TRIPSACUM ANDERSONII IS A NATURAL HYBRID INVOLVING ZEA AND TRIPSACUM: MOLECULAR EVIDENCE

Cytogenetic evidence suggests that Tripsacum andersonii may be a natural hybrid between Zea and Tripsacum. In this paper we show sequences that hybridize to the transposable elements Mul and Spm are found in T. andersonii and all Zea species examined. However, no hybridizable sequences are observed in the five other Tripsacum species surveyed. These results suggest that Mu and Spm elements became components of the Zea genome after the divergence of Zea and Tripsacum, and they strongly support the cytological evidence that T. andersonii is a Zea-Tripsacum hybrid. Examination of nuclear ribosomal genes of T. andersonii also supports the hybridization hypothesis and identifies the Zea parent as Zea luxurians. The Tripsacum parent could not be conclusively identified, but the ribosomal gene data suggest that the species of Tripsacum section Fasiculata most closely resemble T. andersonii. Restriction site patterns of two chloroplast DNA sequences indicate that the maternal parent was a species of Tripsacum. These results are complemented by morphological evidence regarding the origin of T. andersonii.     

COUNTERFEIT HYBRIDS BETWEEN TRIPSACUM AND ZEA (GRAMINEAE)

Diploid (2n = 36) Tripsacum australe Cutler and Anderson var. hirsutum de Wet and Timothy, T. cundinamarce de Wet and Timothy, T. dactyloides (L.) L. var. dactyloides and var. meridonale de Wet and Timothy, and T. laxum Nash were crossed with Zea mays L. (2n = 20) as the pollen parent. True hybrids combine the cytologically nonreduced genome of Tripsacum (36 chromosomes) with the haploid (10 chromosomes) or more rarely diploid (20 chromosome) genome of Zea. Maternal offspring with 2n = 36 Tripsacum chromosomes commonly result from parthenogenetic development of cytologically nonreduced eggs. Some individuals with 2n = 36 Tripsacum chromosomes, however, resemble true hybrids in phenotype. These counterfeit hybrids incorporated Zea genetic material into their Tripsacum genomes without true fertilization having taken place. Offspring of counterfeit hybrids that were grown to maturity resembled their mothers in phenotype, and must have originated parthenogenetically. It is proposed that counterfeit hybrids are also produced in nature, and that this process contributes to origins of variation in gametophytic apomicts, and perhaps also in sexually reproducing species.     

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.     

CYTOLOGY OF MAIZE-TRIPSACUM INTROGRESSION

The American Maydinae genera Zea and Tripsacum cross readily when not isolated from each other by gametophytic barriers, and it has been suggested that intergeneric introgression played a role in the evolution of maize. Four Zea chromosomes pair with members of at least one basic genome of tetraploid Tripsacum, and in hybrids involving octaploid Tripsacum all 10 chromosomes of the basic maize genome frequently compete successfully in synapsis with Tripsacum chromosomes. Hybrids that combine 36 Tripsacum and 10 maize chromosomes are female fertile. When they are pollinated by maize their offspring have 36 Tripsacum and 20 maize chromosomes, or again have 36 Tripsacum and 10 maize chromosomes, but the 10 Zea chromosomes are contributed by the new pollen parent. Later backcross generations also include plants with 36 Tripsacum and 12, 14, 16, or 18 maize chromosomes. Individuals with 2n = 56 produce an abundance of offspring with 18 Tripsacum and 20 maize chromosomes when backcrossed with maize. Further backcrossing results in elimination of Tripsacum chromosomes, and eventually plants with 2n = 20 Tripsacum-contaminated maize chromosomes are obtained. Two generations of selfing restore full fertility to these 2n = 20 plants and eliminate all obvious traces of Tripsacum morphology.     

Malate Dehydrogenase, Alcohol Dehydrogenase, and 6-Phosphogluconate Dehydrogenase Isozymes of Zea mays L. × Tripsacum dactyloides L. Hybrids and Parents

Electrophoretic patterns of malate dehydrogenase (Mdh), alcohol dehydrogenase (Adh), and 6-phosphogluconate dehydrogenase (Pgd) of Zea mays L. × Tripsacum dactyloides L. hybrids and their parents were compared. The components of enzymes specific to T. dactyloides may be used as markers to identify the following T. dactyloides chromosomes in the hybrids: Tr 16 (Mdh 2 and Pdg 1), Tr 7, and/or Tr 13 (Adh 2). The isozymes of Mdh 2 are supposed as a possible biochemical marker to evaluate the introgression of genes, determining an apomictic mode of reproduction from T. dactyloides (localized on Tripsacum 16 chromosome) into Z. mays. The isozymes may be used as markers for the identification of maize chromosomes 1 and 6 in the hybrids as well. Chromosome count taken on the examined hybrids showed the addition of 9 to 13 chromosomes of T. dactyloides to maize chromosome complement.     

Composition of Esterase and Peroxidase Isoenzyme Complex, Zein-2 Protein Fraction, and SDS-Protein Complex of Zea Mays L. × Tripsacum Dactyloides L. Hybrids and Parents

The patterns of esterase and peroxidase isoenzymes, subunits of zein-2 fraction and protomers of SDS-protein complex of Zea mays L. × Tripsacum dactyloides L. hybrids and their parents were compared. The study has been made to detect specific to Tripsacum isoesterases and isoperoxidases, zein subunits and SDS-protein protomers which could be used as markers for introgression of gene loci encoding these proteins from Tripsacum into hybrids of Tripsacum with Zea mays. Isoesterases and isoperoxidases as well protomers of SDS-protein complex specific to Tripsacum were detected in all hybrids analyzed. Zein subunits, specific to Tripsacum were detected in some of the analyzed hybrids which i that introgression frequency of the loci encoding proteins studied was different. Chromosome counts taken on the examined hybrids showed the addition of 9 – 13 Tripsacum chromosomes to maize chromosome complement.     

Rapid amplification and fixation of new restriction sites in the ribosomal dna repeats in the derivatives of a cross between maize and Tripsacum dactyloides

Some of the derivatives of a cross of maize (Zea mays L.) × Tripsacum dactyloides (L) L (2n = 72) have abnormal development leading to strange and striking morphologies. The Tripsacum chromosomes in these “tripsacoid” maize plants (with Tripsacum-like characteristics) were eliminated and the maize chromosomes were recovered through repeated backcrossing to maize. As an initial attempt to analyze the DNA alterations in tripsacoid maize, we have detected a few restriction site changes in the ribosomal DNA repeat of these plants (Hpa II, Bal I, Sst I, Mbo II, and Sph I) and a new Sph I site was mapped to the spacer region between the 26S and 17S genes. Several possible mechanisms for the generation of a new restriction site are discussed, and we propose that the transient presence of Tripsacum genome during the backcrossing in some way induced a rapid amplification and fixation of new restriction sites in a relatively short period of time.     

OBSERVATIONS ON INTROGRESSION OF TRIPSACUM INTO MAIZE

Derivatives of a cross between diploid Zea mays L. and Tripsacum dactyloides (L.) L. (2n = 72) were compared cytologically and morphologically. The objective of this study was to detect introgression from Tripsacum to maize that might have occurred during seven backcross generations with maize. Thirty-three morphological characters were used to analyze variation among aneuploid (20Zm + 2Td), 20-chromosome recovered maize, and the recurrent maize parent plants. Aneuploid and maize checks were extreme types, with 20-chromosome hybrid derivatives being morphologically intermediate. Several recovered maizes clustered with aneuploid plants and these hybrid derivatives have the greatest chance of Tripsacum introgression. Many traits such as endosperm abnormalities, tassel seed, albinos, tunicate glumes, tassel-tipped ears, fasciated and branched ear, and male spikelets between rows of kernels were observed. Although the genetic basis of many traits is unknown, mutations, epistatic effects or expression of Tripsacum chromatin are possible causes. The number of abnormal and tripsacoid traits observed in 20-chromosome recovered maizes indicates genetic transfer from Tripsacum to the maize genome.     

Tripsacum-maize interaction: a novel cytogenetic system

The genera Zea and Tripsacum cross readily when they are not isolated by gametophytic barriers, and it has been postulated that intergeneric introgression played a role in the evolution of maize. The basic x = 9 Tripsacum and x = 10 Zea genomes have little cytological affinity for each other in hybrids that combine 10 Zea with 18 Tripsacum chromosomes. However, one to four Tripsacum chromosomes sometimes associate with Zea chromosomes in hybrids between Z. mays (2n = 20) and T. dactyloides (2n = 72). These hybrids with 10 Zea and 36 Tripsacum chromosomes frequently produce functional female gametes with 36 Tripsacum chromosomes only. When they are pollinated with maize, their offspring again have 36 Tripsacum and 10 maize chromosomes, but the Tripsacum genome is contaminated with maize genetic material. In these individuals, intergenome pairing is the rule, and when they are pollinated with maize, their offspring have 36 Tripsacum and 10, 12, 14, 16, 18, or 20 Zea chromosomes. Plants with 36 Tripsacum and 20 Zea chromosomes behave cytologically as alloploids, although the Tripsacum genome is contimated with maize, and one basic maize genome is contaminated with with Tripsacum genetic material. When they are pollinated with maize, offspring with 18 Tripsacum and 20 Zea chromosome are obtained. Further successive backcrosses with maize selectively eliminate Tripsacum chromosomes, and eventually plants with 2n = 20 Zea chromosomes are recovered. Many of these maize plants are highly "tripsacoid." Strong gametophytic selection for essentially pure Zea gametes, however, eliminates all obvious traces of Tripsacum morphology within a relatively few generations.     

SYSTEMATICS OF SOUTH AMERICAN TRIPSACUM (GRAMINEAE)

The genus Tripsacum is widely distributed between 42°N and 24°S latitude. In South America, the genus extends around the Amazon and Orinoco basin, and from the Caribbean coast south to Brazil and Paraguay. The most common South American taxon is T. dactyloides (L.) L. var. meridonale de Wet and Timothy (2n = 36), which differs from North American representatives of the species in having subdigitate recemes usually appressed with the apical male sections typically curved. Closely related to T. dactyloides, but usually occupying more seasonally moist and dry habitats, is T. australe Cutler and Anderson. This species is typically robust with the basal leaf sheaths tomentose, and the much elongated culms becoming decumbent in older plants. Smaller plants, with essentially erect culms and leaf sheaths on the culms hirsute rather than tomentose, are recognized as T. australe var. hirsutum de Wet and Timothy. The two varieties of T. australe are both diploid (2n = 36) and they cross to produce fertile hybrids. They also cross with T. dactyloides var. meridonale (2n = 36), but these hybrids are partially sterile. Tripsacum cundinamarce de Wet and Timothy (2n = 36) is a robust species with glaucus leaves. It resembles robust specimens of T. dactyloides in having glabrous leaf sheaths, but can always be recognized by inflorescences that are composed of racemes arranged along a several-noded primary axis. This species is confined to moist habitats, while T. dactyloides occupies a range of habitats in South America. Tripsacum peruvianum de Wet and Timothy is a gametophytic apomict with 2n = 72, 90 or 108 chromosomes. It is characterized by an erect growth habit and strongly hirsute leaf sheaths. The cultivated Guatemala grass, T. andersonii Gray, occurs spontaneously in the mountains of Venezuela, Colombia, and Peru. This sexually sterile species is characterized by 2n = 64, and may combine 54 Tripsacum and 10 Zea chromosomes in its genome. Electrophoretic patterns of seed storage proteins confirm the validity of recognizing T. cundinamarce as distinct from T. dactyloides, and T. peruvianum as distinct from T. australe.     

MORPHOLOGY OF SOME MAIZE-TRIPSACUM HYBRIDS

Diploid (2n = 20) and tetraploid (2n = 40) Zea mays L. were crossed with diploid (2n = 36) and tetraploid (2n = 72) Tripsacum dactyloides (L.) L. to produce a series of hybrids combining different numbers of haploid genomes from each parent. Eight hybrid groups and three parental groups were studied morphologically. Twenty-nine quantitative characters were recorded for each sample. Data were analyzed by univariate analysis of variance, multivariate analysis of variance, and discriminant function analysis, in an attempt to evaluate hybrid differences objectively and determine which morphological characters contribute statistically to group separation. The overall MANOVA F test was significant, establishing the presence of real differences between the hybrids; discriminant function analysis indicated that the percent of paired pistillate spikelets/cupule in the lateral inflorescence was the main variable which differentiated hybrids. Duncan's Multiple Range Tests for significant differences between means were applied to five variables contributing maximally to group discrimination, using the appropriate univariate ANOVAs. Pronounced maize-like attributes of backcross hybrids, as compared with corresponding F₁'s possessing similar genome constitutions, gave possible evidence of gene transfer between Zea mays and Tripsacum during backcrossing to 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.     

MORPHOLOGY OF TEOSINTOID AND TRIPSACOID MAIZE (ZEA MAYS L.)

Modern races of maize (Zea mays L.) are characterized by indurated glume and rachis tissues. The archaeological record, as well as experimental studies indicate that in North America this induration is associated with hybridization between domesticated maize and its closest wild relative Z. mays subsp. mexicana (Schrad.) Iltis (teosinte). Similar induration can also be introduced into maize through introgression from Tripsacum. North and South American indurated races of maize are not all closely allied morphologically. They evolved independently under domestication. Teosinte is absent from South America, but Tripsacum is widely sympatric with maize from about 42 N to 42 S latitude. For these reasons it has been postulated that induration in South American races may be the result of Tripsacum introgression. However, barriers restricting gene exchange between Zea and Tripsacum are difficult to overcome in nature. It is maintained that indurated South American races of maize were derived from indurated Mexican races, and that the presence or absence of such induration is due to different degrees of expression by intermediate alleles of the tunicate locus.     

A cross between two maize relatives:Tripsacum dactyloides andZea diploperennis (Poaceae)

Crosses betweenTripsacum dactyloides and teosinte (Zea diploperennis) using standard pollination technique have been successfully attempted and six highly fertile hybrid plants obtained. Previous research had shown other teosintes to be cross-incompatible with Tripsacum and maize to be crossable but highly intersterile withTripsacum. Some investigators believe thatTripsacum played a prominent role in the origin of maize; theTripsacum-diploperennis hybrid provides evidence to support that idea. Ears produced by the hybrid have paired kernel rows, a distinctive characteristic of the oldest archaeological maize that none of the wild relatives have. This unique hybrid is described and discussed in terms of its possible role in the origin and evolution of 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.     

Comparative isozyme polymorphisms of north American eastern gamagrass,Tripsacum dactyloides var.dactyloides and maize,Zea mays L.

Random samples, consisting of at least 100 individual seedlings, were taken from the diploid (2n=2x=36) eastern gamagrass (Tripsacum dactyloides var.dactyloides) and assayed to determine which of 12 enzyme marker loci and isozyme systems would be most informative in providing satisfactory resolution of both maize andTripsacum isozyme systems. For comparison, eight maize inbreds were included in the study to aid evaluation and comparison of the various isozyme systems. In addition, evaluations were conducted to identify if the identified optimum isozyme system could be used to detectTripsacum introgression in maize following a maize ×Tripsacum backcrossing scheme. Using the established isozyme techniques for maize (Zea mays L.), theAdh, Pgd, Cat, Est, B-Glu, Got, Idh, Tpi isozyme systems detected no polymorphism among theTripsacum individuals assayed. TheEst andB-Glu systems forTripsacum were unscorable due to poor staining and resolution. TheAcp, Mdh, Pgm, andPhi isozyme systems were found to be satisfactory markers for differentiating between eastern gamagrass individuals as well as detectingTripsacum introgression in maize. The availability of useful isozyme systems which can simultaneously provide significant isozyme resolution of maize,Tripsacum and maize-Tripsacum backcross hybrids, on a single gel system, will be useful for the detection of marker assistedTripsacum introgression into maize. In addition, the identification of a set of variable biochemical markers should also assist breeding, selection and genetic manipulations in eastern gamagrass.The use of company names in this publication does not imply endorsement by the USDA-ARS, or the product names of criticism of similar ones not mentioned. All programs and services of the U.S. Department of Agriculture are offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, age, marital status, or handicap.     

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.     

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.     

Distribution of sequences related to the Bg transposable element of maize in Zea and related genera

Thirty-four accessions from Zea and 10 accessions from related genera were assayed for the presence of Bg, a transposable element originally found in maize (Zea mays ssp. mays). Bg-like sequences, identified as hybridizing bands on Southern blots, were visualized in all Zea accessions and were present in approximately equal numbers in teosinte and maize. With the exception of Tripsacum dactyloides, all accessions from related genera failed to hybridize with the Bg probes, even at reduced stringency. A comparison of the restriction patterns of related inbred lines revealed numerous common hybridizing fragments. An index of molecular similarity (MS) was used to determine the degree of similarity between pairs of inbred lines. Computed MS values endorse an inbred relationship and are in good agreement with published results of cluster analysis on these inbred lines.     

SYSTEMATICS OF TRIPSACUM SECTION FASCICULATA (GRAMINEAE)

Tripsacum section Fasciculata is characterized by staminate spikelet pairs in which one spikelet is sessile and the other is supported by a long and slender pedicel. In section Tripsacum both spikelets of a staminate pair are sessile, or one is supported by a short and stout pedicel. Section Fasciculata includes five closely allied species. Tripsacum lanceolatum Ruprecht ex Fournier (2n = 72) extends from Durango in Mexico to the Huachuca mountains of southern Arizona. It resembles T. jalapense de Wet & Brink spec. nov. (2n = 72) from Guatemala in having terminal inflorescences with 3–10 racemes, but they differ in growth habit and are genetically isolated. Terminal inflorescences of the remaining three species have 15–50 racemes. Tripsacum laxum Nash (2n = 36) from the eastern escarpment of the Central Mexican Plateau is the only species of the group with essentially glabrous basal leaf-sheaths. It resembles the more widely distributed T. maizar Hernandez & Randolph (2n = 36, 72) in respect to inflorescence morphology, but is genetically isolated from this species. The widely distributed T. pilosum Scribner & Merrill (2n = 72) was divided into var. pilosum and var. guatemalense de Wet & Brink var. nov.