2n = 36) and a species of Zea (2n = 20). Comparative morphology and flavonoid chemistry fail to identify the Zea species involved in this intergeneric hybrid. Chromosome morphology suggests that it was either Z. mays L. subsp. mays (domesticated maize) or subspecies mexicana (Schrad.) Iltis (annual teosinte). The Tripsacum parent probably was T. latifolium Hitchc. of Central America. It resembles T. andersonii in vegetative morphology. Tripsacum maizar Hernandez et Randolph and T. laxum Nash, which resemble T. andersonii in flavonoid chemistry, are eliminated as possible parents on the basis of growth habit and the morphology of their hybrids with maize.     

Dynamic evolution of bz orthologous regions in the Andropogoneae and other grasses

Genome structure exhibits remarkable plasticity within Zea mays. To examine how haplotype structure has evolved within the Andropogoneae tribe, we have analyzed the bz gene‐rich region of maize (Zea mays), the Zea teosintes mays ssp. mexicana, luxurians and diploperennis, Tripsacum dactyloides, Coix lacryma‐jobi and Sorghum propinquum. We sequenced and annotated BAC clones from these species and re‐annotated the orthologous Sorghum bicolor region. Gene colinearity in the region is well conserved within the genus Zea. However, the orthologous regions of Coix and Sorghum exhibited several micro‐rearrangements relative to Zea, including addition, truncation and deletion of genes. The stc1 gene, involved in the production of a terpenoid insect defense signal, is evolving particularly fast, and its progressive disappearance from some species is occurring by microhomology‐mediated recombination. LTR retrotransposons are the main contributors to the dynamic evolution of the bz region. Common transposon insertion sites occur among haplotypes from different Zea mays sub‐species, but not outside the species. As in Zea, different patterns of interspersion between genes and retrotransposons are observed in Sorghum. We estimate that the mean divergence times between maize and Tripsacum, Coix and Sorghum are 8.5, 12.1 and 12.4 million years ago, respectively, and that between Coix and Sorghum is 9.3 million years ago. A comparison of the bz orthologous regions of Zea, Sorghum and Coix with those of Brachypodium, Setaria and Oryza allows us to infer how the region has evolved by addition and deletion of genes in the approximately 50 million years since these genera diverged from a common progenitor.     

Population genetic structure of a specialist leafhopper on Zea: likely anthropogenic and ecological determinants of gene flow

Corn leafhopper, Dalbulus maidis DeLong & Wolcott (Hemiptera: Cicadellidae), is a specialist herbivore on the genus Zea (Poaceae). The genera Dalbulus and Zea evolved in central Mexico. We sought to determine whether population genetic structuring is prevalent in corn leafhoppers inhabiting three of its host plants: (1) the highland species perennial teosinte (Zea diploperennis Iltis, Doebley & Guzman), (2) the mid‐ to lowland‐species Balsas teosinte (Zea mays ssp. parviglumis Iltis & Doebley), and (3) the ubiquitous domesticated maize (Zea mays ssp. mays L.). We used amplified fragment length polymorphisms to detect population structuring and genetic differentiation among corn leafhoppers on the three host plants in western‐central and ‐northern Mexico. Our results showed that corn leafhopper in Mexico is composed of at least two genetically discrete populations: an ‘Itinerant’ population associated with the annual hosts maize and Balsas teosinte, which appears to be widely distributed in Mexico, and a ‘Las Joyas’ population restricted to perennial teosinte and confined to a small mountain range (Sierra de Manantlán) in western‐central Mexico. Our results further suggested that population structuring is not due to isolation by distance or landscape features: Las Joyas and Itinerant corn leafhopper populations are genetically distinct despite their geographic proximity (ca. 4 km), whereas Itinerant corn leafhoppers separated by hundreds of kilometers (>800 km), mountain ranges, and a maritime corridor (Sea of Cortez) are not genetically distinct. Based on our results and on published ethnohistorical and archaeological data, we propose pre‐Columbian and modern scenarios, including likely ecological and anthropogenic influences, in which the observed genetic population structuring of corn leafhopper could have originated and could be maintained. Also, we hypothesize that after evolving on the lowland Balsas teosinte, corn leafhopper expanded its host range to include maize and then the highland perennial teosinte, following the domestication and spread of maize within the last 9 000 years.     

RFLP analysis of the wild potato species,Solanum acaule Bitter (Solanum sect. Petota)

Summary Intraspecific variation of a wild potato species, Solanum acaule Bitt., was analyzed by RFLPs of genomic DNA. One hundred and five accessions were selected throughout the distribution area, including all subspecies, i.e., ssp. albicans (hexaploid), ssp. punae (tetraploid), ssp. acaule (tetraploid) and ssp. aemulans (tetraploid). Twenty-seven low-copy DNA clones (probes) were Southern hybridized with EcoRI, EcoRV, HindIII, and XbaI digests of total DNA of all accessions. In total, 238 RFLPs were detected from 94 enzyme x probe combinations. Among them, 49 RFLPs were specific to ssp. albicans, suggesting that the additional third genome is distinct from its two other genomes. RFLPs between and within subspecies were analyzed by principal component analysis. DNA similarities between subspecies coincided with a former taxonomic treatment in the sense that ssp. albicans is the most distantly related to ssp. acaule and ssp. aemulans is distantly related. Subspecies acaule and ssp. punae were indistinguishable. In addition, RFLPs could be used to distinguish groups within subspecies. Subspecies aemulans, confined to Argentina, was divided into two populations, one from the province of La Rioja and the other from the province of Jujuy. In ssp. acaule, some accessions from the southernmost distribution area were clearly distinguishable, while the others varied continuously, showing a geographical cline from Peru to Argentina.Reference to a specific brand or firm name does not constitute endorsement by the US Department of Agriculture over others of similar nature not mentioned     

DNA contents,giemsa banding,and systematics inScilla bifolia,S. drunensis,andS. vindobonensis (Liliaceae)

DNA contents have been determined cytophotometrically in the three Central European, relatedScilla speciesS. bifolia (2n = 18, 2 x, 1 C = 6.2 pg),S. drunensis (2n = 36, 4 x, 1 C = 12.8 pg), andS. vindobonensis (2n = 18, 2 x, 1 C = 9.4 pg). The tetraploid speciesS. drunensis contains twice as much DNA as the diploidS. bifolia. However, the diploid speciesS. vindobonensis differs in DNA content fromS. bifolia by a factor of about 1.5. This difference is largely due to euchromatic DNA, although the higher DNA content inS. vindobonensis is combined with higher heterochromatin content. The data indicate thatS. bifolia andS. drunensis on the one hand, andS. vindobonensis on the other hand are phyletically well separated. Previous taxonomic conclusions from morphology as well as C-banding are thus corroborated.Evolution ofScilla and Related Genera, V.     

TAXONOMY OF ZEA (GRAMINEAE). II. SUBSPECIFIC CATEGORIES IN THE ZEA MAYS COMPLEX AND A GENERIC SYNOPSIS

A compromise classification of the genus Zea, reflecting both phylogeny and practical needs, recognizes six taxa, as follows: Section Luxuriantes : Zea perennis. Zea diploperennis, Zea luxurians. Section Zea : Zea mays ssp. mexicana (Neo-volcanic Plateau), Zea mays ssp. parviglumis Iltis & Doebley ssp. n. var. parviglumis (Rio Balsas drainage, Pacific slope from Guerrero to Jalisco), Zea mexicana ssp. parviglumis var. huehuetenangensis Iltis & Doebley var. n. (Pacific slope, western Guatemala, Prov. Huehuetenango), Zea mays ssp. mays. The new subspecies is distinguished by smaller spikelets and rachis joints, the varieties by different habitats, blooming dates and their genetic behavior in relation to cultivated Zea mays. Zea mays ssp. mexicana is the ancestor of corn.     

Higher expression of induced defenses in teosintes (Zea spp.) is correlated with greater resistance to fall armyworm,Spodoptera frugiperda

Selection for plant traits important for agriculture can come at a high cost to plant defenses. While selecting for increased growth rate and yield, domestication and subsequent breeding may lead to weakened defenses and greater susceptibility of plants to herbivores. We tested whether expression of defense genes differed among maize, Zea mays ssp. mays L. (Poaceae), and its wild relatives Zea mays ssp. parviglumis Iltis & Doebley and Zea diploperennis Iltis et al. We used two populations of Z. mays ssp. parviglumis: one expected to express high levels of an herbivore resistance gene, wound‐inducible protein (wip1), and another expected to have low expression of wip1. To test whether maize and wild Zea differed in induction of defenses against Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae), we quantified expression of several genes involved in plant defense: wip1, maize protease inhibitor (mpi), pathogenesis‐related protein (PR‐1), and chitinase. Moreover, we compared growth, development, and survival of caterpillars on maize and wild Zea plants. We found that maize expressed low levels of all but one of the genes when attacked by caterpillars, whereas the wild relatives of maize expressed induced defense genes at high levels. Expression of wip1, in particular, was much greater in the Z. mays ssp. parviglumis population that we expected to naturally express high levels of wip1, with expression levels 29‐fold higher than in herbivore‐free plants. Elevated expression of defenses in wild plants was correlated with higher resistance to caterpillars. Larvae were 15–20% smaller on wild Zea compared with maize, developed 20% slower, and only 22% of them survived to pupation on Z. mays ssp. parviglumis with high levels of wip1. Our results suggest that domestication has inadvertently reduced the resistance of maize, and it is likely that expression of wip1 and other genes associated with defenses play an important role in this reduction in resistance.     

Restriction site variation in the zea chloroplast genome

Nineteen accessions selected from the four species and three subspecies of the genus Zea and one accession from the related genus Tripsacum were surveyed for variation with 21 restriction endonucleases. In all, 580 restriction sites were assayed in each chloroplast (cp)DNA, this representing 2.2% of the genome. Twenty-four of the 580 sites were variable in one or more of the cpDNAs. The number of nucleotide substitutions per site (p) between Zea and Tripsacum (0.0056) approximates that between other closely related angiosperm genera. The range in values of p among Zea species (0.0003-0.0024) is on the lower end of the range reported for other angiosperm genera. Analysis of the distribution of restriction site mutations throughout the genome indicated that the inverted repeat evolves more slowly than either the small or large unique sequence regions. Parsimony phylogenetic analysis of the restriction site data produced a tree consistent with isoenzymatic and morphological measures of affinity among the species. Chloroplast DNA analysis was not useful in discriminating the subspecies within Zea mays. The lack of any detectable differences between the cpDNA of maize (Z. mays subsp. mays) and some teosintes (Z. mays subsps. mexicana and parviglumis ) is consistent with the hypothesis that maize is a domesticated form of teosinte. Comparison of the degree of sequence divergence for Z. mays cpDNA and the Adh1 locus suggests the latter may be evolving at 10 times the rate of the former. Comparison of rates of sequence evolution for the mitochondrial and chloroplast genomes was inconclusive and could not clarify whether these two genomes have dissimilar rates of sequence evolution.     

Dryinid (Hym.: Dryinidae) parasitoids ofDalbulus leafhopper (Hom.: Cicadellidae) in Mexico

Little is known about the natural enemies of the leafhopperDalbulus spp. (Homoptera: Cicadellidae). Searches for its dryinid (wasps) parasitoids were made in Jalisco, Mexico. Jalisco contains the greatest number ofDalbulus species, and is considered to be near to the center of origin of this leafhopper genus and its host plants: maize, teosintes (Zea spp.) and gamagrasses (Tripsacum spp.). The dryinidGonatopus bartletti was found parasitizingD. maidis on maize and on annual teosinteZea mays spp.parviglumis. G. flavipes was found parasitizingD. elimatus on perennial teosinteZ. perennis; and a new speciesG. moyaraygozai andAnteon ciudadi parasitizingD. quinquenotatus onTripsacum pilosum andT. dactyloides. Parasitism by dryinids was found at altitudes of 680–2,000 m.Dalbulus maidis, the leafhopper species which causes the greatest losses in maize in Latin America, was found to be parasitized from 680–1,760 m. TheDalbulus species associated with annual host plants (maize andZ. mays spp.parviglumis) were parasitized by dryinids during the rainy season, while theDalbulus species associated with perennial host plants (Z. perennis andTripsacum) were parasitized by dryinids during both the rainy and dry season. The greatest diversity of dryinid parasitoids ofDalbulus spp. and the highest levels of parasitism were recorded from perennial plants, indicating that such species are reservoirs of natural enemies ofDalbulus spp.