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Development and Morphogenesis |
2Department of Biology, University of Northern Iowa, Cedar Falls, Iowa 50614 USA; 3Department of Biological Sciences, Emporia State University, Emporia, Kansas 66801 USA
Received for publication March 8, 2002. Accepted for publication June 4, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: development • inflorescence • Mexico • organogenesis • Poaceae • teosinte • Zea
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Galinat, 1992
; Iltis, 2000
; Benz, 2001
), other hypotheses about the origin and evolution of maize are still being debated (Mangelsdorf, 1974
; Bird, 1979
; Wilkes, 1979
; Eubanks, 1995
, 1997
, 2001
; Provan et al., 1999
; MacNeish and Eubanks, 2000
; Bennetzen et al., 2001
). One unresolved issue in the controversy of maize evolution is the origin of the maize ear, which has sparked developmental investigation (Orr et al., 2001
). Previous studies of inflorescence development in the genera Tripsacum and Zea (teosintes and maize; Poaceae: Andropogoneae) have shown a developmental pattern common to the ears and tassels of teosinte (Sundberg and Orr, 1986
, 1990
; Sundberg, 1987
; Orr and Sundberg, 1994a
, b
), small-eared "primitive" maizes (Sundberg, LaFargue, and Orr, 1995
; Sundberg and Orr, 1996
), and eastern gamagrass, T. dactyloides (L.) L. (Orr et al., 2001
). Both femininity and masculinity were derived from a common developmental background, although key variations have been uncovered that differentiate teosinte inflorescences from maize inflorescences and Zea ears from Zea tassels (Orr and Sundberg, 1994a
). This understanding of inflorescence morphogenesis has been useful in articulating the sexual translocation theory (Iltis, 2000
), which argues that the maize ear was derived via a sexual translocation (i.e., feminization) of the central spike of the primary branch tassel (see Iltis, 2000
, Figs. 14–16).
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), although no detailed morphological race assignment has been made to date. The Toluca population is unique because it grows at 2500–2750 m, 100 m higher than the highest Chalco teosinte at Amecameca in the Valley of Mexico, and much higher than altitudes reported for the Balsas teosintes, which grow at 500–1950 m (Wilkes, 1979
; Sanchez Gonzalez et al., 1998
; Vibrans and Estrada Flores, 1998
).
Doebley (1990)
has said that current molecular evidence suggests a domestication of all races of maize from a Balsas teosinte, Z. mays subsp. Parviglumis Iltis Doebley, found in lowland south-central Mexico (Sanchez Gonzalez et al., 1998
). Our interest in the new high-altitude Toluca teosinte arose in part because Galinat (1992
, 1995
, 2001
) proposed that maize derives from two independent domestications: a lowland Balsas teosinte and an upland (high-altitude) Chalco teosinte, Z. mays subsp. mexicana (Schrader) Iltis. Interestingly, Eagles and Lothrop (1994)
have argued that highland Mexican maize probably was derived from a higher altitude population. Their studies of race formation in wild plants and of the distribution of cultivated maize led them to also hypothesize an ancient origin of high-altitude maize from a Chalco teosinte, Z. mays subsp. mexicana. This argument reilluminates Wilkes's (1979)
earlier thesis that maize originated in the Mexican highlands and, following its domestication there, was hybridized with Z. diploperennis Iltis, Doebley & Guzman. Thus, Wilkes postulated, all modern maize and Mexican annual teosintes evolved from the hybrids.
In this paper we describe ear and tassel ontogeny in the Toluca highland teosinte, and we compare it with corresponding patterns of development in Zea and Tripsacum inflorescences.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The teosinte plants grew in and adjacent to fields of a highland maize, Cónico. Cónico belongs to the high-elevation Mexican pyramidal complex of maizes that are farmed in the coldest regions of Mexican maize agriculture (Eagles and Lothrop, 1994
). Toluca teosinte is an excellent maize mimic and, thus, difficult to control in maize fields (Wilkes and Taba, 1993
). Vibrans and Estrada Flores (1998)
have placed Toluca in the race Chalco because its fruit cases are like those of the Valley of Mexico population (Wilkes and Taba, 1993
). The central plateau race occurs in the highlands of central Mexico (Eagles and Lothrop, 1994
) and in the eastern and southern part of the Valley of Mexico (Eagles and Lothrop, 1994
; Wilkes, 1977
).
Terminology
In Toluca staminate (tassel) inflorescences terminate the main axis (A1) and consist of a central tassel spike and one or more proximal tassel branches. Pistillate (ear) inflorescences (A2) arise as lateral clusters to the main axis. Each ear cluster is composed of a series of branchlets of higher order (A2, A3, etc.). Each branchlet is tipped by a pistillate inflorescence. This arrangement of Toluca inflorescences is similar to that described for other teosintes (Iltis, 2000
).
We used a codification for the teosinte branching pattern that was illustrated by Sundberg and Orr (1986)
, a scheme also employed for maize inflorescences (Orr, Haas, and Sundberg, 1997
), and was used to compare maize and teosinte organogenesis (cf. fig. 25 in Orr and Sundberg, 1994a
). This codification is comparable to that used by Cámara-Hernández and Gambino (1990)
and by Iltis (2000)
for teosinte and maize, except these authors labeled the terminal teosinte inflorescence A0 and the axillary branchlets A1, A2. The codification of the inflorescence branching pattern in Tripsacum, another member of the Tripsacinae, is similar to the axial designation used in this paper (Orr et al., 2001
).
Growth and examination
Toluca teosinte seeds were sown and placed into a Conviron CMP 3020 plant-growth chamber (Conviron, Winnipeg, Manitoba, Canada) under a day/night temperature environment of 24°/18°C and a long-day (LD) photoperiod of 15 h light/9 h dark. When the plants reached the fifth-ligule (V5) stage of leaf growth, the photoperiod in the growth chamber was changed to a short-day (SD) period of 8 h light/16 h dark to induce inflorescence formation and promote flowering (Orr and Sundberg, 1994a
). Photosynthetic photon flux density (PPFD) was maintained at 600–700 µmol·s–1·m–2 at the top of the leaf canopy during both LD and SD photoperiods. Photosynthetic photon flux density was measured twice a week using a Li-Cor 185 (Li-Cor, Lincoln, Nebraska, USA) equipped with a quantum sensor.
Toluca tassel inflorescences were harvested from the tip of the primary axis (A1), and ear inflorescences were harvested from the tip of the secondary (A2) or higher order (A3, A4) branchlets. Tassel and ear inflorescences in various stages of development were selected, and the outermost leaves removed. Remaining inflorescence leaves were removed, and spikelets were dissected using an Olympus SZH dissecting microscope (Olympus Optical, Tokyo, Japan). Inflorescences were fixed in formalin-acetic acid-alcohol fixative (Jensen, 1962
), dehydrated in a graded ethanol-acetone series (Liang and Tucker, 1989
), and stored in 100% acetone. Samples were critical-point dried in a Samdri-790 critical-point dryer (Tousimis Research, Rockville, Maryland, USA), mounted on a metal stub, and gold-palladium sputter-coated in a Technics Hummer VII sputter coater (Technics, Alexandria, Virginia, USA). Examination and photography of inflorescences was done using a Hitachi S-570 scanning electron microscope (Hitachi, Tokyo, Japan) at 20 kV. Inflorescences from more than 80 Toluca plants were observed. Approximately 50 ears and 30 tassels were examined at various stages of inflorescence development, and representative organogenic stages were photographed using Polaroid type 55 4 x 5 black and white film.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Sundberg, 1987
; Cámara-Hernández and Gambino, 1990
), Z. mexicana and Z. parviglumis (Sundberg and Orr, 1990
), and Z. perennis (Hitchc.) Reeves & Mangelsdorf (Orr and Sundberg, 1994a
). Each rank of spikelet pair primordia (Fig. 5) in the central spike region typically contained pairs of sessile and pedicellate spikelets (Figs. 6, 7). Each spikelet meristem produced in series first (outer) and second (inner) glumes (Figs. 7, 8), and first (outer) lemma (Fig. 8) and second (inner) lemma (Fig. 9). A lower floret formed in the axil of the first lemma (Fig. 8), and the terminal meristem of the original spikelet became the upper floret (Fig. 8). The upper and lower florets were separated by an upper palea (Fig. 9). The upper floret, developmentally in advance of the lower floret at this stage, produced three stamen initials (Fig. 9). The upper floret first formed two lateral stamen primordia, then one adaxial stamen primordium (Fig. 9). The ovary wall ring meristem (gynoecial ridge) was formed on the adaxial surface of the upper floret (Fig. 10) and was the final lateral primordium produced by the upper floret. The lower floret was developmentally behind the upper floret, but the pattern of organogenesis was similar to that in the upper floret (Fig. 10), except that the third stamen primordium and the ovary wall ring meristem were formed abaxial to the ovule of the lower floret (Fig. 10). The single ovary primordium formed in each floret was subsequently arrested (Fig. 11), leaving two staminate florets per spikelet.
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Ear
Initiation of the A2 inflorescence began when the vegetative meristem (Fig. 12) elongated into the transition stage (Fig. 13). Branch primordia (Figs. 13, 14) were the first observed appendages produced along the rachis of the A2 inflorescence, and almost all later branch primordia functioned as spikelet pair primordia. Each spikelet pair primordium gave rise to a pair of spikelets (Fig. 14). These functional spikelet pair primordia arose acropetally along the axis in one of three orthostichous patterns: distichous, tristichous, and tetrastichous. In the population of 51 plants we examined, 47 ear inflorescences were purely distichous (Fig. 14), three ears exhibited an intermediate phyllotaxy (two/three- or two/four-ranked) arrangement of spikelet pair primordia (Fig. 15), and one ear had four ranks of paired spikelets (Fig. 16). In two ears with an intermediate (two/three-ranked) phyllotaxy arrangement of paired spikelets, the third rank arose on the adaxial surface of the rachis at approximately 90° angle to the distichous arrangement of spikelet pair primordia (Fig. 15). Of the two ears with four proximal ranks of paired spikelets, only one ear formed four ranks of paired spikelets along the entire rachis (Fig. 16), whereas the other ear switched from a four-rank production of spikelets to a two-ranked production (Fig. 17). In female inflorescences that exhibited an intermediate phyllotaxy, the two-ranked condition was at the distal end of the rachis. Thus, both female and male Toluca inflorescences are able to developmentally switch orthostichy from polystichous (three or four ranks of spikelet pairs) to a more typical distichous pattern during the formation of spikelet pair primordia.
Each female spikelet pair primordium gave rise to a pair of spikelet primordia—one sessile and one pedicellate (Figs. 14, 15). The sessile spikelet primordium was usually larger and developmentally precocious compared with its pedicellate partner. Typically, each spikelet (pedicellate/sessile) primordium formed in acropetal succession outer (first) and inner (second) glumes (Fig. 14) and outer (first) and inner (second) lemmas (Fig. 18) before the developmental arrest and subsequent abortion of the pedicellate spikelet (Fig. 14). However, the developmental arrest of the pedicellate spikelet was observed in some cases (Fig. 19) prior to the initiation of lemmas or after the initiation of the lower floret (Fig. 20). Arrest of pedicellate spikelets was not observed in the polystichous (three- or four-ranked) zone. Organogenesis of pedicellate spikelets in the polystichous zone was similar to sessile spikelet development (Fig. 21).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Dorweiler and Doebley, 1997
; Orr, Haas, and Sundberg, 1997
), taxonomy (Bird, 1978
; Doebley and Iltis, 1980
; Iltis and Doebley, 1980
), archaeological morphology (Bird, 1994
; Benz, 2001
), and molecular biology (Doebley, 1990
; Buckler, Ippolito, and Holstford, 1997
; Provan et al., 1999
), among many other studies, has significantly contributed to the understanding of maize evolution. Elucidation of inflorescence development in the Poaceae, especially the genera Zea and Tripsacum, has illuminated organogenic patterns of maize and teosinte inflorescence evolution, which has helped clarify phylogenetic relationships (Sundberg and Orr, 1990
; Orr and Sundberg, 1994a
; Sundberg, LaFargue, and Orr, 1995
; Pizzolato and Sundberg, 1999
; Orr et al., 2001
). The primary goal of the research reported here was to determine inflorescence organogenesis in a newly described population of high-altitude teosinte growing in the Valley of Toluca, Mexico, an endeavor that may shed light on the hypothesis of a double domestication of maize (Galinat, 1992
; Eagles and Lothrop, 1994
).
Spikelets and florets
As predicted from our previous work, the development of spikelets and florets in Toluca teosinte inflorescences followed a pattern we observed in two annual teosintes (Z. mays subsp. parviglumis and subsp. mexicana) and two perennial teosintes (Z. perennis and Z. diploperennis) (cf. Fig. 25 in Orr and Sundberg, 1994a
). In all cases early development was similar in both the A1 (tassel) and the A2 (ear) inflorescences. Later female inflorescences underwent the arrest and abortion of pedicellate spikelets and the subsequent arrest and abortion of stamens floret of A2 sessile spikelets. In tassel spikelets, ovules were aborted in both sessile and pedicellate spikelets. The arrest and subsequent abortion of pedicellate spikelets in A2 inflorescences is the earliest developmental event that can be used to discriminate the sexuality of teosinte inflorescences (Sundberg and Orr, 1990
; Orr and Sundberg, 1994a
). This developmental marker is also found in Tripsacum inflorescences (Orr et al., 2001
). However, it is not observed in U.S. varietal maize (Cheng, Greyson, and Walden, 1983
; Stevens et al., 1986
), or in the Latin American landraces Chapalote and Argentine popcorn (Sundberg, LaFargue, and Orr, 1995
; Sundberg and Orr, 1996
). The marker does not imply a causal relationship between the developmental fates of Toluca pedicellate spikelet primordia and Toluca inflorescence organ type primordia (Orr and Sundberg, 1994a
); these may be two separate organogenic events in Zea as well as in Tripsacum.
Nonabortive spikelet primordia of Toluca teosinte developed additional primordia chronometrically—outer glume, inner glume, outer lemma, lower floret, inner lemma, upper floret, and palea—in a sequential pattern common to other teosintes, maizes, and Eastern gamagrass (Orr et al., 2001
). Both the upper and lower florets of these functional spikelets produced stamen and ovule primordia. The pattern of arrested and aborted development of lower florets in A2 sessile spikelets of Toluca was similar to the abortion of the lower florets in other teosintes maizes (Cheng, Greyson, and Walden, 1983
; Stevens et al., 1986
), landrace maize (Sundberg, LaFargue, and Orr, 1995
; Sundberg and Orr, 1996
), and Eastern gamagrass (Orr et al., 2001
). These observations of Toluca inflorescence development clearly support our earlier hypothesis that both femininity and masculinity share a common pattern of inflorescence organogenesis in the Tripsacineae (Sundberg and Orr, 1990
; Orr and Sundberg, 1994a
). Moreover, the evidence from this study further substantiates the view that virtually all the andropogonoid grasses share a common pattern of inflorescence development (Francis, 1990
). Inflorescence development in the Toluca teosinte is also consistent with another interesting observation: similarity in spikelet development and the formation of unisexual florets indicates a common mechanism for sex determination in the tribe Andropogoneae (Le Roux and Kellogg, 1999
).
Orthostiches
The characteristic initiation of spikelet pairs along the central rachises of male and female teosinte inflorescences occurs in a distichous arrangement (Sundberg and Orr, 1986
, 1990
; Sundberg, 1987
; Cámara-Hernández and Gambino, 1990
; Orr and Sundberg, 1994a
). The developmental pattern of spikelet initiation observed in 51 Toluca female A2 inflorescences was, except for four specimens, in accord with this two-ranked pattern. However, the reverse was evident in male (A1) inflorescences. In the 30 tassel specimens we examined, only 5 exhibited a purely distichous pattern of spikelet formation along the central spike, whereas 25 tassels exhibited polystichy in the central spike, but not along the lateral branches, where spikelets were always distichously arranged. In fact, one of the most interesting observations in this study was the occurrence of atypical teosinte orthostiches along the central spike of Toluca inflorescences: intermediate (two-ranked/four-ranked) tassels, pure (four-ranked) tassels, intermediate (two/three-ranked) ears, and pure (four-ranked) ears. Interestingly, a polystichous arrangement of the long branches that are proximal to the central spike has been seen in a newly discovered teosinte from Nicaragua (Bird, 2000
, and personal observation). However, in these plants all the ears and the male central spikes were two-ranked (distichous). In Toluca female inflorescences only one purely tetrastichous ear was observed, whereas there were 15 tassels with central spikes showing only the tetrastichous condition. Moreover, only one Toluca plant exhibited the polystichous condition in both male and female inflorescences.
In polystichous tassels and ears that exhibited mixed orthostiches, including a two-ranked condition of spikelet pair primordia, the distichous arrangement was expressed distal to the polystichous condition. In these mixed polystichous inflorescences, especially the male, the orthostichous developmental pattern switched along the rachis of the central spike during the formation of spikelet pair primordia. These data indicate that the number of ranks of spikelets in the central spike of the Toluca tassel, and its homolog the ear, may not be determined at the initial production of spikelet pair primordia, as is also true in the maize Argentine popcorn (Sundberg, LaFargue, and Orr, 1995
). Whenever the early production of spikelet ranks was greater than two, there was a developmental tendency to revert to a standard teosinte distichous pattern. This developmental plasticity, manifested in the transition from polystichy to distichy, may have resulted from an altered expression of a major genetic locus, such as RANK, or one or more minor loci affecting rank number (Doebley and Stec, 1991
). However, the plasticity in the number of ranks of spikelet pair primordia between ears and the central tassel spike in the Toluca population as a whole, as well as between tassel and ears on many individual Toluca plants, needs to be determined.
The plasticity of spikelet pair primordia development was manifested in Toluca ears by a de novo production of spikelet pair primordia (Fig. 27), a doubling or tripling of spikelets with a common outer glume (Figs. 28, 29), and perhaps a second bifurcation of a spikelet pair primordia (Fig. 21). The occurrence of a second bifurcation in Toluca ears is consistent with a new model, derived from scanning electron microscopy, of the origin of polystichy in maize (Sundberg and Doebley, 1990
; Sundberg, LaFargue, and Orr, 1995
)—namely, additional spikelet pair primordia bifurcation is the mechanism that increases the number of ranks (thus, more rows) on the inflorescence—rather than the unsupported condensation-twisting model of Collins (1919)
. The plasticity in spikelet pair primordia production in the Toluca female inflorescence is also evident in the appearance of de novo primordia (Fig. 27). It is clear that these de novo primordia are out of the bilateral distichous plane typical for a teosinte inflorescence (Orr and Sundberg, 1994a
). Thus, a de novo initiation of lateral primordia may result in a change in phyllotaxy. Interestingly, the novel occurrence of two/three spikelets coupled with a common outer glume appeared to be derived from an additional branching of either the sessile or pedicellate spikelet (Fig. 28). The occurrence of triplet spikelets has been observed in modern wild-type maize inflorescences (Bonnett, 1953
), landrace maize (Sundberg, LaFargue, and Orr, 1995
; Sundberg and Orr, 1996
), and maize-teosinte hybrids (Doebley, 1995
). However, additional proliferation of row number was likely best attained by bifurcation of spikelet pair primordia (Sundberg, LaFargue, and Orr, 1995
; Pizzolato and Sundberg, 1999
), rather than through additional branching of spikelet primordia.
A distichous inflorescence is a definitive characteristic of teosinte (Evans and Grover, 1940
; Orr and Sundberg, 1994a
; Iltis, 2000
). Yet Toluca teosinte commonly have three or more ranks. Why? One possibility is that polystichous inflorescences are the result of a mutational event; another is that they are a manifestation of introgression between maize and Toluca teosinte. The latter explanation may be the most parsimonious one. That teosinte and maize hybridize in natural situations has been well documented, starting almost 85 yr ago (Collins, 1919
; Wilkes, 1977
; Doebley, 1984
; Srinivasan and Brewbaker, 1999
; Benz, 2001
). Wilkes (1977)
identified agricultural sites in Mexico where maize and teosinte readily hybridize. Further, he reported that although all the collection sites harbored teosinte-maize hybrids, they were most frequently found in the Chalco, Central Plateau, and Nobagame regions. Taba (1995)
stated that teosinte is adaptation to mid- and high-elevation regions in Mexico and recounted that the race Chalco grows in both Chalco and Toluca localities.
The seed used in our investigation was collected from Toluca teosinte growing at elevations higher than reported for Chalco and Balsas teosinte. The Toluca plants were adjacent to fields of the highland maize Cónico. Maize-Toluca teosinte hybrids are common in and adjacent to areas of cultivated highland maize (Vibrans and Estrada Flores, 1998
). Toluca teosinte, an agricultural weed, is able to survive to the reproductive stage in, and adjacent to, fields of maize because of its excellent vegetative mimicry of maize (Wilkes and Taba, 1993
). Thus, the occurrence of orthostiches greater than two in some individual Toluca inflorescences may reflect the introgression of highland maize genomic information into a Toluca background. Wilkes (1977
, Fig. 11A) reported F1 maize x teosinte hybrids with polystichous ears. However, the frequency of tetrastichy in the hybrid population was not indicated. As noted above, we observed a very little polystichy in Toluca ears. Srinivasan and Brewbaker (1999)
constructed F1 hybrids between maize and teosinte that produced ears with traits intermediate between the two parents, including hybrid ears that were four-rowed but retained a bilateral symmetry. This pattern is unlike the radial symmetry we observed in 2 Toluca ears and 15 Toluca tassels. Although we have opted for the introgression hypothesis to explain our unusual orthostichies in Toluca inflorescences, we are aware of Doebley's (1984)
argument that evidence for a long-lasting maize introgression into teosinte is largely circumstantial. However, the two oldest maize cobs in the New World, from Guila Naquitz Cave, Mexico, are from a primitive maize that exhibits a teosinte influence (Benz, 2001
; Piperno and Flannery, 2001
). Furthermore, evidence of maize x teosinte F1 hybrids was found in Ramero's Cave, Tamaulipas, Mexico (Wilkes, 1977
).
Conclusions and perspectives
We elucidated the pattern of inflorescence development in the high-altitude Toluca teosinte and compared it with the pattern in other teosintes, maize and gamagrass. As expected, the results supported our hypothesis that both femininity and masculinity share a common pattern in the Poaceae, and they confirm a common mechanism of sex determination in the Andropogoneae (Le Roux and Kellogg, 1999
). However, we unexpectedly demonstrated a plasticity in the initiation of the orthostichy pattern and an alteration of the distichous arrangement of spikelet pair primordia. If we assume that introgression between local highland maize and Toluca teosinte is occurring, our investigation is the first study of inflorescence development in natural maize-teosinte hybrids.
If high-altitude Toluca teosinte was an important source of germplasm in the domestication of highland maize, a determination of inflorescence development in Toluca teosinte plants may encourage the use of highland maize from central Mexico in breeding programs. We hope this investigation offers insight into the effect(s) on inflorescence development when different developmental programs are recombined in maize-teosinte hybrids. Further molecular evidence is crucial to these studies to determine lineage/progenitor species.
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FOOTNOTES |
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4 E-mail for reprint requests: orr{at}uni.edu
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LITERATURE CITED |
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