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.     

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.     

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.     

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.     

SYSTEMATICS OF TRIPSACUM DACTYLOIDES (GRAMINEAE)

Tripsacum dactyloides (L.) L. extends across the range of this genus from about 42°N to 24°S latitude in the New World. It is recognized to include T. dactyloides var. dactyloides (North America), var. meridonale deWet et Timothy (South America), var. hispidum (Hitchc.) deWet et Harlan comb. nov. (Mesoamerica) and var. mexicanum deWet et Harlan var. nov. (Mesoamericana). The genus is divided into sections Tripsacum and Fasciculatum. Mesoamerican members of section Tripsacum are classified into T. bravum Gray, T. dactyloides (L.) L., T. intermedium deWet et Harlan spec, nov., T. latifolium Hitchc., T. manisuroides deWet et Harlan spec. nov. and T. zopilotense Hern,*** et Randolph. A key to the species of section Tripsacum is presented.     

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.     

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.     

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.     

Transposon signatures: species-specific molecular markers that utilize a class of multiple-copy nuclear DNA

Transposable elements are mobile sequences found in nuclear genomes and can potentially serve as molecular markers in various phylogenetic and population genetic investigations. A PCR-based method that utilizes restriction site variation of element copies within a genome is developed. These patterns of site variation, referred to as transposon signatures, are useful in differentiating between closely related groups. Signature data using the magellan retrotransposon, for example, is useful in examining relationships within the genus Zea and Tripsacum. This method allows transposable elements, or even other multiple-copy nuclear DNA sequences, to be generally utilized as molecular markers in discriminating between other closely related species and subspecies.     

Zeins as indicators of maize-Tripsacum introgression

A survey of zeins in tripsacoid and non-tripsacoid races of maize from Mesoamerica and from South America, annual teosinte, perennial species of Zea and species of Tripsacum revealed at least 33 zein proteins as determined by isoelectric focusing. Zea and Tripsacum and generally also species within these genera are characterized by distinct combinations of zein proteins. Maize is extensively heterogenous, and spans the complete spectrum of zeins present in wild Zea taxa. A comparison of zein proteins failed to distinguish between introgression of maize with Tripsacum or teosinte. The ease with which maize crosses naturally with wild Zea taxa, and the rarity of hybrids with Tripsacum essentially rule out natural Tripsacum introgression as a mode of racial evolution in maize.     

CYTOLOGICAL EVIDENCE OF NATURAL INTERTRIBAL HYBRIDIZATION OF TRIPSACUM AND MANISURIS

A plant with the morphological aspect of Tripsacum zopilotense, collected at Acahuizotla, State of Guerrero, showed a series of irregularities during meiosis. In diakinesis, 18 bivalents and 9 univalents were observed. In the following stages the univalents stayed separated, like a genome with very different chromosomes. The study of irregularities during meiosis suggests that the plant could be a hybrid which originated from a fusion of an unreduced gamete of T. zopilotense having 36 chromosomes with a reduced gamete of Manisuris cylindrica having 9 chromosomes.     

Molecular analysis of crosses between Tripsacum dactyloides and Zea diploperennis (Poaceae)

 DNA fingerprinting verified hybrid plants obtained by crossing Eastern gamagrass, Tripsacum dactyloides L., and perennial teosinte, Zea diploperennis Iltis, Doebley & R. Guzmán. Pistillate inflorescences on these hybrids exhibit characteristics intermediate to the key morphological traits that differentiate domesticated maize from its wild relatives: (1) a pair of female spikelets in each cupule; (2) exposed kernels not completely covered by the cupule and outer glumes; (3) a rigid, non-shattering rachis; (4) a polystichous ear. RFLP analysis was employed to investigate the possibility that traits of domesticated maize were derived from hybridization between perennial teosinte and Tripsacum. Southern blots of restriction digested genomic DNA of parent plants, F₁, and F₂ progeny from two different crosses were probed with RFLP markers specifically associated with changes in pistillate inflorescence architecture that signal maize domestication. Pairwise analysis of restriction patterns showed traits considered missing links in the origin of maize correlate with alleles derived from Tripsacum, and the same alleles are stably inherited in second generation progeny from crosses between Tripsacum and perennial teosinte. Received: 11 October 1996/Accepted:8 November 1996     

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.     

Heterochronic features of the female germline among several sexual diploid <Emphasis Type="Italic">Tripsacum</Emphasis> L. (Andropogoneae,Poaceae)

One element of gametophytic apomixis is unreduced embryo sac (ES) formation, which often occurs precociously displacing or replacing meiosis and causing apospory or diplospory, respectively. This study evaluated a premise that apomixis may evolve in hybridogenous plants that contain duplicate sets of allelically divergent ovule development heterochrony genes. The duplicate sets of genes would belong to duplicate genomic regions that are recombinationally isolated from each other (no gene flow) by allopolyploidy or paleopolyploidy, and this isolation would genetically stabilize apomixis. For apomixis to evolve, the ancestral donors of the duplicate regions must have differed from each other in timing of megasporogenesis, ES formation and embryony such that epigenetic misexpressions, or competitions in expression, of the duplicate heterochrony genes in hybridogenous derivatives would cause apomixis. Herein, we report substantial heterochrony in onset timing of germline stages among several sexual diploid Tripsacum genotypes, which may have been progenitors of apomictic polyploid Tripsacum. Tripsacum floridanum and Tripsacum zopilotense genotypes entered meiosis early. The former advanced rapidly through ES formation, but the latter entered a lengthy lag phase prior to ES formation. In two Tripsacum dactyloides var. dactyloides genotypes, meiosis occurred late and was followed by a distinct lag phase prior to ES formation. Likewise, the T. dactyloides var. meridonale genotype entered meiosis late, but the lag phase was brief. These differences appear to reflect allelic diversity at loci responsible for onset timing of different germline development stages within and across species and possibly across the recombinationally isolated duplicate chromosome regions in the Tripsacum paleopolyploid haplome (x = 18). Unique combinations of divergent alleles in hybridogenous plants coupled with polyploidy induced gene misexpressions may be required for apomixis to evolve. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

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.     

A comparison of apomictic reproduction in eastern gamagrass (Tripsacum dactyloides (L.) L.) and maize-Tripsacum hybrids

The expression of gene(s) governing apomictic reproduction inTripsacum provides the best foundation for comparing the effectiveness of apomictic reproduction in a series of maize-Tripsacum hybrids. Several 38-chromosome, apomictic maize-Tripsacum hybrids are available which possess the gene(s) conferring apomictic reproduction fromTripsacum. Without a base line for comparison, studies directed towards discerning the successful transfer or effectiveness of gene expression in a maize background are hampered. The objectives of this study are to compare the reproductive features found in apomicticTripsacum with those in apomictic maize-Tripsacum hybrids. In addition, this study determined the feasibility of utilizing these maize-Tripsacum hybrid materials to continue an attempt to transfer the genes into a pure maize background. The frequency and occurrence of five unique reproductive features found in apomictic accessions ofTripsacum dactyloides were compared to the reproductive behaviours exhibited in the maize-Tripsacum hybrids. Results indicate the genes controlling apomixis in tetraploidTripsacum are fully functional in maize-Tripsacum hybrids with diploid and triploid maize constitutions. The ability of theTripsacum apomictic genes to retain full expression provides evidence to continue their transfer to a diploid or tetraploid maize background.The use of company names in this publication does not imply endorsement by the USDA-ARS, or the product names or 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.     

Dosage effects in the endosperm of diplosporous apomictic Tripsacum (Poaceae)

 Imprinting in the endosperm of angiosperms, a phenomena by which expression of alleles differs depending on whether they originate from the male or female parent, has been shown to explain most failure of interploidy or interspecific crosses in plants. Because of imprinting, seeds develop normally only if a specific dosage is represented in the endosperm, with the relative contributions of genomes in the ratio of two maternal doses to one paternal dose (2m:1p). In Tripsacum, a wild relative of maize, all polyploids reproduce through the diplosporous type of apomixis. Diplospory results from meiotic failure in megasporocytes that develop into eight-nucleate unreduced female gametophytes. The male gametophytes remain unaffected. Flow cytometry was used to determine ploidy levels in the endosperm of both apomictic and sexual Tripsacum accessions. In both cases, fertilization appeared to involve only one sperm nucleus. Therefore, endosperm of apomictic Tripsacum develops normally even though the ratio of genomic contributions deviates from the normal 2m:1p ratio. Ratios of 2:1, 4:1, 4:2, 8:1 and 8:2 were observed, depending on both the ploidy level of the parents and the mode of reproduction. Thus, specific dosage effects are seemingly not required for endosperm development in Tripsacum. These findings suggest that evolution of diplosporous apomixis might have been restricted to species with few or no imprinting requirements, and the findings have strong implications regarding the transfer of apomixis to sexually reproducing crops. Received: 17 February 1997 / Revision accepted: 7 July 1997     

En/Spm-like transposons in Poaceae species: Transposase sequence variability and chromosomal distribution

Belonging to Class II of transposable elements, En/Spm transposons are widespread in a variety of distantly related plant species. Here, we report on the sequence conservation of the transposase region from sequence analyses of En/Spm-like transposons from Poaceae species, namely Zingeria biebersteiniana, Zingeria trichopoda, Triticum monococcum, Triticum urartu, Hordeum spontaneum, and Aegilops speltoides. The transposase region of En/Spm-like transposons was cloned, sequenced, and compared with equivalent regions of Oryza and Arabidopsis from the gene bank database. Southern blot analysis indicated that the En/Spm transposon was present in low (Hordeum spontaneum, Triticum monococcum, Triticum urartu) through medium (Zingeria bieberstiana, Zingeria trichopoda) to relatively high (Aegilops speltoides) copy numbers in Poaceae species. A cytogenetic analysis of the chromosomal distribution of En/Spm transposons revealed the concurence of the chromosomal localization of the En/Spm clusters with mobile clusters of rDNA. An analysis of En/Spm-like transposase amino acid sequences was carried out to investigate sequence divergence between 5 genera — Triticum, Aegilops, Zingeria, Oryza and Arabidopsis. A distance matrix was generated; apparently, En/Spm-like transposase sequences shared the highest sequence homology intra-generically and, as expected, these sequences were significantly diverged from those of O. sativa and A. thaliana. A sequence comparison of En/Spm-like transposase coding regions defined that the intra-genomic complex of En/Spm-like transposons could be viewed as relatively independent, vertically transmitted, and permanently active systems inside higher plant genomes. The sequence data from this article was deposited in the EMBL/GenBank Data Libraries under the accession nos. AY707995-AY707996-AY707997-AY707998-AY707999-AY708000-AY708001-AY708002-AY708003-AY708004-AY708005-AY708005-AY265312.