- © 2000 by AASP Foundation
The results of a pollen study of 77 American species of Acacia (tribe Acaciae) characterizes pollen among the different New World taxa of the compound-pollen Mimosoideae on the basis of the most relevant morphologic and structural features.
Two pollen types and five subtypes are recognized and summarized in the diagnostic key presented herein. As a result of the species studied, some newly observed structural features are now utilized; these features modify the previous characterization of the pollen of Acacia, and include frequent ornamentation of the suprasubreticulate or supramicrorugulate exine, presence of subpseudocolpi in some species, and presence of a bistratified granular infractectal layer in the subgenus Aculeiferum but not in the section Filicinae.
Morphological and structural diversity of the compound pollen of the Mimosoideae, which form tetrads or polyads, is a character that is often used for systematic differentiation in the group. It is also a character for which there exists a considerable amount of phylogenetic information (Caccavari, 1986a, b, 1987, 1988; Guinet, 1969, 1981, 1986; Guinet and Rico, 1988; Guinet and Hernández, 1989; Guinet and Caccavari, 1992; Guinet and Grimes, 1997; Hernández and Guinet, 1990; Nielsen et al., 1984a, b; Sorsa, 1969; Vassal and Guinet, 1972).
Increased knowledge during the last few years regarding this palynological diversity has shed light on the botanical affinities of fossil pollen (Graham, 1977, 1988, 1991; Graham and Jarzen, 1969; Guinet and Salard-Cheboldaeff, 1975; Guinet et al., 1987; Barreda and Caccavari, 1992; Cavagnetto and Guinet, 1994 and Caccavari and Barreda (in press). This, in turn, has resulted in more precise paleobiogeographic and paleobioclimatologic interpretations, and brings additional evidence to the table regarding our understanding the origin and dispersion of Acacia.
However, palynologic characters of this group have typically been “over-generalized,” which tends to obscure its botanical affinity. This is particularly true for the American genera (Caccavari, 1996), most likely as a result of the lack of a summarized treatment of their palynological diversity.
This paper is the first contribution to the identification and diagnosis of the more relevant morphologic and structural characters for the compound pollen of American Mimosoideae taxa, including American species of the tribe Acacieae (Elias, 1981). Within the Mimosoideae, the Tribe Acacieae comprises those taxa having numerous stamens (more than 50 per flower), and filaments that are free down to their bases. It includes the genera Faidherbia A. Chev. and Acacia Miller, the latter being the only Mimosoideae genus occurring on the American continent (Elias, 1981; Vassal, 1981).
Acacia is distributed in tropical and subtropical areas of America, Asia, Africa and Australia (Elias, 1981). Its species range through a diverse group of habitats; their growth habits vary from trees of different sizes, to bushes (erect or climbing), to vines. The genus is subdivided into three subgenera (Vassal, 1981) which contain about 1200 species. The subgenera are Acacia (see Vassal, 1972), Aculeiferum (see Vassal, 1972) and the predominantly Australian Phyllodinae (see Pedley, 1978, 1986). These subgenera can typically be identified using pollen characters (Guinet, 1986).
MATERIALS AND METHODS
The pollen of 77 American species of Acacia was analyzed and typified on the basis of morphological and structural characters. Material was obtained from specimens in the herbaria at the Instituto de Botánica Darwinion (SI), Museo Argentinode Ciencias Naturales “B. Rivadavia” (BA), the Instituto de Recursos Biológicos INTA Castelar (BAB), Buenos Aires, Argentina, and from the Facultad de Agronomía de Maracay, Venezuela (MY). The collection data are summarized in Table 1⇓. In addition, data concerning the habitat and locality of species are provided for potential use in future palaeoenvironmentals or palaeobiogeographic studies. The habitat for each species is given based on data obtained from a variety of taxonomic and biogeographical papers (Bentham, 1876; Britton and Rose, 1928; Britton and Killip, 1936; Standley and Calderón, 1941; Mc Bride, 1943; Standley and Steyermark, 1946; Woodson and Schery, 1950; Burkart, 1952, 1979; Isely, 1969, 1973; Cárdenas de Guevara, 1974; Elias, 1974; Walter, 1977; Klein, 1978; Cabrera and Willink, 1980; Cialdella 1984, 1996; Grimes and Barneby, 1985; Lewis, 1987; Mc Vaugh, 1987; Seigler and Ebinger, 1988, 1995; Clarke et al., 1990; Fortunato and Cialdella, 1996; Turner, 1996).
One hundred sixty-five samples were acetolyzed (Erdtman, 1960) and mounted in either glycerin gelatin or glycerin for study with the light microscope (LM). Observations were made using a Leitz Laborlux 12 microscope and photomicrograps were taken with a Leitz Diaplan and a Leitz Laborlux microscope. A minimum of twenty pollen grains was observed for each sample. Measurements correspond to the mean of the recorded values.
Selected material was treated as indicated in Caccavari and Dome (2000) for study with the Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM).
A Jeol T-100 SEM and a Leitz AMR 1200 were used at 15 Kilovolts. The TEM photomicrographs were obtained with a Jeol JEM 100C microscope at 80 Kilovolts.
General Morphology and Structure
The pollen of Acacia (Barth and Yoneshigue, 1965; Guinet, 1969, 1981, 1986; Sorsa, 1969; Caccavari, 1970) is dispersed in acalymate polyads which are biconvex, and circular to slightly elliptical in outline (in the sense of the plane); the polyads are formed by 8 or 16 symmetrically-distributed pollen grains (bitetrads or tetratetrads), or by 32 irregularly-distributed pollen grains.
The pollen grains are heteropolar, pyramidal, and their base makes up the distal face. Distal and proximal openings are present on most grains. Proximal aperture always composed of pores; distal apertures are pores or colpores. The distal pseudocolpi are numerous as a consequence of a thinning or interruption of the tectum only in the species Acacia phyllodinae; and they are circular or quadrangular in shape (Guinet, 1986). In some Acacia of the subgenus Aculeiferum, subpseudocolpi have been detected that are the result of a fracture area inside the infratectal layer (Caccavari and Dome, 2000). When the distal face is colporate, the infratectal layer of the exine is columellar; otherwise it is granular (Barth, 1965; Guinet and Lugardon, 1976; Guinet and Ferguson, 1989).
Pollen Types of the American Species of Acacia
On the basis of morphological and structural characters, it is possible to differentiate two basic pollen types, and five subtypes:
Polyads with colpori in distal faces of pollen grains (porate in proximal faces, Plate 1, fig. 4⇓). Exine 2 μm or more in thickness, tectate baculate on the distal face and often intectate, very scarcely baculate on the proximal face. Surface of the distal exine is irregular and smooth to supramicroreticulate. The tectum is perforated, with perforations less than 0.5 μm in diameter and frequently varying according to the species (Plate 1, fig. 5⇓). Sexine thickness is double or more than the thickness of the nexine. The colpi is without a sexine.
Under SEM (Plate 2, figs. 1–7⇓), the ornamentation of the distal exine appears supramicrorugulate, and observation of the perforations becomes difficult (Plate 2, fig. 4⇓). When the exine surface is smooth to irregular (Plate 2, fig. 6⇓), the perforations can be seen in depressions of varying in depth in the tectum, thereby imparting a characteristic undulate appearance to the tectum (Plate 2, figs. 3, 6, 7⇓).
With TEM (Plate 4, figs. 1, 2⇓), the foot layer is noted to be quite thick and, in some species, it appears as a lower band with a hiatus. The tectum is compact. The columellae are variable in height depending on where they occur on the distal face of the pollen grain, and they have broad bases that can fuse with one another. Towards the colpi, the total thickness of the ectexine layers is reduced. The endexine is irregular along its inner margin; at the level of the ora the endexine forms lamellae which are open towards the center of the opening. On the proximal face, occuring on top of the endexine, broken vestigial intectate sexine with small columellae are frequently observed (Plate 4, fig. 1⇓).
Subtype A. 1.
(Table 2⇓). Pollen polyads generally consisting of 16 grains (Plate 1, figs. 1, 2, 4, 5⇓; Plate 2, figs. 1–4, 7⇓). Colpi Y-shaped on the distal face of the central pollen grains, and H-shaped on distal face of the peripheral grains. Ora situated near the free ends of the colpi in an interangular or angular subdistal position (Plate 2, fig. 3⇓). For the central pollen grains of the polyads, the distal exine is nearest to the symmetry axis of the polyad, and presents a triangular or quadrangular appearance that is variable between species (Plate 2, figs. 3, 4⇓).
Subtype A. 2.
(Table 2⇓). (Plate 1, fig. 3⇓; Plate 2, figs. 5–6⇓) This subtype includes polyads with 32 pollen grains. The higher number of grains results in a loss of symmetry, the regular shape of the polyad, the symmetrical distribution of grains, and the shape of the colpi which may have 2–4 free ends.
Polyads with porate pollen grains. Subdistal pores at the angles of the distal face. On the distal face the exine is usually less than 2 μm thick. Tectum is with or without perforations; infratectal layer is very thin. The surface of the exine is smooth, scabrate or slightly sculptured (undulate or suprasubreticulate with circular lumina).
Under TEM (Plate 5, figs. 1–5⇓), a compact tectum is clearly seen; it has perforations, and a granular infratectal layer characterized by two granular strata which are differentiated by the disposition, size, and shape of the granules at the inner or lower stratum. This suggests that the inner strata may represent the foot layer with granules fused at the level of contact with the endexine. The thickness of the ectexine may be greater or lesser than that of the endexine.
Subtype B. 1.
(Plate 1, figs. 6–8, 13⇓; Plate 3, figs. 2, 5, 8, 9⇓). This subtype includes species assigned to the section Monacanthea Vassal p. p. (Table 2⇓), and contains species with 16 pollen-grain polyads. Pollen grains with pores occur in more typical positions and on the proximal areas (Plate 1, fig. 13⇓); the sexine is thicker than the nexine. Uniform exine ornamentation occurs over the entire surface of the distal face.
Subtype B. 2.
This subtype (Plate 1, figs. 9, 10⇓) includes species of the section Monacanthea Vassal p. p. (Table 2⇓), which has the general characteristicss of the polyad type, but with subpseudocolpi (Caccavari and Dome, 2000).
With LM, some random pollen grains of the polyads appear sunken (Plate 1, figs. 9, 10⇓). Under SEM these depressions coincide with an area that exhibits a change in the type of ornamentation on the exine (Plate 3, figs. 1, 3, 6⇓).
With TEM, the change in ornamentation of the exine (Plate 5, fig. 1⇓) can be demonstrated to coincide with an abrupt thinning of the infratectal layer. This corresponds with an abrupt change in the thickness of the granular strata, which thereby creates a fracture area parallel to the equatorial edge of the distal face that is evident on the surface of the tectum. The presence of this fracture allows the expansion or contraction of the distal polar area without affecting the structure of the polyad.
Subtype B. 3.
This group presents several interesting morphological and structural differences within the basic characters of the polyad type. Polyads are represented by eight pollen grains (bitetrads). The tetrads of two bitetrad hemispheres have four pollen grains opposed “two-to-two” in a longitudinal sense (Plate 1, fig. 12⇓), two central grains in contact with each other, and two lateral grains; proximal pores are lacking. Surface of the exine is smooth to slightly sculptured (undulate or suprasubreticulate with circular lumina).
Under the SEM, a number species which appear psilate under LM are demonstrated to have suprasubreticulate ornamentation (Plate 3, fig. 7⇓).
With TEM (Plate 5, figs. 3, 5⇓), the grains display a number of characters, including a compact tectum with perforations that does not traverse the surface entirely, a thin infratectal layer without granular strata, and with a vestigial interrupted foot layer.The thickness of the ectexine is usually less than that of the endexine.
The pollen of Acacia is one of the more extensively studied with regard to its morphological aspects (Guinet, 1964, 1969, 1981, 1986, 1990). The genus contains approximately 1200 species (Vassal, 1981), of which about 900 are restricted to the greater Australian region. The pollen of the American Acacia species have been the subject of a significantly smaller amount of research (Barth and Yoneshigue, 1965; Guinet, 1969; Caccavari, 1970). In addition, detailed information regarding the exine structure of Acacia is relatively scarce (Barth, 1965; Guinet and Lugardon, 1976; Caccavari and Dome, 2000).
In this study, new morphological and structural pollen analyses were made for a considerable number of American Acacia species (Table 2⇓); the results of these analyses are typified in order to precisely characterize the pollen and to differentiate it from other American genera of Mimosoideae with compound pollen.
In the subgenus Acacia, the exine surface is not always smooth; the frequent presence of a supramicrorugulate exine is demonstrated using SEM. This feature can also be interpretated from the TEM illustrations of A. farnesiana in Guinet and Lugardon (1976).
In the subgenus Aculeiferum, it is not possible to sustain the premise that the character of exine surface is uniquely smooth or undulate; for example, suprasubreticulate surface ornament, which has been verified from both sculpture and structural features, is very frequent.
A foot layer formed by fusion of granula in the inner stratum where it is in contact with the endexine has also been demonstrates, and this changes our understanding regarding infratectal structure and the concept of a constant presence of two granular strata. In A. catharinensis (see Barth, 1965), the presence of the foot layer is also unmistakeable.
The marked structural heteromorphism of A. bahiana (see Guinet and Lugardon, 1976) does not appear to be a very constant character.
In the section Monacanthea, the character referred to as subpseudocolpi (Caccavari and Dome, 2000) is the result of a reduction and modification of the strata of the infratectal layer, and it is generally marked by sculptural differences in the exine (Table 2⇓). This character has been observed in some species (Table 2⇓); it may also be present in additional species belonging to this subgenus, but more studies are needed to demostrate its occurrence.
In the section Filicinae, the analysis of structures provides new characters for taxonomic differentiation, including presence of perforations in the tectum (which do not extend completely through it) reduced granular infratectal layer without strata differentiation and an interrupted foot layer, a considerable thickness of the nexine (which is greater than that of the ectexine), and a predominantly suprasubreticulate exine ornamentation.
The subgenus Phyllodinae (Heterophyllum in Vassal 1972), with polyads frequently characterized by the ornamentation of the suprasubreticulate exine, and by quadrangular or circular pseudocolpi (the product of a reduction of the sexine) on the distal face of the pollen grains (Guinet, 1986), is not extant among American species. However, a palynomorph with these morphological characters has been reported in the Patagonian Miocene in Argentina (Barreda and Caccavari, 1992).
These observations demonstrate that an expanded knowledge of the pollen structure of Acacia results in greater potential precision for its morphological characterization. Following the concepts developed by Vezey et al. (1991) in the study of Swartzieae pollen, it is highly likely that a more complete knowledge of the structural characters (especially within the subgenus Aculeiferum) is critical to a refined definition of the the generic limits for Acacia.
The observations presented herein imply the need to modify subgeneric characterizations within the genus Acacia. Accordingly, an identification key based on morphological and structural features is detailed below.
Generic Characterization of American Acacia Pollen
Polyads are acalimate, biconvex, with 8 or 16 pollen grains, radially symmetrical, 20–51 μm in greatest diameter; or with 32 pollen grains, asymmetrical, elliptical in outline, 60–68 μm in greatest diameter. Pollen grains are orate or distally colporate, typically with proximal pores. Exine 1–3 μm thick, smooth, undulate to suprasubreticulate or supramicroreticulate on the distal face, and only a nexine with vestigial sexine on the proximal face.
The exine structure is smooth to suprasubreticulate, or supramicrorugulate ornamentation; tectum perforate, infratectal layer columellar or granular ................ Acacia
Key to the Pollen Types and Subtypes of American Acacia
|1 – Polyads colporate; colpi generaly Y-H shaped; ora in free ends of colpi; exine 2–3 μm, collumelate, with surface irregular and densely perforated. Exine structure with collumelar infratectum and a conspicuous foot layer; ornamentation smooth or supramicrorugulate||Type A|
|2 – Polyads of 16 pollen grains||Subtype A.1|
|2′ – Polyads of 32 pollen grains||Subtype A.2|
|1′ – Polyads porate; pores in distal angular pattern; exine 1–2 μm, not collumelate, with surface smooth to undulate or suprasubreticulate and scarcely perforated; exine structure with granular infratectum and granular foot layer; ornamentation smooth to suprasubreticulate||Type B|
|3 – Polyads of 16 pollen grains; exine structure with granular bistratified infratectum; ectexine thicker than endexine.|
|4 – Sculpture with uniform ornamentation on the distal face||Subtype B.1|
|4′ – Sculpture of the exine with subpseudocolpi||Subtype B.2|
|3′ – Polyads with 8 pollen grains; pollen grains of tetrads in decussate disposition; exine surface smooth to suprasubreticulate. Exine structure with granular infratectum no stratified, thiner than tectum; ectexine thinner than endexine||Subtype B.3|
We would like to thank the curators of the following Institutions for permitting the examination of specimens: Facultad de Agronomía de Maracay, Venezuela (MY), Instituto de Botánica Darwinion (SI), Instituto de Recursos Biológicos, INTA Castelar (BAB) and Museo de Ciencias Naturales “Bernardino Rivadavia” (BA). We are also grateful to Dr. Arquímedes Bolondi of the CICV-INTA Castelar Laboratory of Electronic Microscopy for the use of the laboratory facilities, and to Mr. Natalio De Vicenzo for his assistence with the Scanning Electronic Microscopy. We appreciate the critial review and suggestions of Dr. A. Graham. Thanks are also due to Drs. R. Van Pelt, E. Cushing and G. Jones for valuable comments on the paper.