In spite of advances in describing morphological characters within Myrtaceae, the taxonomy of the family remains complex and incomplete (Barroso & Peron 1994), especially with regard to distinguishing genera (Landrum & Kawasaki 1997). Numerous studies have found that the distinction of infrafamilial groups is not necessarily clear (Candolle 1828; Schauer 1841; Berg 1855-1856; 1857-1859; Niedenzu 1893; Kausel 1966; McVaugh 1968; Schmid 1980; Johnson & Briggs 1984; Wilson et al. 2001; 2005; Lucas et al. 2005; 2007; Biffin et al. 2007; 2010; Murillo-A. et al. 2013).
Johnson & Briggs (1984) were the first to study the phylogeny of Myrtaceae using inflorescence structure. Subsequently, Wilson et al. (2001; 2005) investigated the morphology and taxonomy of Myrtaceae based on molecular data, and proposed two subfamilies, the Myrtoideae with 15 tribes, and Psiloxyloideae with two tribes. Myrteae is the largest tribe of Myrtoideae and includes all the American representatives of Myrtaceae except for Metrosideros stipularis, which is included in Metrosidereae (Lucas et al. 2007).
The traditional subdivision of Myrteae into three subtribes was based on embryo morphology: Myrtinae, with a bent/coiled embryo and small cotyledons; Myrciinae, with an embryo consisting of leaf-like folded cotyledons; and Eugeniinae, with a food-storing embryo and plane-convex cotyledons (Berg 1855-1856; 1857-1859). Lucas et al. (2007) revealed the polyphyly of Eugeniinae and Myrtinae, while Myrciinae (excluding Myrceugenia) continues to be considered monophyletic. These authors also argued that the subtribal classification reported by Berg (1855-1856; 1857-1859) does not reflect monophyletic groups. Seven informal clades generated by their molecular analysis were proposed: “Australasian group”, “Eugenia group”, “Pimenta group”, “Myrteola group”, “Myrcia group”, “Myrceugenia group”, and “Plinia group”. Additionally, a subsequent phylogenetic analysis by Murillo-A et al. (2013) indicated that the subtribes sensu Berg are not monophyletic and proposed slightly different subgroups for Myrtinae.
Although phylogenetic analyses have made considerable progress in understanding the relationships of Myrteae, morphological, anatomical and ontogenetic studies still need to be brought to bear on the phylogeny of the family. Different floral characters have been investigated with regard to the phylogeny Myrtaceae, such as studies of the gynoecium (Pimentel et al. 2014) and floral anatomy (Schmid 1972b; c; Volgin & Stepanova 2001; 2002a; b; 2004; Lopes 2008; Costa et al. 2010; Pimentel 2010; Moreira-Coneglian 2011; Pires & Souza 2011).
The inflorescences of Myrteae were found to be of the panicle type, or reductions of this, and developed to form a racemic, dichasial or single flower (Briggs & Johnson 1979; Wilson 2011). The flowers can be uni- or bisexual (Wilson et al. 2005; Wilson 2011), and usually possess numerous stamens with tetra- or bilocular anthers (Wilson et al. 2005). The ovary varies from being inferior to being semi-inferior (Wilson et al. 2005; Wilson 2011).
The main purpose of this investigation was to determine the usefulness of floral characters of five species of Myrteae (Myrtaceae), which represent five of the six South American clades of the family, for: a) understanding species morphology and taxonomy; and b) identifying character states of phylogenic relevance to the tribe.
Materials and methods
Floral buds and flowers of five species representing five clades of Myrteae (sensu to Lucas et al. 2007) were collected in Cerrado (South American savanna) of “Parque Estadual de Vila Velha - PEVV” (25º14’09”W, 50º00’17”S) and “Parque Estadual do Guartelá - PEG” (50°10”W, 24°37”S), in the municipalities of Ponta Grossa and Tibagi. Voucher specimens were deposited in the Herbarium (HUPG) of Universidade Estadual de Ponta Grossa (Tab. 1).
|Pimenta||Campomanesia adamantium||PEVV/PEG||HUPG 19421, 19751||L. Martos & A. T. O. F. Galan 24 A. T. O. F. Galan & L. Martos 66|
|Eugenia||Eugenia pitanga||PEVV/PEG||HUPG 19442, 19444||A. T. O. F. Galan & L. Martos 54 L. Martos & A. T. O. F. Galan 14|
|Myrceugenia||Myrceugenia alpigena||PEVV/PEG||HUPG 19400, 19753||A. T. O. F. Galan; N. C. Machado & L. Martos 41 A. T. O. F. Galan & L. Martos 74|
|Myrcia||Myrcia multiflora||PEVV/PEG||HUPG 19401, 19754||N. C. Machado; A. T. O. F. Galan & L. Martos 10 MRB Carmo 1809|
|Plinia||Myrciaria cuspidata||PEVV/PEG||HUPG 19454, 19781||L. Martos & A. T. O. F. Galan 21 L. Martos et al. 78|
Analyses of floral buds and flowers were done using material fixed in FAA 50 (formaldehyde, acetic acid and ethylic alcohol), following the protocol of Johansen (1940). Fixed material was embedded in historesin (Guerrits 1991), sectioned (cross- and longitudinal sections) in a rotation microtome and stained in Toluidine Blue (0.1 M acetate buffer, pH 4.7) (O’ Brien et al. 1964). Photographs were taken using a Leica EZ4D digital camera coupled to a light microscope, and subsequently processed using the software Leica Application Suite version 1.8.
Perianth - Both surfaces of the sepals have a stomatous uniseriate epidermis. Non-glandular trichomes were not found in Eugenia pitanga (Fig. 1A), but all the other species are hairy with glandular trichomes (Fig. 1B-C). Trichomes can be distinguished on both epidermal faces of Myrceugenia alpigena trichomes (Fig. 1C), while in Myrcia multiflora (Fig. 1B), Campomanesia adamantium and Myrciaria cuspidata they are restricted to the adaxial surface. Trichomes were found to be composed of one (C. adamantium) or two cells (M. multiflora and M. cuspidata), or are T-shaped (Fig. 1D) (M. alpigena). Mesophyll is parenchymatous, varying slightly in cell shape and length (Fig. 1A-C), with the presence of secretory cavities (all species) and druse idioblasts (M. alpigena and M. cuspidata). Sepal vasculature can consist of a single collateral bundle (Fig. 1B) (M. multiflora and M. cuspidata); three bundles, in which the main bundle is centrally located (C. adamantium); or several vascular bundles (E. pitanga and M. alpigena) (Fig. 1A and C).
Petals have a glabrous uniseriate epidermis, and spongy parenchymatous mesophyll (Fig. 1E-F), interspersed with secretory cavities. Druse idioblasts occur in the mesophyll of only in Myrceugenia alpigena (Fig. 1F). One collateral vascular bundle (Fig. 1F) runs through the midvein of all species.
Hypanthium - The hypanthium consists of glabrous uniseriate outer and inner epidermis, and homogeneous parenchyma with secretory cavities (Fig. 2A-B). One or two layers of parenchyma cells adjacent to the epidermis of Myrciaria cuspidata are composed of mostly elongated cells; their secretory cavities are conspicuous and account for almost 2/3 of the thickness of the hypanthium. The hypanthia of Myrceugenia alpigena and Campomanesia adamantium are much thicker than those of the other species and contain the perianth and vascular traces of the androecium (Fig. 2A). Druse idioblasts are distinguishable in M. cuspidata hypanthium.
Androecium - Different types of vascular bundles may be recognized in the filaments of the studied species, including an amphicribral bundle in Eugenia pitanga (Fig. 2C), a U-shaped bundle in Campomanesia adamantium (Fig. 2D) and Myrciaria cuspidata, and a collateral bundle in Myrceugenia alpigena (Fig. 2E) and Myrcia multiflora. Anthers are tetrasporangiate and their immature wall consists of a uniform or interrupted epidermis, an endothecium, a middle layer, and a secretory tapetum (Fig. 2F). Only in C. adamantium does the middle layer consists of only one or, in some regions, two cell layers (Fig. 2F). The occurrence of druses and a conspicuous secretory cavity is notable in the connectives of M. alpigena, M. multiflora and M. cuspidata.
Gynoecium - Ovary is globose with axillar placentation, and exhibits morphological variation among all species. Two carpels and locules are found in Eugenia pitanga, Myrcia multiflora and Myrciaria cuspidata (Fig. 3A-B), whereas Campomanesia adamantium possesses seven to nine (Fig. 3C) and Myrceugenia alpigena has three or four (Fig. 3D). The ovules vary in number and position: numerous ovules of each locule can be connected to the same portion of the middle region of the septum (E. pitanga) (Fig. 3A); many ovules per locule can be arranged in two longitudinal rows (Fig. 3C) (M. alpigena and C. adamantium); or two ovules per locule can be connected to the same portion of the basal third of the septum (the other species).
The ovary is characterized by having uniseriate outer epidermis, which consists of squared, rounded to cylindrically-shaped cells in cross-section. The epidermis is glabrous (Fig. 3A-C), but Myrceugenia alpigena shows T-shaped trichomes (Fig. 3D). Mesophyll of Eugenia pitanga and Myrciaria cuspidata is composed of homogeneous parenchyma, whereas Myrcia multiflora has radially elongated cells beneath the epidermis. Collenchyma and parenchyma were observed in the mesophyll of M. alpigena, while three tissue regions can be recognized in the mesophyll of C. adamantium, including an outer region with tangentially elongated cells near the outer epidermis, a middle region composed of large cells, and an inner region composed of short cells. Secretory cavities are always present in the mesophyll, usually near the outer epidermis (Fig. 3A-C); idioblasts with druses and monocrystals are found only in M. multiflora and M. alpigena.
The septum of the ovary of Eugenia pitanga, Myrcia multiflora and Myrciaria cuspidata consists of glabrous uniseriate epidermis and parenchyma with vascular traces of the ovules, and transmitting tissue. The septum of Campomanesia adamantium and Myrceugenia alpigena is nearly the same as in the other species, although it is narrower (Fig. 3C-D). Especially notable is the compitum (Fig. 3A), which interrupts the septum, thereby communicating the locules.
A more detailed analysis of the serial sections of the ovaries of the studied species indicates that in all species the compitum (Fig. 3A), characterized by the interruption of the septum, consists of a slit communicating the locules and having a aperture of variable size. The position in which the cleft becomes evident is related to the distribution of ova in the ovary, being median in Eugenia, from the median region to the apex in Campomanesia and Myrceugenia and basal in Myrcia and Myrciaria (Tab. 2).
|Species/ Characters||Sepal vasculature||Petal vasculature||Number of carpels||Ovary vasculature||Number of ovules/locule||Degree of rift opening in compitum||Secretory cavity in the connective|
|E. pitanga||>1||1||2-carpels||Two rings of bundles (the outer with greater caliber)||>2 ovules/locule||Septum filled by transmitting tissue||Absent|
|C. adamantium||>1||1||>2-carpels||Three rings of bundles (the central with greater caliber)||>2 ovules/locule||Reduced rift||Absent|
|M. alpigena||>1||1||>2-carpels||Two rings of bundles (the outer with greater caliber)||>2 ovules/locule||Reduced rift||Present|
|M. multiflora||1||1||2-carpels||One ring of bundle||2 ovules/locule||Wide rift||Present|
|M. cuspidata||1||1||2-carpels||Two rings of bundles (the outer with greater caliber)||2 ovules/locule||Wide rift||Present|
Ovary vasculature of E. pitanga, M. alpigena and M. cuspidata consists of two bundle rings, the outer one with comprised of collateral and/or bicollateral bundles interspersed with smaller bundles, and the inner ring comprised of small bundles (Fig. 3A, D). In C. adamantium there are three rings of collateral vascular bundles, in which the main bundles run into the middle region of the ovary wall (Fig. 3C). The M. multiflora ovary shows a single ring of collateral vascular bundles (Fig. 3B).
The style is solid with a single strand of transmitting tissue, (Fig. 3C, G) although a central rift or intercellular spaces in the tissue of M. multiflora (Fig. 3E) has been observed. Beneath the uniseriate epidermis of the style (Fig. 3E-G) there is parenchyma with secretory cavities (Fig. 3F-G) and druse idioblasts (Fig. 3E-F). The style of M. alpigena resembles that of the other species with the exception that it also exhibits collenchyma near the surface. The most remarkable histological distinction among the styles is the number and type of vascular bundles: E. pitanga, M. multiflora and M. cuspidata have two collateral bundles (Fig. 3E), M. alpigena has six to eight collateral bundles (Fig. 3F), whereas C. adamantium has seven or nine collateral, amphicribral or U-shaped bundles (Fig. 3G).
Green sepals may be structurally similar to foliage leaves (Fahn 1990; Endress 1991). The latter are isobilateral or centric in Myrtaceae (Metcalfe & Chalk 1957), but contrary to interpretation of the green calyx by Fahn (1990) and Endress (1991), the sepals analyzed here have homogeneous mesophyll. Based on the literature, Endress (1991) refers to a widely accepted hypothesis that the sepals of angiosperms are evolutionarily derived from bracts and the petals from stamens, with the sepals being vascularized by three traces and the petals by one.
Compared with the results of previous studies of Myrteae (Volgin & Stepanova 2001; 2002a; b; 2004; Lopes 2008; Moreira-Coneglian 2011; Pires & Souza 2011), the sepal vasculature of the species investigated here exhibit a tendency for reduction to just one vascular bundle, based on the phylogeny of Lucas et al. (2007). The petal vasculature of all of the species analyzed herein have one vascular bundle, which is very different from the 11 vascular bundles found in petals of Eugenia uniflora (Lopes 2008). If considered exclusively, perianth vasculature is insufficient for forming phylogenetic hypotheses.
Davis (1966) defined four types of anther wall formation, with the Dicotyledoneous and Monocotyledoneous types possessing four layers, both derived from the Basic type by suppression of the periclinal divisions in the inner and the outer secondary parietal layers, respectively. Dahlgren (1991) reported the Dicotyledoneous type for Myrtales (Myrtaceae), but ontogenetic studies are needed to determine the type of anther wall formation in the species investigated species here.
Secretory cavities of the anther wall do not occur in Eugenia pitanga (Eugenia Group), are absent or, if present, their number can vary in the species belonging to the Pimenta Group, as observed for Campomanesia adamantium in the present work and by Landrum & Bonilla (1996) for this species and 298 other species of Myrtinae. In Myrceugenia, Myrcia and Myrciaria, secretory cavities were always present, as observed in Myrceugenia alpigena, Myrcia multiflora and Myrciaria cuspidata. The results of the present study, combined with data from other studies such as Landrum & Bonilla (1996), suggest a tendency towards the maintenance of secretory cavities in the anther in the most derived clades of Myrteae sensuLucas et al. (2007).
Most species of Myrtaceae have flowers with mostly inferior ovaries (Schmid 1972a), but Wilson et al. (2005) describe species with half-inferior and superior ovaries. Half-inferior ovaries are present in Myrteae, particularly in Luma apiculata (Belsham & Orlovich 2003), Myrcia rosangelae (Lughadha et al. 2012) and Myrciaria floribunda (Pimentel et al. 2014). On the other hand, the species studied here along with other species of Myrteae (Lopes 2008; Pimentel 2010; Moreira-Coneglian 2011) have inferior ovaries. In contrast to M. floribunda (Pimentel et al. 2014), which has half-inferior ovaries, the ovaries of M. cuspidata are inferior, indicating variation in ovary position within the genus.
Bicarpellate and biloculate ovaries are present in Eugenia pitanga, Myrcia multiflora and Myrciaria cuspidata, while there is a variation in the locule/carpel number in the other studied species: three to four in Myrceugenia alpigena and seven to nine in Campomanesia adamantium. The range of diversity in locule/carpel number is especially pronounced in the gynoecium of other species of the same genera, such as having two locules (Eugenia punicifolia) (Silva & Pinheiro 2006), three locules (Myrcia venulosa), six to seven locules (Campomanesia pubescens) and two to six locules (Psidium) (Bünger et al. 2012).
Pimentel et al. (2014) found that most South American species have two carpels and an apical septum in the ovary, whereas exceptions regarding the number of carpels have been documented in Myceugenia myrcioides (three), Ugni molinae (three) and Psidium (four). In agreement with these data, it is suggested that the evolution of South American clades involved a decrease in carpel number. Considering the condition of three carpels to be a plesiomorphic feature; one carpel could have been lost in Blepharocalyx salicifolius, the “Eugenia” group and the common ancestor of the “Myrcia” and “Plinia” groups. The presence of more than three carpels was only observed in the “Pimenta” group.
The reduced number of ovules per locule in the species investigated here reinforces the hypothesis of an evolutionary trend towards a reduction in the number of ovules. This was suggested by Lucas et al. (2007) in explaining variation in the number of locules in Myrteae, from unilocular locules in Rhodamnia to up to 20 locular in Campomanesia, while in the “Myrcia group” and some genera of the "Plinia group" two ovules/locule are found. Based on this, Lucas et al. (2007) concluded that the optimization of this character indicates that ancestral species of Myrteae had between 12 and 70 ovules.
Another interpretation of ovule number is that the report of up to six ovules in the Neomitranthes/“Plinia” group (Landrum & Kawasaki 1997) constitutes strong evidence in support of the statement by Lucas et al. (2007) that: the ovule number is either a polymorphic character or clearly represents a reversal of this character within the group “Plinia”, because this genus has a terminal phylogenetic position. Pimentel et al. (2014) provided support for the close relationship of the “Plinia” and “Myrcia” groups, while Vasconcelos et al. (2015) noted the same strongly incurved pre-anthetic stamen position.
All five species of Myrteae have a compitum in the ovary, which is a frequently used character among species of Myrtaceae that have axile placentation (Lughadha & Proença 1996). Carr & Carr (1961) considered the compitum (eu-syncarpous ginoecium) as a connection between the carpels that allows pollen tubes from grains germinating on any stigma or part of the stigma to fertilize ovules belonging to more than one carpel. Armbruster et al. (2002) found eu-syncarpous gynoecia for all species of Myrtaceae they analyzed, however, there is variation in the degree of development of the compitum.
Harthman (2016) proposed a classification of compitum types for the 21 species investigated by her based on the degree of its development, which varied from the presence of a slit to its absence (filled by transmitter tissue), not observing a standard on the phylogeny of Lucas et al. (2007). However, the results of the present study demonstrate that all species have a compitum consisting of a crevice, and what varies is their degree of opening. Furthermore, there seems to be a relationship between the position of the crevice and the distribution of the ovules in the ovary. These differences in the interpretation of these characters highlight the need for more careful investigation.
The origin of the inferior ovary, which was found in the species of Myrteae analyzed here, is a very important question from a phylogenetic point of view, with the established theories in the literature being the appendicular or receptacle theories (Dickison 2000). The vascular supply of the inferior ovary may be a good tool for the morphological interpretation of the inferior ovary, with the presence of recurrent vascular bundles strongly supporting the receptacle origin of the inferior ovary (Roth 1977). The present study suggests an appendicular origin of the inferior ovary for Myrceugenia alpigena, Myrcia multiflora and Myrciaria cuspidata.Pimentel et al. (2014) proposed the appendicular origin of the inferior ovary for Syzigium, Blepharocalyx salicifolius and the “Myrcia” and “Plinia” groups, and also suggested that the common ancestor of Myrteae had an inferior ovary of receptacular origin. Harthman (2016) also reported the appendicular origin of the inferior ovary in another 10 species belonging to the Eugenia, Myrcia, Myrceugenia, Plinia and Pimenta groups proposed by Lucas et al. (2007).
Solid styles were found in four of the species of Myrteae studied here, while Myrcia multiflora was found to possess a reduced stylar canal. Endress (1991) introduced a very detailed classification of the pollen tube transmitting tract for Angiosperms, according to which the tract of the four species of Myrteae may be considered to be of the type defined as “Epidermis weakly secretory, path of pollen tubes internal in cell walls of the transmitting tissue (transmitting tissue several cell layers thick)”.
On the other hand, according to Endress (1991), M. multiflora may be characterized as having the type described as “Stylar canal with small lumen and small inner surface”. Endress (1991) also comments that a gynoecium with many ovules, (observed in Eugenia pitanga, Myrceugenia alpigena and Campomanesia adamantium) tends to have a several-layered tract.
Comparative analysis of the flowers of the species of Myrteae examined and described here (Tab. 2) point to characters, such as the histology of sepal, petal and floral vasculature, and the degree of compitum opening, that if investigated in a large number of species of the tribe can reveal evolutionary patterns.
Considering the contribution of morphology to the phylogenetic study of Myrtaceae, a notable character is the floral vascular supply, which may be of great utility for the morphological interpretation of the inferior ovary since the main argument in favor of this character is the presence of recurrent vascular bundles. This seems to be a promising character, worthy of further exploration for a better understanding of the relationships among the different clades/groups of Myrtaceae.
This work also demonstrated the presence of a compitum consisting of a crevice in the five studied species, however, the variable degree of opening seems to be related to the position of the crevice, as well as the distribution of ovules in the ovary.