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Anatomy and Morphology |
Department of Botany, Claremont Graduate University and Rancho Santa Ana Botanic Garden, 1500 N. College Ave., Claremont, California 91711-3157 USA
Received for publication 5 May 2007. Accepted for publication 16 November 2007.
ABSTRACT
Unisexual flowers have evolved repeatedly in the angiosperms. In Poaceae, multiple transitions from bisexual to unisexual flowers are hypothesized. There appear to be at least three distinct developmental mechanisms for unisexual flower formation as found in members of three subfamilies (Ehrhartoideae, Panicoideae, Pharoideae). In this study, unisexual flower development is described for the first time in subfamily Chloridoideae, as exemplified by Bouteloua dimorpha. Scanning electron microscopy (SEM) and anatomy were used to characterize the development of male (staminate) and female (pistillate) flowers, spikelets, and inflorescences. We found the developmental pathway for staminate flowers in B. dimorpha to be distinct from that described in the other three subfamilies, showing gynoecial arrest occurs at a different stage with possible loss of some cellular contents. However, pistillate flowers of B. dimorpha had some similarity to those described in other unisexual-flowered grasses, with filament and anther differentiation in abortive stamens. Comparing our findings with previous reports, unisexual flowers seem to have evolved independently in the four examined grass subfamilies. This analysis suggests the action of different genetic mechanisms, which are consistent with previous observations that floral unisexuality is a homoplasious condition in angiosperms.
Key Words: Chloridoideae • development • dioecy • evolution • monoecy • Poaceae • unisexual flowers
The evolution of the separation of the sexes is a topic of general interest to biologists because it is an important mechanism for outbreeding and a vital source of genetic variation. In plants, this process is demonstrated by the shift from bisexual (hermaphrodite) to unisexual flowers and from bisexual to unisexual individuals. Unisexual flowers can be distributed on plant species in various arrangements resulting in a spectrum of sexual forms. Two important examples of this are monoecy, characterized by staminate (male) and pistillate (female) flowers on the same individual plant, and dioecy, characterized by male and female flowers on different individual plants. Dioecy is of particular evolutionary significance because it is analogous to the sexual systems found in most animals and is therefore a striking example of parallel evolution across the plant and animal kingdoms. Most of our knowledge about the genetics of sex determination in angiosperms has come from studies of the model crop Zea mays L. (Poaceae). Although the genetic and molecular basis of sex determination has only been described in maize (and its close relative Tripsacum dactyloides L.), the developmental pathways for unisexual flower formation have been characterized in multiple grass subfamilies. Because of the available data at the genetic, molecular, and developmental levels, Poaceae are ideal for comparative studies concerning the evolution of unisexual flowers at the familial level, which may give insight into the parallel nature of this trait.
Approximately 20–30% of angiosperm species produce unisexual flowers (Yampolsky and Yampolsky, 1922; Richards 1997
). It is generally thought that unisexual flowers have evolved as a mechanism to promote outcrossing (Charlesworth and Charlesworth, 1978; Thomson and Barrett, 1981) and/or to allow for sexual resource allocation and specialization (Bawa 1980; Charlesworth and Charlesworth, 1981; Thomson and Brunet, 1990; Brunet and Charlesworth, 1995). Based on the widespread taxonomic distribution of unisexual-flowered species, it is evident that this trait has evolved independently numerous times (Lewis 1942; van der Pijl, 1978; Lloyd 1982; Charlesworth 1985; Richards 1997).
Additional evidence for the parallel evolution of this trait is the diversity of genetic and developmental mechanisms known to be involved in the formation of unisexual flowers. Unisexual flower development has been broadly categorized into two main types (Mitchell and Diggle, 2005). In type I development, which is more frequently observed, stamens and pistils are initiated in both staminate and pistillate flowers. As flowers develop, only the androecium or gynoecium reaches functional maturity, and development of the ultimately nonfunctional, rudimentary floral organs is arrested. This type of development has been described in numerous species including Asparagus officinalis L. (Asparagaceae; Bracale et al., 1991; Caporali et al., 1994), Silene latifolia L. (Caryophyllaceae; Grant et al., 1994), and Zea mays (Cheng et al., 1983). In plants with type II development, flowers appear to be unisexual from inception and only initiate development of either stamens or pistils. Mature flowers of this type do not have rudimentary organs. This pathway has been documented in Mercurialis annua L. (Euphorbiaceae; Durand and Durand, 1991), Spinacia oleracea L. (Amaranthaceae; Sherry et al., 1993), and Thalictrum dioicum L. (Ranunculaceae; Di Stilio et al., 2005). In T. dioicum, sex determination may involve the differential regulation of B and C class floral homeotic genes (Di Stilio et al., 2005).
Type I development has been demonstrated for several unisexual-flowered species of grasses (Poaceae), and there appear to be more than one mechanism of floral organ arrest in this family (Table 1). In some species, organ arrest involves a process of cellular vacuolization and/or cell death, whereas in others, arrest occurs without evidence of cellular breakdown. In Panicoideae, certain cell layers become vacuolated in the aborting gynoecium of staminate flowers that is characterized by the loss of cytoplasmic organelles and the breakdown of nuclei (Cheng et al., 1983; LeRoux and Kellogg, 1999). This process has been well documented at the level of gene expression in the panicoid Zea mays and its close relative Tripsacum dactyloides (DeLong et al., 1993; Li et al., 1997). In staminate maize flowers, stamens and pistils are both initiated, but the gynoecium arrests early in development at the time of formation of the gynoecial ridge, the primordial ovary (carpel) wall surrounding the developing nucellus. Cessation of development involves vacuolization and loss of organelles in cells near the summit of the gynoecium. In pistillate flowers of maize, stamens are initiated but growth is arrested at the time of formation of the style in the functional gynoecium. Transmission electron microscopy (TEM) has revealed that stamen arrest in Z. mays also involves cellular vacuolization and loss of cytoplasmic organelles beginning in the anther lobes and progressing throughout the entire anther (Cheng et al., 1983).
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) described the development of unisexual flowers in three species of tribe Andropogoneae, which includes Z. mays and T. dactyloides, of Panicoideae: Bothriochloa bladhii (Retz.) S. T. Blake, Heteropogon contortus (L.) P. Beauv. ex Roem. & Schult., and Hyparrhenia hirta (L.) Stapf. They also described the development of staminate flowers in Panicum repens L. of Paniceae, the other major tribe of Panicoideae. In the three species of Andropogoneae, staminate flower development is very similar to that of Z. mays and T. dactyloides, with an apparent programmed cell death mechanism. The gynoecium arrests at the time of formation of the gynoecial ridge, with cellular vacuolization, loss of cytoplasmic organelles, and dissolution of nuclei. A similar pattern of development was found in P. repens, although cell death occurs in different cell layers. In another developmental study that included Panicum maximum Jacq., the same histological pattern was found in the staminate flower of P. maximum, although subtle differences in the timing and location of vacuolization were detected in P. maximum relative to P. repens (Reinheimer et al., 2005). In the three species of Andropogoneae, cell death takes place in the central cells of the gynoecium, whereas in P. repens and P. maximum it occurs in other cell layers. The authors suggest that a cell death mechanism in staminate flower development may occur in species throughout Panicoideae, and this may point to a common genetic mechanism in this subfamily.
Unlike the species of Panicoideae, staminate flower development in Zizania aquatica L. (Ehrhartoideae) occurs without apparent cell death in the aborted gynoecium (Zaitchik et al., 2000). In Z. aquatica, gynoecial arrest occurs at a later stage than seen in species of Panicoideae. The gynoecium develops beyond formation of the gynoecial ridge and does not abort until after formation of the nucellus and integument. Based on histology, there is no evidence of cellular vacuolization or death. This finding suggests that the mechanism of male flower development in Z. aquatica is distinct from that found in Panicoideae and probably involves a different genetic mechanism (Zaitchik et al., 2000). Unisexual spikelet development has also recently been examined in Pharoideae, an early-diverging grass subfamily, in which all species are monoecious (Clark and Judziewicz, 1996). Staminate and pistillate spikelet development has been examined in two species of Pharus P. Br. (Sajo et al., 2007). The arrested pistils in staminate flowers of Pharus resemble those found in Bothriochloa bladhii (Le Roux and Kellogg, 1999), although it is unclear if vacuolization occurs in the cells of this structure. Also in Pharus, vacuolization is not implicated in the arrested stamens of pistillate spikelets that go on to form six staminodes.
Placing these developmental data in a phylogenetic context, it appears that at least three pathways for the development of staminate flowers have evolved in Poaceae (Fig. 1; Kellogg 2000). At least one developmental pathway likely evolved in Ehrhartoideae (BEP lineage; subfamilies Bambusoideae, Ehrhartoideae, and Pooideae; GPWG 2001), at least one in the Panicoideae (PACCMAD lineage; subfamilies Panicoideae, Arundinoideae, Chloridoideae, Centothecoideae, Micrairoideae, Aristidoideae, and Danthonioideae; Sánchez-Ken et al., 2007), and a third in the Pharoideae (an early diverging lineage; GPWG 2001). In Panicoideae, the developmental mechanism involves cell death in the arresting gynoecium. Development in Pharoideae resembles that of Panicoideae, although it is unclear if there is vacuolization or cell death. In Zizania aquatica (Ehrhartoideae), gynoecial arrest occurs at a much later stage without any evidence of cell death.
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). Poaceae are a logical choice for such studies because of the prevalence and phylogenetic distribution of unisexual-flowered species. These species, which occur in a variety of sexual forms, are present in ca. 142 of the some 700 genera (Watson and Dallwitz, 1992 onward) and in nine of the 13 subfamilies (Fig. 1). This distribution indicates numerous shifts between unisexual and bisexual flowers throughout the evolutionary history of the family, although it is currently unknown if the ancestral state of Poaceae is unisexual or bisexual flowers (Malcomber and Kellogg, 2006). However, in Ecdeiocoleaceae, the putative sister family to Poaceae (Michelangeli et al., 2003), and in another closely related family, Restionaceae, all species have unisexual flowers (Stevens 2001 onward; Rudall et al., 2005).
The current study aims to add to the knowledge of the evolution of unisexual flowers by describing the pathway of floral development in the grass Bouteloua dimorpha Columbus (syn. Opizia stolonifera J. Presl) of Chloridoideae, one of the PACCMAD subfamilies. Bouteloua dimorpha is a stoloniferous, turf-forming species native to tropical North America (primarily Mexico) and has both monoecy and dioecy (Columbus et al., 1998, Columbus et al., 2000). In addition, the species has sexual dimorphism of the inflorescence and spikelet (Columbus 1994). The staminate inflorescence (Fig. 2) consists of an elongated culm that serves as the main inflorescence axis and positions 1–6 distichous primary branches above the foliage of the plant. Each branch bears 10–25 spikelets arranged in two rows along one side of the branch axis. The staminate spikelet of B. dimorpha possesses a single awnless floret with two large fleshy lodicules at anthesis that are involved in separating the lemma and palea to facilitate stamen exsertion. Staminate inflorescences resemble those of other diclinous (bearing unisexual flowers) and hermaphrodite species of Bouteloua Lag. (Columbus et al., 1998, Columbus et al., 2000). It is hypothesized that the morphology of the staminate inflorescence has been selected to enhance male function and has evolved independently in several lineages in Bouteloua (Columbus et al., 2000).
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). The main axis of the pistillate inflorescence is shorter than that of the staminate inflorescence. Spikelets are borne singly on 10–25 short inflorescence branches that are arranged distichously along the main axis. In contrast to the staminate spikelet, the pistillate spikelet has two florets, the lemma is three-awned, and the lodicules are rudimentary and scale-like; however, only the proximal floret develops a flower. It is thought that the position and morphology of the pistillate inflorescence in diclinous species of Bouteloua are important for pollen capture and seed dispersal (Quinn et al., 1994; Columbus et al., 2000). However, unlike the staminate inflorescence, the pistillate inflorescences of these species vary considerably from one another (Columbus et al., 2000). In this study we describe the development of staminate and pistillate flowers, spikelets, and inflorescences of Bouteloua dimorpha, addressing the following: (1) How does development differ in the staminate and pistillate morphs? (2) How does unisexual flower formation compare to that described for other grass species? (3) Is a process of cell death implicated in unisexual flower formation? (4) Are additional pathways for the development of unisexual flowers implicated in the grass family, therefore suggesting the actions of different genetic mechanisms?
MATERIALS AND METHODS
Specimen collection and fixation
Specimens of Bouteloua dimorpha were collected from five populations in Mexico (Appendix). Caryopses were sown in individual pots, and plants were grown to maturity in either a controlled environment chamber or greenhouse at Rancho Santa Ana Botanic Garden. Multiple individuals grown from each population were examined, and young, developing inflorescences were dissected from plants previously determined as male, female, or monoecious. Immature inflorescences at a range of developmental stages were dissected from shoots above the most distal vegetative node, usually enveloped by two or three leaf sheaths. Inflorescences were immediately fixed in FPA (1:1:18 37% formaldehyde:propionic acid:70% ethanol) for 24 h at 4°C and subsequently transferred to 70% ethanol for storage at 4°C.
Micromorphology
Samples were dehydrated in an ethanol series and critical-point dried in CO2 using a Pelco E3000 critical-point dryer (Ted Pella, Redding, California, USA). Samples were then affixed to aluminum stubs with carbon conductive tape and coated in gold using a Pelco SC-7 sputter coater. Inflorescences were examined using an International Scientific Instruments (Milpitas, California, USA) WB-6 scanning electron microscope at 10 kV. Photographs were taken with Polaroid Type 55 film (Polaroid, Cambridge, Massachusetts, USA).
Anatomy
Inflorescences were dehydrated, embedded in paraffin, sectioned longitudinally, and ribbons were affixed to slides largely following Columbus (1998). Samples were sectioned at 7–8 µm. For visualization of nuclei and cytoplasm, a modified version of Heidenhain 1894 hematoxylin staining protocol was used (Johansen 1940). Hematoxylin stains nuclei dark gray to black, and with counterstaining the cytoplasm appears buff gray (Ruzin 1999). Slides were placed in a 3% w/v aqueous ferric ammonium sulfate solution for 1 h, rinsed in distilled water (dH2O), and then placed in a 0.5% w/v aqueous hematoxylin solution for 12–24 h. Slides were rinsed again in dH2O and destained in a 2% w/v aqueous ferric ammonium sulfate solution, followed by counterstaining in a solution of orange G/tannic acid, and a final rinse in dH2O. After dehydrating via an ethanol series, coverslips were placed on slides following standard protocols (Ruzin 1999). Sections were visualized using a Leitz Laborlux D light microscope (Leica Instruments, Milton Keynes, UK) and digital images were acquired using a SPOT digital camera with SPOT 3.2.4 imaging software (Diagnostic Instruments, Sterling Heights, Michigan, USA). The slides are deposited at Rancho Santa Ana Botanic Garden.
RESULTS
Staminate flower, spikelet, and inflorescence development
A developmental series for staminate flowers, which develop mature stamens and aborted pistils, is presented in Figs. 4–10. The youngest inflorescences dissected had distinct primary branches, each with two alternating rows of spikelet primordia (Fig. 4). Observations of developmental stages showed spikelet initiation to be acropetal, and maturation to be basipetal. The gynoecium and at least two stamens are initiated at about the same time (Figs. 5, 7). The glumes, lemma, and palea have already initiated by this stage and begin to enclose the floral organs (Fig. 5). At a slightly later stage, two lodicules are observed at the base of the flower, and the anthers begin to differentiate (Fig. 8). The gynoecium continues to develop and the ovary wall begins to elongate giving the gynoecium a cup-shaped appearance in longitudinal section (Fig. 8). The ovary wall and styles develop extensively but the ovule may not form (Figs. 6, 9, 10). Also, the anthers continue to develop rapidly and are near maturity before the filaments begin to elongate. Development of the gynoecium begins to arrest at this point, and there appears to be pale staining of the cytoplasm in the outer cell layers at the base and middle of the gynoecium (Figs. 9, 10). This could indicate possible vacuolization. However, there is dark staining of the cytoplasm and nuclei at the apex and core of the aborted gynoecium. Most of the pale staining cells retain their nuclei (stained dark gray; Figs. 9, 10), although nuclei may be lost in a small number of these cells. Development of the gynoecium largely ceases at this stage, but some growth continues at the apex that could be elongation of the ovary wall and/or the beginning of the formation of the style (Fig. 10).
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. The newly formed pistillate inflorescence is composed of a main axis with distichous branch primordia (Fig. 11). Each inflorescence branch develops a solitary spikelet (Figs. 12, 13). Initiation of branch primordia is acropetal, and maturation is basipetal. As the inflorescence branch develops, components of the spikelet and flower are initiated (Fig. 12), and stamen and gynoecium primordia are evident (Fig. 14). The lemma and palea of the proximal floret begin to enclose the floral organs while the distal, sterile floret, which develops later, is not evident at this stage (Fig. 13); the lemma is three lobed, each lobe eventually forming a long awn (Figs. 3, 13). As the lemma encloses the flower, the gynoecium appears to develop more rapidly than the stamens (Fig. 14). Some anther differentiation is observed in the abortive stamens, although it is not clear if filaments differentiate (Figs. 16–18). Later, the gynoecium differentiates into the ovary wall and ovule (Fig. 15). The ovary wall elongates, enclosing the ovule, and stamen development ceases (Fig. 16). There is no indication of cellular vacuolization in the arrested stamens, due to the uniform dark staining of these structures (Figs. 15–18). The ovary wall continues to extend and form the stylar tube, and complete stamen arrest is evident. Diminutive lodicules are visible below the rudimentary stamens as the flower and spikelet reach maturity (Fig. 18).
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Comparison of staminate and pistillate flowers and spikelets of Bouteloua dimorpha
The androecium and gynoecium are initiated in both staminate and pistillate flowers of B. dimorpha ; therefore this species meets the criteria for type I development of unisexual flowers (Mitchell and Diggle, 2005). Lodicules are apparent and comparably larger in the early stages of development in staminate flowers and become large and fleshy at maturity, whereas in pistillate flowers they are greatly reduced and do not become evident until later, and they remain diminutive. It is generally thought that lodicules function in many grasses at anthesis to push apart the lemma and palea to facilitate stamen exsertion (Soreng and Davis, 1998). The observations in this study are consistent with this theory. The differences in lodicule development and size in male and female flowers of B. dimorpha are likely the result of selection for specialized sexual function. The large lodicules in male flowers may facilitate pollen dispersal, whereas the diminutive lodicules in female flowers may have no significant function. Similarly, in Pharus, male florets have two lodicules and in female florets, lodicules are completely lacking.
The arrest of the gynoecium in male flowers of B. dimorpha may involve some loss of cytoplasm and nuclei; however, details of the histology and development suggest a mechanism of development that is different from that seen in Panicoideae and other grass subfamilies. In female flowers, the ovary wall and the ovule initiate and proceed to maturity with no evidence of cellular breakdown at any point throughout development. Also, there is no indication of cellular vacuolization or breakdown in the arresting stamens in pistillate flowers. This observation may point to different developmental processes for organ arrest and sex determination in male and female flowers of B. dimorpha and suggests that different genetic mechanisms are at work.
Comparison to other unisexual-flowered species
Bouteloua dimorpha follows the same developmental pattern as other type I unisexual-flowered species. However, the patterns of gynoecial arrest in B. dimorpha and other grasses are very different than what is seen in Asparagus officinalis and Silene latifolia, two other type I species. In A. officinalis the pistil develops to an advanced stage and occasionally forms a mature megagametophyte (Lazarte and Palser, 1979). In S. latifolia a rod of undifferentiated cells develops in the position of the pistil, bearing no resemblance to the functional gynoecium in female flowers (Grant et al., 1994; Farbos et al., 1997). In Ecdeiocolea monostachya F. Muell. (Ecdeiocoleaceae, the putative sister family to Poaceae; Michelangeli et al., 2003), the development of the gynoecium appears to be arrested at a much earlier stage than in grasses resulting in a minute pistillode with no clear organ differentiation (Rudall et al., 2005).
Development of staminate flowers of B. dimorpha is more similar to that reported for species of Panicoideae and Pharoideae (Table 1), with the gynoecium being arrested at an early stage. However, in Panicoideae and Pharoideae the gynoecial ridge encloses a visible nucellus, which was not observed in B. dimorpha. Additionally, in B. dimorpha there is more extensive development of the ovary wall and the style. In species of Panicoideae, there is a clear pattern of vacuolization and loss of nuclei. In B. dimorpha, there is possible loss of cytoplasm in the outer cell layers near the middle of the gynoecium, and most of these cells retain their nuclei. Further study employing an additional nuclei-specific stain, such as DAPI (4'-6-diamidino-2-phenylindole), would allow for a more accurate comparison. Gynoecial arrest in B. dimorpha, Panicoideae, and Pharoideae is very different from what has been reported for the grass Zizania aquatica (Ehrhartoideae; Zaitchik et al., 2000), in which arrest occurs at a later stage with no evidence of vacuolization.
In pistillate flowers of B. dimorpha, stamens are initiated and there appears to be some anther differentiation and possible filament differentiation. In this respect, stamen arrest in B. dimorpha is similar to that reported for other grass species as well as Ecdeiocoleaceae (Rudall et al., 2005). In Zea mays, cellular vacuolization is observed in the aborted stamens (Cheng et al., 1983), but in Heteropogon contortus, as in B. dimorpha, stamen arrest occurs without evidence of loss of cellular contents or breakdown of nuclei. Le Roux and Kellogg (1999) demonstrated that nuclei remain intact in abortive anthers of H. contortus with DAPI staining, and B. dimorpha also appears to retain nuclei in abortive anthers in the current study. However, it should be kept in mind that TEM studies of anther degeneration have only been done in Z. mays. Because anther degeneration was characterized using different methods in these three species, it is difficult to make comparisons of the developmental mechanisms.
The ability to produce unisexual flowers has evolved repeatedly in the angiosperms. The lability of this trait is exemplified in Poaceae in which multiple transitions from bisexual to unisexual flowers are implicated (Malcomber and Kellogg, 2006). Developmental data are now available for four subfamilies (Chloridoideae, Ehrhartoideae, Panicoideae, and Pharoideae), and in each of the four subfamilies different developmental processes are implicated. These data indicate that unisexual flowers are likely independently derived in each of these subfamilies and are a convergent trait in the grasses (see also Fig. 1). Floral unisexuality in the grasses is another example of a homoplasious morphological feature that can be distinguished at the developmental level (Gleissberg and Kadereit, 1999; Bharathan et al., 2002).
The results of this study build on previous research demonstrating the existence of multiple developmental pathways for the formation of unisexual flowers in grasses and across angiosperms. In addition, the developmental data from Bouteloua dimorpha support the hypotheses of Malcomber and Kellogg (2006) that multiple genetic mechanisms regulating this process may have evolved even within grasses and that staminate and pistillate flowers seem to be controlled by separate developmental and genetic programs. In Zea mays and Tripsacum dactyloides (Andropogoneae), a short-chain alcohol dehydrogenase protein, Tasselseed 2 (Ts 2), is required for cell death and gynoecial abortion in staminate flowers (DeLong et al., 1993; Li et al., 1997). However, the regulation of cell death is intimately tied to the Silkless 1 (Sk 1) gene that appears to play a role in preventing cell death in tissues in which Ts 2 is expressed (Calderon-Urrea and Dellaporta, 1999). Although the role, if any, of these genes in sex determination is currently unknown for other grass species, the histological pattern of cell death in the aborted gynoecia of Z. mays and T. dactyloides is very similar to that seen in other species of Panicoideae (Le Roux and Kellogg, 1999). These findings suggest that the Ts 2/Sk 1 pathway may play a wider role in sex determination in Panicoideae. The histological pattern found in B. dimorpha does show possible cellular vacuolization, but this process occurs in different cell layers and at a different developmental stage. Studies of molecular evolution of putative orthologs of Ts 2 have been performed across a wide range of grass species including B. dimorpha (Kinney et al., 2003; Malcomber and Kellogg, 2006). The results indicate that Ts 2 is highly conserved throughout the grasses and is generally under strong purifying selection. In addition, expression studies of Ts 2 in Oryza sativa L., Sorghum bicolor (L.) Moench, and Z. mays have shown that Ts 2 is expressed in floral and vegetative tissues, suggesting that the gene has a more widespread developmental role than originally thought (Malcomber and Kellogg, 2006). The developmental data from the current study indicate that Ts 2 could have a sex-determination role in B. dimorpha (i.e., is involved in possible vacuolization), although it probably would be just one piece of a more complex genetic program (e.g., Ts 2/Sk 1 pathway). If vacuolization is occurring in aborted gynoecia of Chloridoideae, it is likely the result of convergent evolution involving numerous genetic factors and not simply from the effect of a single sex-determining gene.
Appendix 1. Source and voucher information for the specimens of Bouteloua dimorpha used in this study. All are collections by J. T. Columbus from Mexico deposited at the herbarium of Rancho Santa Ana Botanic Garden (RSA).
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FOOTNOTES
1 The authors thank the staff and students of Rancho Santa Ana Botanic Garden (RSABG). This work was supported by grants from the Andrew W. Mellon Foundation, the Hardman Award for Native Plant Research, the Goldhamer Award, the C. L. Smith Botany Award, and the RSABG Alumni Foundation. They also thank two anonymous reviewers for helpful comments. 
2 Author for correspondence (e-mail: kinneym{at}missouri.edu); current address: Division of Biological Sciences, University of Missouri-Columbia, 311 Life Sciences Center, 1201 Rollins St., Columbia, MO, 65211-7310
LITERATURE CITED
Bawa, K. 1980. Evolution of dioecy in flowering plants. Annual Review of Ecology and Systematics 11: 15–39.[CrossRef][Web of Science]
Bharathan, G., T. E. Goliber, C. Moore, S. Kessler, T. Pham, AND N. R. Sinha. 2002. Homologies in leaf form inferred from KNOX1 gene expression during development. Science 296: 1858–1860.
Bracale, I., E. Caporali, M. G. Gaili, C. Longo, G. Marziani-Longo, G. Rossi, A. Spada, E. Soave, A. Falavigna, F. Raffaldi, E. Maestri, F. M. Restivo, AND F. Tassi. 1991. Sex determination and differentiation in Asparagus officinalis (L.). Plant Science 80: 67–77.
Brunet, J., AND D. Charlesworth. 1995. Floral sex allocation in sequentially blooming plants. Evolution; International Journal of Organic Evolution 49: 70–79.
Calderon-Urrea, A., AND S. L. Dellaporta. 1999. Cell death and cell protection genes determine the fate of pistils in maize. Development 126: 435–441.[Abstract]
Caporali, E., A. Carboni, M. G. Galli, G. Rossi, A. Spada, AND G. P. Marziani-Longo. 1994. Development of male and female flowers in Asparagus officinalis. Search for point of transition from hermaphroditic to unisexual developmental pathway. Sexual Plant Reproduction 7: 239–249.[Web of Science]
Charlesworth, D. 1985. Distribution of dioecy and self-incompatibility in angiosperms. In P. J. Greenwood, P. H. Harvey, and M. Slatkin [eds.], Evolution: essays in honour of John Maynard Smith, 237–268. Cambridge University Press, Cambridge, UK.
Charlesworth, D., AND B. Charlesworth. 1978. Population genetics of partial male-sterility and evolution of monoecy and dioecy. Heredity 41: 137–153.[CrossRef][Web of Science]
Charlesworth, D., AND B. Charlesworth. 1981. Allocation of resources to male and female function in hermaphrodites. Biological Journal of the Linnean Society 15: 57–74.[CrossRef][Web of Science]
Cheng, P. C., R. I. Greyson, AND D. B. Walden. 1983. Organ initiation and the development of unisexual flowers in the tassel and ear of Zea mays. American Journal of Botany 70: 450–462.[CrossRef][Web of Science]
Clark, L. G., AND E. J. Judziewicz. 1996. The grass subfamilies Anomochlooideae and Pharoideae (Poaceae). Taxon 45: 641–645.[CrossRef][Web of Science]
Columbus, J. T. 1994. The female inflorescence of Opizia (Gramineae: Chloridoideae): Organization and homologies. American Journal of Botany 81 suppl.: 149.
Columbus, J. T. 1998. Morphology and leaf blade anatomy suggest a close relationship between Bouteloua aristidoides and B. (Chondrosium) eriopoda (Gramineae: Chloridoideae). Systematic Botany 23: 467–478.[CrossRef][Web of Science]
Columbus, J. T., M. S. Kinney, R. Pant, AND M. E. Siqueiros Delgado. 1998. Cladistic parsimony analysis of internal transcribed spacer region (nrDNA) sequences of Bouteloua and relatives (Gramineae: Chloridoideae). Aliso 17: 99–130.
Columbus, J. T., M. S. Kinney, M. E. Siqueiros Delgado, AND J. M. Porter. 2000. Phylogenetics of Bouteloua and relatives (Gramineae: Chloridoideae): Cladistic parsimony analysis of internal transcribed spacer (nrDNA) and trn L–F (cpDNA) sequences. In S. W. L. Jacobs, and J. Everett [eds.], Grasses: systematics and evolution, 189–194. CSIRO Publishing, Melbourne, Victoria, Australia.
DeLong, A., A. Calderon-Urrea, AND S. L. Dellaporta. 1993. Sex determination gene TASSELSEED2 of maize encodes a short-chain alcohol dehydrogenase required for stage-specific floral organ abortion. Cell 74: 757–768.[CrossRef][Web of Science][Medline]
Di Stilio, V. S., E. M. Kramer, AND D. A. Baum. 2005. Floral MADS box genes and homeotic gender dimorphism in Thalictrum dioicum (Ranunculaceae)—a new model for the study of dioecy. The Plant Journal 41: 755–766.[CrossRef][Web of Science][Medline]
Durand, B., AND R. Durand. 1991. Sex determination and reproductive organ differentiation in Mercurialis. Plant Science 80: 49–65.
Farbos, I., M. Oliveira, I. Negrutiu, AND A. Mouras. 1997. Sex organ determination and differentiation in the dioecious plant Melandrium album (Silene latifolia): a cytological and histological analysis. Sexual Plant Reproduction 10: 155–167.[CrossRef][Web of Science]
Gleissberg, S., AND J. W. Kadereit. 1999. Evolution of leaf morphogenesis: evidence from developmental and phylogenetic data in Papaveraceae. International Journal of Plant Sciences 160: 787–794.[CrossRef][Web of Science]
Grant, S., A. Houben, B. Vyskot, J. Siroky, W. H. Pan, J. Macas, AND H. Saedler. 1994. Genetics of sex determination in flowering plants. Developmental Genetics 15: 214–230.[CrossRef][Web of Science]
Grass Phylogeny Working Group. 2001. Phylogeny and subfamilial classification of the grasses (Poaceae). Annals of the Missouri Botanical Garden 88: 373–457.[CrossRef][Web of Science]
Heidenhain, M. 1894. Neue Untersuchungen über die Centralkörper und ihre Beziehungen zum Kern- und Zellen-Protoplasma. Archiv fur Mikroskopisch-Anatomische 43: 423–758.
Johansen, D. A. 1940. Plant microtechnique. McGraw-Hill, New York, New York, USA.
Kellogg, E. A. 2000. The grasses: A case study in macroevolution. Annual Review of Ecology and Systematics 31: 217–238.[CrossRef][Web of Science]
Kinney, M. S., J. T. Columbus, AND E. A. Friar. 2003. Molecular evolution of the maize sex-determining gene TASSELSEED 2 in Bouteloua (Poaceae). Molecular Phylogenetics and Evolution 29: 519–528.[CrossRef][Web of Science][Medline]
Lazarte, J. E., AND B. F. Palser. 1979. Morphology, vascular anatomy and embryology of pistillate and staminate flowers of Asparagus officinalis. American Journal of Botany 66: 753–764.[CrossRef][Web of Science]
Le Roux, L. G., AND E. A. Kellogg. 1999. Floral development and the formation of unisexual spikelets in the Andropogoneae (Poaceae). American Journal of Botany 86: 354–366.
Lewis, D. 1942. The evolution of sex in flowering plants. Biological Reviews of the Cambridge Philosophical Society 17: 46–67.
Li, D., C. A. Blakely, C. Dewald, AND S. L. Dellaporta. 1997. Evidence for a common sex determination mechanism for pistil abortion in maize and its wild relative Tripsacum. Proceedings of the National Academy of Sciences, USA 94: 4217–4222.
Lloyd, D. G. 1982. Selection of combined versus separate sexes in seed plants. The American Naturalist 120: 571–585.[CrossRef]
Malcomber, S. T., AND E. A. Kellogg. 2006. Evolution of unisexual flowers in grasses (Poaceae) and the putative sex-determination gene, TASSELSEED2 (TS2). The New Phytologist 170: 885–899.[CrossRef][Medline]
Michelangeli, F. A., J. I. Davis, AND D. W. Stevenson. 2003. Phylogenetic relationships among Poaceae and related families as inferred from morphology, inversions in the plastid genome, and sequence data from the mitochondrial and plastid genomes. American Journal of Botany 90: 93–106.
Mitchell, C. H., AND P. K. Diggle. 2005. The evolution of unisexual flowers: morphological and functional convergence results from diverse developmental transitions. American Journal of Botany 92: 1068–1076.
Quinn, J. A., D. P. Mowrey, S. M. Emanuele, AND R. D. B. Whalley. 1994. The ‘Foliage is the Fruit’ hypothesis: Büchloe dactyloides (Poaceae) and the shortgrass prairie of North America. American Journal of Botany 81: 1545–1554.[CrossRef][Web of Science]
Reinheimer, R., R. Pozner, AND A. C. Vegetti. 2005. Inflorescence, spikelet, and floral development in Panicum maximum and Urochloa plantaginea (Poaceae). American Journal of Botany 92: 565–575.
Richards, A. J. 1997. Plant breeding systems, 2nd ed. Chapman and Hall, London, UK.
Rudall, P. J., W. Stuppy, J. Cunniff, E. A. Kellogg, AND B. G. Briggs. 2005. Evolution of reproductive structures in grasses inferred by sister group comparisons with their putative closest living relatives, Ecdeiocoleaceae. American Journal of Botany 92: 1432–1443.
Ruzin, S. E. 1999. Plant microtechnique and microscopy. Oxford University Press, Oxford, UK.
Sajo, M. G., H. Longhi-Wagner, AND P. J. Rudall. 2007. Floral development and embryology in the early divergent grass Pharus. International Journal of Plant Sciences 168: 181–191.[CrossRef][Web of Science]
Sánchez-Ken, J. G., L. G. Clark, E. A. Kellogg, AND E. E. Kay. 2007. Reinstatement and emendation of subfamily Micrairoideae (Poaceae). Systematic Botany 32: 71–80.[CrossRef][Web of Science]
Sherry, R. A., K. J. Eckard, AND E. M. Lord. 1993. Flower development in dioecious Spinacia oleracea (Chenopodiaceae). American Journal of Botany 80: 283–291.[CrossRef][Web of Science]
Soreng, R. J., AND J. I. Davis. 1998. Phylogenetics and character evolution in the grass family (Poaceae): Simultaneous analysis of morphological and chloroplast DNA restriction site character sets. Botanical Review 64: 1–85.[CrossRef]
Stevens, P. F. 2001 [onward]. Angiosperm phylogeny website, version 6, more or less continuously updated http://www.mobot.org/MOBOT/research/APweb/
Thomson, J. D., AND S. C. H. Barrett. 1981. Selection for outcrossing, sexual selection, and the evolution of dioecy in plants. The American Naturalist 118: 443–449.[CrossRef]
Thomson, J. D., AND J. Brunet. 1990. Hypotheses for the evolution of dioecy in seed plants. Trends in Ecology &Evolution 5: 11–16.[CrossRef][Web of Science]
van der Pijl, L. 1978. Reproductive integration and sexual disharmony in floral functions. In A. J. Richards [ed.], The pollination of flowers by insects, 79–88. Academic Press, London, UK.
Watson, L., AND M. J. Dallwitz. 1992 [onward]. Grass genera of the world: descriptions, illustrations, identification, and information retrieval; including synonyms, morphology, anatomy, physiology, phytochemistry, cytology, classification, pathogens, world and local distribution, and references. Website http://biodiversity.uno.edu/delta/
Yampolsky, C., AND H. Yampolsky. 1922. Distribution of sex forms in the phanerogamic flora. Bibliotheca Genetica 3: 1–62.
Zaitchik, B. F., L. G. Le Roux, AND E. A. Kellogg. 2000. Development of male flowers in Zizania aquatica (North American wild-rice: Gramineae). International Journal of Plant Sciences 161: 345–351.[CrossRef][Web of Science][Medline]
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  Compiled by, F. Tooke, T. Chiurugwi, and N. Battey Flowering Newsletter bibliography for 2008 J. Exp. Bot., June 23, 2009; (2009) erp154v1. [Full Text] [PDF]
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