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(American Journal of Botany. 1998;85:1237-1242.)
© 1998 Botanical Society of America, Inc.

Cytological and molecular evaluation of the reproductive behavior of Tripsacum andersonii and a female fertile derivative (Poaceae)1

C. L. Dewald2,a and B. K. Kindigera

a USDA, ARS, Southern Plains Range Research Station, Woodward, Oklahoma 73801


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Research was conducted to characterize the reproductive behavior of the highly sterile Tripsacum andersonii Gray and its viable progeny through breeding, cytological, and molecular studies. Four progeny were obtained from open-pollinated seeds of clones (M-34445, M-34450 and M-34455) of T. andersonii maintained at the USDA-ARS National Germplasm Repository, Miami, Florida. One of the progeny had 64 chromosomes, which is typical of T. andersonii, and probably resulted from apomictic reproduction. Karyotypes of the other three progeny indicated a tetraploid Tripsacum genomic constitution (2n = 4x = 72) plus a haploid set of Zea (1n = 1x = 10) chromosomes. Two of these progeny were completely sterile, whereas one (95-51) produced ~5% seed set when crossed with diploid (2n = 36) T. dactyloides (L.)L. The partially fertile 95-51 produced four progeny, one with 2n = 72 (elimination of 10 Zea chromosomes), two with 2n = 82 (apomictic reproduction) and one with 2n = 100 (sexual polyploidization). Polymerase Chain Reaction - Random Amplified Polymerase DNA analysis verified that T. andersonii accessions from seven countries were genetically uniform, and that its progeny were derived through apomixis and sexual polyploidization. This analysis also confirmed that chromosome elimination, apomixis, and sexual polyploidization reproductive behaviors occur in the T. andersonii derivative 95-51.

Key Words: apomixis • Poaceae • sexual polyploidization • Tripsacum • Zea


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Tripsacum andersonii Gray, guatemala grass, is a vigorous perennial species vegetatively propagated as a cultivated forage in tropical Mesoamerica, South America, and the West Indies (Hernandez, 1970; Randolph, 1970). Levings, Timothy, and Hu (1976) first reported the chromosome numbers of T. andersonii to be 2n = 64. Subsequent morphological (deWet et al., 1983) and molecular (Talbert et al., 1990; Larson and Doebley, 1994) evidence supports the hypothesis that T. andersonii is an intergeneric hybrid between Tripsacum and Zea with 54 Tripsacum chromosomes (2n = 3x = 54) and a haploid set (n = 10) of Zea chromosomes.

Morphological uniformity across a wide distribution range suggests this unique species is of single hybrid origin and of ancient derivation (deWet et al., 1983). The origin of this unique natural intergeneric hybrid is of interest from an evolutionary prospective and also because of its potential relationship to the important grain crop maize (Zea mays L. ssp. mays). In its genus, the anomalous T. andersonii most nearly resembles T. latifolium Hitchc., particularly in spikelet morphology and stoloniferous growth habit (deWet et al., 1983). Molecular evidence strongly supports Zea luxurians Iltis as the Zea parent involved in the cross and a Tripsacum species as the female parent (Talbert et al., 1990). Larson and Doebley (1994) used combined rDNA and cpDNA data to indicate T. laxum Nash or a related species as the Tripsacum maternal parent of T. andersonii).

T. andersonii is completely male sterile, but can set an occasional seed; deWet, Harlan, and Randrianasolo (1978) pollinated some 21 000 female florets and recovered only eight viable progeny from T. andersonii crossed with various maize races. The resulting hybrids were 2n = 74, indicating fertilization of unreduced egg cells with n = 10 chromosome pollen. Fertility of the 2n = 74 chromosome hybrids has not been reported. This study was conducted to explore reproductive mechanism(s) of the highly sterile T. andersonii and its subsequent progeny.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Spikelets (female fruitcases) collected from 20 open-pollinated accessions of T. andersonii were obtained from the USDA-ARS National Germplasm Repository (NGR), Miami, Florida, in 1994. Seed (caryopses) were hand dissected from fertile spikelets at the Southern Plains Range Research Station, Woodward, Oklahoma, and subjected to germination procedures as previously described (Dewald and Kindiger, 1994). Four seedlings resulted and were field transplanted to observation nurseries during June 1995 where they remained until mid-October at which time they were excavated, planted in 19-L containers, and maintained in a greenhouse at 25° ± 5°C.

Only one of the T. andersonii progeny, 95-51, produced inflorescences during the 1995 winter breeding season. This individual was cross-pollinated by diploid, 2n = 36, T. dactyloides using techniques as previously described (Dewald et al., 1987). During 1996, T. andersonii progeny 95-51, 95-52, and 95-53 were pollinated with several Tripsacum species, Zea mays spp. mays, and Zea luxurians for female fertility determinations.

Rootstock of eight accessions of T. andersonii from seven different countries were obtained from NGR for cytological and molecular evaluation. The principal plant materials evaluated in this study are listed in Table 1.


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  		Table 1. Designation, collection site or hybrid origin, and chromosome numbers of T. andersonii and derivatives evaluated in this study.

 
Mitotic root tip chromosome counts were used to verify the chromosome numbers in the eight T. andersonii accessions and to obtain chromosome numbers of T. andersonii OP progeny and progeny of 95-51 x diploid T. dactyloides. Mitotic chromosome counts used the procedure of Sallee and Kimber (1983). Root tips were taken from actively growing plants obtained under greenhouse conditions.

Female fertility estimates of derivatives of 95-51 were determined from 1996 crosses by harvesting seed heads at maturity and extracting caryopses by hand. Percentage seed set was determined by dividing the number of spikelets containing a caryopses by the total number of spikelets harvested and then multiplying by 100. In this study, seed set determinations are from greenhouse-controlled cross-pollinations using an excess of freshly collected pollen.

Polymerase Chain Reaction (PCR)-Random Amplified Polymerase (RAPD) DNA analysis was performed on DNA samples obtained from lyophilized leaf tissue harvested from greenhouse-grown plants. DNA extractions were made following the method of Saghai-Maroof et al. (1984). Individuals were analyzed by RAPD techniques using the protocol developed by Williams et al. (1990). Reactions were conducted on ~5 ng plant DNA. Ampli-Taq DNA polymerase and 10 X reaction buffer were purchased from Perkin Elmer (Branchburg, New Jersey).

Primers used in the RAPD study were decamer oligonucleotides available as kits C and D, purchased from Operon Technologies (Alameda, California). PCR amplifications and band separation and visualization were as described by Kindiger and Dewald (1996). Faint bands were disregarded and only bright reproducible bands were considered during evaluations. Additional reamplifications were carried out on individuals that exhibited questionable or variant profiles to verify their reproducibility.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Cytological studies
Chromosome counts on eight T. andersonii accessions originally collected from seven countries in Latin America were verified to be 2n = 64 (Table 1). Chromosomes of both Tripsacum and Zea were identifiable by morphological characters from root tip cells at metaphase verifying earlier cytological studies by deWet et al. (1983).

Five caryopses were found in 1253 spikelets (fruitcases) from open-pollinated T. andersonii clones (M 34445, M 34450, and M 34455) maintained at the USDA-ARS National Germplasm Repository, Miami, Florida. Four seedlings were obtained and one, 4713, a progeny of M 34450, had a chromosome number of 64 (Fig. 1), presumably as a result of apomictic reproduction. The other three T. andersonii progeny (95-51, 95-52, and 95-53) had 82 chromosomes (Fig. 2) and karyotypes indicated they possessed a tetraploid (2n = 4x = 72) Tripsacum genomic constitution plus a haploid (1n = 1x = 10) set of Zea chromosomes.



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  		Figs. 1–3. Somatic metaphase chromosomes obtained from root tips. Scale bar = 1 um. 1. Sixty-four chromosomes of T. andersonii seedling 4713 showing chromosome Tr 16 of Tripsacum and chromosome 6 of Zea identifiable by their microsatalites (arrows). 2. Eighty-two chromosomes of T. andersonii progeny 95-51 derived by sexual polyploidization (2n + n mating) with larger chromosomes generally representing Zea and smaller chromosomes representing Tripsacum . 3. Seventy-two chromosomes of 95-51 progeny 97-4 with Zea chromosomes missing that are present in T. andersonii and derivitive 95-51.

 
One of the T. andersonii progeny, 95-51, is partially female fertile and produced four progeny from crosses with a diploid T. dactyloides pollinator in 1995. Two progeny (97-1 and 97-2) possessed 82 chromosomes and were likely generated by apomixis, one (97-3) had 100 chromosomes, probably due to a 2n + n mating event, and one (97-4) carried 72 chromosomes. A karyotype of the 72 chromosome individual (97-4) indicated a loss of Zea chromosomes present in its maternal parent 95-51 (Fig. 3).

Seed set determination
T. andersonii progeny 95-52 and 95-53 failed to set seed during the 1996 breeding season and are considered completely sterile. Progeny 95-51 is male sterile, but partially female fertile and produced seed ( seed set = 3.9%) and seedlings when pollinated by five Tripsacum spp., Zea mays spp. mays, and Zea luxurians during 1996 (Table 2).


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  		Table 2. Number of spikelets and caryopsis, mean percentage seed set, number total seedlings and twin seedlings obtained from T. andersonii progeny 95-51 crossed with various pollen parents.

 
Percentage seed set of T. andersonii progeny 95-51 was higher when pollinated by Tripsacum spp. compared to Zea species (8.2 vs. 1.3%). Increased seed set was obtained with pollen from T. maizar Hern. and Randolph (17.8%) compared to the other Tripsacum spp. ( = 7.57). Pollen from Zea luxurians stimulated increased seed set (3.4%) compared to pollen from Zea mays spp. mays (0.2%), indicating increased compatibility between T. andersonii progeny 95-51 and Zea luxurians.

Approximately 7% of the seedlings produced from T. andersonii progeny 95-51 during 1996 were twin seedlings resulting from polyembryony. Polyembryony has been documented in apomictic Tripsacum polyploids (Farquharson, 1955) and in apomictic maize x Tripsacum hybrids (Fokina, 1976; Kindiger, Sokolov, and Dewald, 1996).

Molecular studies
RAPD analysis using 40 decamer primers, OPC 1-20 and OPD 1-20, did not detect any variation in banding profiles among the eight T. andersonii accessions from diverse locations, indicating the species maintains a uniform genetic constitution (Table 3).


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  		Table 3. Decamer primers that gave variable band patterns, loss or gain of bands, in specific plant material comparisons.

 
Banding patterns from 13 decamer primers indicated that the three 82-chromosome progeny (95-51, 95-52, and 95-53) gained bands compared to their maternal parent, T. andersonii, as a result of 2n + n matings (Fig. 4). Several decamer primers produced different band patterns between the three 82-chromosome progeny. This is to be expected since they are products of different 2n + n mating events.



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  		Figs. 4–5. 4. A RAPD-PCR profile of molecular mass marker 0X174 RF DNA/Hae (lane 1); six T. andersonii accessions (lanes 2-7); three T. andersonii progeny (lanes 8, 9, and 10); and four progeny of 95-51 (lanes 11-14) amplified with decamer primer OPC-15. Arrows A1 and A2 on lane 8 (95-51, 2n = 82) show bands not present in T. andersonii accessions, indicating a 2n + n mating. Arrow B on lane 13 (97-3, 2n = 100) depicts a band not present in its maternal parent 95-51 nor T. andersonii accessions, again indicating a 2n + n mating with its subsequent paternal contribution. 5. A partial RAPD-PCR profile of molecular mass marker 0X174 RF DNA/Hae (lane 1); two T. andersonii accessions M 34453 and M 34455 (lanes 2 and 3); T. andersonii progeny 95-51, 95-52, and 95-53 (lanes 4, 5, and 6, respectively); and progeny of 95-51 (lanes 7-10) amplified with decamer primer OPD-8. Arrow A in lane 4 (95-51) shows a band in common with the T. andersonii accessions (lanes 2 and 3) that is missing in its 72 chromosome progeny (lane 10), presumably as a result of Zea chromosome elimination. The B arrows in lane 4 depict two new bands in 95-51 that are not present in T. andersonii and are the result of a 2n + n mating event.

 
Progeny with 82 chromosomes (97-1 and 97-2) derived from the partially female-fertile parent, 95-51, gave banding patterns identical to 95-51 and to each other, indicating apomictic reproduction capabilities in 95-51 (Fig. 5). Several decamer primers gave one or more additional bands in the 100-chromosome progeny (97-3) compared to the maternal 95-51 parent, indicating sexual polyploidization (2n + n) reproductive capabilities. The 72-chromosome individual (97-4), which presumably lost its Zea genome, lost bands present in the parent 95-51 (Fig. 5). The absence of new bands indicated the lack of a paternal genomic contribution.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
T. andersonii, an intergeneric Tripsacum x Zea hybrid, is sexually sterile due to genomic imbalance (deWet, Harlan, and Randrianasolo, 1978), but occasionally produces a few seed. Five caryopses were found in open-pollinated spikelets (fruitcases) of T. andersonii, and four viable seedlings were obtained. Chromosome counts and RAPD analysis indicate that one of the seedlings (2n = 64) was a result of apomictic reproduction and that the three additional seedlings (2n = 82) likely resulted from fertilization of unreduced egg cells (2n + n matings). Apomictic reproduction in Tripsacum polyploids and maize x Tripsacum hybrids is common, being characterized as diplosporus pseudogamy of the Antennaria type (Brown and Emery, 1958; Burson et al., 1990; Sherman et al., 1991; LeBlanc et al., 1995). Sexual polyploidization (2n + n matings) represents a reproductive alternative in Tripsacum polyploids as well as in apomictic maize x Tripsacum hybrids (Kindiger and Dewald, 1994; Kindiger, Sokolov, and Dewald, 1996).

Among four individuals obtained in 1995 from T. andersonii progeny 95-51, two 82-chromosome derivatives (97-1 and 97-2) are likely products of apomixis, the 100-chromosome derivative (97-3) a result of sexual polyploidization (2n + n mating), and the 72-chromosome individual (97-4) resulted from an apparent elimination of the Zea group present in its maternal parent 95-51. PCR-RAPD analysis failed to detect any bands not present in the maternal parent of 97-4, indicating fertilization of the egg by the pollen source did not occur. Embryological and molecular studies have identified the infrequent occurrence of a partial meiotic first division restitution (FDR) event in Tripsacum polyploids, which defines the Taraxacum form of diplosporus apomixis (LeBlanc et al., 1995; Kindiger and Dewald, 1996). During this event, the opportunity for a low level of chromosome pairing and recombination may occur (Gustaffson, 1946). Perhaps partial pairing of the four Tripsacum genomes with elimination of the Zea chromosome from the lower dyad of 95-51 may account for the 72-chromosome individual.

The upper leaf sheaths of T. andersonii are glabrous, whereas its 82-chromosome progeny derived from open pollinations have densely pubescent leaf sheaths. This seems to implicate a Tripsacum spp. with pubescent leaf sheaths, i.e., T. maizar or T. pilosum, as the unknown pollen parents of the 82-chromosome progeny. Also, because the hairy leaf sheath trait is recessive in Tripsacum species hybrids (Galinat and Randolph, 1968) we would not expect it to appear in the progeny of the glabrous sheathed parent unless the gene(s) for this trait were already present, but suppressed in T. andersonii. This implies a hybrid origin, possibly an allotriploid derivation, of the Tripsacum genome components in T. andersonii with at least one parent having the hairy leaf sheath trait.

Of the 16 recognized species of Tripsacum, the stoloniferous growth habit is exclusive to T. andersonii and T. latifolium only. This suggests a relationship such as a common ancestor if not the direct involvement of T. latifolium in the parentage of T. andersonii as proposed by deWet et al. (1983). Although similarities in chloroplast DNA indicate T. laxum as the female parent of T. andersonii (Larson and Doebly, 1994), this does not exclude the involvement of T. latifolium in the parentage. An allotriploid hybrid between diploid T. laxum and tetraploid T. latifolium as the pollen parent followed by sexual polyploidization with the Zea parent could produce a 64-chromosome product with cpDNA similar to T. laxum and morphological characteristics similar to T. latifolium. Fertile triploid hybrids have been obtained from intermatings of diploid x tetraploid T. dactyloides, presumably as a result of apomictic reproduction (Dunfield, 1991; Dewald, Taliaferro, and Dunfield, 1992; Dewald and Kindiger, 1994). Fertile allotriploids have also been generated between diploid T. dactyloides and tetraploid T. maizar and T. zopilotense Hern. and Randolph (Dewald and Kindiger, 1994, unpublished data). Fertile triploids reproduced by apomixis, 2n + 0, and sexual polyploidization, 2n + n (Dewald and Kindiger, 1994; Kindiger and Dewald, 1994) and as such may have exerted major influences on Tripsacum evolution and speciation by providing an intermediate step for the creation of new and genetically unique germplasm in higher polyploids. The most direct route to the origin of T. andersonii appears to be through such a triploid bridge followed by sexual polyploidization with a Zea ancestor. All progeny of 95-51, with the exception of 97-4, appear identical in early vegetative morphology, indicating that its higher ploidy has a buffering effect against change. Derivative 97-4 has narrower leaves, a slower growth rate, and is more typical of Tripsacum than its predecessors and sibs. Derivative 97-4 and progeny of 95-51 produced in 1996 have not flowered at this time.

Considering the uniformity of accessions across a wide area of distribution in tropical Latin America, deWet et al. (1983) speculated that this anomalous species is of single hybrid origin of ancient derivation. Our PCR-RAPD analysis supports the hypothesis that T. andersonii accessions from seven countries are genetically uniform and probably represent a one-time intergeneric hybrid event.

The antiquity of T. andersonii is a matter of speculation, but its genetic uniformity across a wide area of distribution indicates an ancient origin as suggested by deWet et al. (1983). Relic genetic combinations that have been latent for centuries may be retrievable through horizontal gene flow as demonstrated in this study.


   FOOTNOTES
 
1 The authors thank Dr. R. J. Schnell, II, of the USDA-ARS National Clonal Germplasm Repository, Miami, Florida, for supplying plant materials used in this study; Verl Louthan and Gerald Murlin for their assistance in breeding studies; and Mary Ager for assisting in the PCR-RAPD analysis.

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.

Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. Back

2 Author for correspondence. Back


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Brown, W. V., and W. H. P. Emery.1958.Apomixis in the Gramineae: Panicoideae. American Journal of Botany 45: 253–262. [CrossRef][ISI]

Burson, B. L., P. W. Voigt, R. A. Sherman, and C. L. Dewald.1990.Apomixis and sexuality in eastern gamagrass. Crop Science 30: 86–89. [Abstract/Free Full Text]

Dewald, C. L., B. L. Burson, J. M. J. deWet, and J. R. Harlan.1987.Morphology, inheritance and evolutionary significance of sex reversal in Tripsacum dactyloides (Poaceae). American Journal of Botany 74: 1055–1059. [CrossRef][ISI]

———, and B. Kindiger.1994.Genetic transfer of gynomonoecy from diploid to triploid eastern gamagrass. Crop Science 34: 1259–1262. [Abstract/Free Full Text]

———, C. M. Taliaferro, and P. C. Dunfield.1992.Registration of fertile triploid germplasm of eastern gamagrass. Crop Science 32: 504.[Free Full Text]

deWet, J. M., G. B. Fletcher, K. W. Hilu, and J. R. Harlan.1983.Origin of Tripsacum andersonii (Gramineae). American Journal of Botany 70: 706–711. [CrossRef][ISI]

———, J. R. Harlan, and A. V. Randrianasolo.1978.Morphology of teosintoid and Tripsacoid maize (Zea mays L.). American Journal of Botany 65: 741–747. [CrossRef][ISI]

Dunfield, P. C.1991.Cytological and reproductive characteristics of diploid and tetraploid eastern gamagrass plants and their F1 hybrids. Ph.D. dissertation, Oklahoma State University, Stillwater, OK.

Farquharson, L. I.1955.Apomixis and polyembryony in Tripsacum dactyloides. American Journal of Botany 2: 737–743.

Fokina, E. S.1976.Frequency of polyembryony in the offsprings of maize-Tripsacum apomicts. In L. I. Laikova and H. B. Sukhareva [eds.], Apomixis and its significance in evolution and selection, 91–98. Trudy Biological Institute, Siberian Department of the Academy of Sciences of the USSR, Novosibirsk.

Galinat, W. C., and L. F. Randolph.1968.Recessiveness of hairy leaf sheaths in Tripsacum species hybrids. Maize Genetics Cooperative News Letter 42: 98–99.

Gustaffson, A.1946.Apomixis in the higher plants. 1. The mechanism of apomixis. Lunds University Arsskrift Foer Modersmalslararnas Forening 42: 1–67.

Hernandez, E.1970.Apuntes sobre la exploracion etnobtanica y su metodologia. College Postgraduate Escuel National Agricultural Publication, Chapingo, Mexico.

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———, and ———.1996.A system for genetic change in apomictic eastern gamagrass. Crop Science 36: 250–255. [Abstract/Free Full Text]

———, V. Sokolov, and C. Dewald.1996.A comparison of apomictic reproduction in eastern gamagrass (Tripsacum dactyloides (L.) L.) and maize-Tripsacum hybrids. Genetica 97: 103–110. [CrossRef][ISI]

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Levings, C. S., III, D. H. Timothy, and W. W. L. Hu.1976.Cytological characteristics and nuclear buoyant densities of corn, teosinte, Tripsacum and corn-Tripsacum hybrids. Crop Science 16: 63–66.

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