Research Article |
Corresponding author: Victoria Sosa ( victoria.sosa@inecol.mx ) Academic editor: Alice Calvente
© 2019 Catalina Ruiz-Domínguez, Andrew P. Vovides, Victoria Sosa.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Ruiz-Domínguez C, Vovides AP, Sosa V (2019) Systematic relevance of pollen morphology in tribe Hylocereeae (Cactaceae). PhytoKeys 128: 121-140. https://doi.org/10.3897/phytokeys.128.35842
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Hylocereeae is one of the nine tribes in the subfamily Cactoideae (Cactaceae), for which the limits and recognition of genera have been controversial. Essentially, this group comprises epiphytic and hemiepiphytic genera with stems modified as climbing structures. The aim of this paper is to examine pollen attributes in representative species of genera of Hylocereeae, focusing on Selenicereus whose current circumscription comprises Hylocereus and three Weberocereus species, to find whether significant potentially apomorphic and/or autapomorphic systematic characters can be discovered. Utilizing SEM and light microscopy, 25 pollen characters were observed and measured. Tribe Hylocereeae is stenopalynous, with pollen grains isopolar and radially symmetrical monads, mostly tricolpate, except in Kimnachia, Pseudoripsalis and Weberocereus, whose pollen grains are pantocolpate. Seven attributes (five qualitative and two continuous) exhibited useful variation and were coded. The character of brevicolpate pollen grains was shared by Kimnachia ramulosa and Pseudorhipsalis amazonica. Convex quadrangular outline in the polar view was shared by Weberocereus tunilla and S. glaber. The absence of spinules on the exine was shared by S. minutiflorus and S. stenopterus. The largest pollen grain, found in Selenicereus megalanthus, might be correlated with polyploidy. Selenicereus is the taxon with the highest variation in pollen attributes, including species with an exine with or without spinules and variable polar area index and shape (subprolate or oblate-spheroidal).
Hylocereus, pantocolpate, Selenicereus, stenopalynous, tricolpate
Hylocereeae is one of the nine tribes in subfamily Cactoideae (Cactaceae), in which the limits and recognition of genera have been controversial (
Morphological variation in tribe Hylocereeae. A Aporocactus martianus (Photo S. Avendaño) B Disocacus ackermannii (Photo C. Ruiz) C Kimnachia ramulosa subsp. ramulosa (From Flora de Nicaragua, O.M. Montiel) D Selenicereus atropilosus (From Hunt 2006) E Epiphyllum hookeri (Photo C. Ruiz) F Pseudorhipsalis amazonica (From Mobot - Hammel 24524) G Acanthocereus tetragonus (Photo C. Gómez-Hinostrosa) H Selenicereus (Hylocereus) undatus (Photo C. Ruiz) I Weberocereus tunilla (From Mobot – B. Hammel 22442).
Five taxonomical studies have introduced classifications for tribe Hylocereeae in the last century. First, the group was considered a subtribe in tribe Cereeae by
The aim of this paper is to examine pollen attributes in representative species of genera of tribe Hylocereeae focusing on the current concept of Selenicereus that includes Hylocereus and three species of Weberocereus to find whether potentially apomorphic and/or autapomorphic character states can be discovered. Traditionally, pollen has provided valuable and significant characters in plant taxonomy (
Leuenberger (1976) compiled the most complete description of pollen morphology of 600 cactus species, and found that aperture ratio was one of the most variable and useful characters at different taxonomic levels, from subfamily to genus. In addition,
The study of the pollen of tribe Hylocereeae is part of our current project on the evolution and systematics of the Hylocereus clade. We aim to incorporate pollen characters with morphological and molecular data to better understand the limits and phylogenetic position of this group, along with phylogenetic relationships of its species, the evolution of chemical and fruit characters. In this paper, the pollen attributes of 27 representative taxa of the genera comprising tribe Hylocereeae, with a main focus in the current concept of Selenicereus (including Hylocereus and three Weberocereus species), are examined to determine whether character states are shared or are exclusive in these taxa.
Pollen grains of 27 species of tribe Hylocereeae, corresponding to 8 genera according to classifications of this group, were collected (
The acetolysis method proposed by
For the species studied, fourteen qualitative characters were coded: 1) shape of pollen grain, 2) type of polar area, 3) aperture (based in polar area index = apocolpium/ equatorial diameter in polar view), 4) outline of the pollen grain polar view (amb), 5) pollen unit, 6) pollen type (according to polar axis longitude), 7) polarity, 8) aperture class (colpate or brevicolpate), 9) number of colpi, 10) symmetry, 11) tectum (perforate or imperforate), 12) exine (tectate or semitectate), 13) exine spinules (present or absent), 14) margo (present or absent) (Fig.
Quantitative and qualitative characters from pollen grains: A, B optical Microscope Photographs (800×) A polar view: Apocolpium (A), Mesocolpium (M), Equatorial diameter in polar view (EDP), Exine length (EL), Amb, Polarity, Symmetry, Aperture length, Polar Area Index (PAI) = A/EDP B equatorial view: Equatorial diameter (ED), Polar axis (PA), Pollen unit, Aperture (number, position, character), Pollen type, Shape class (PE) = PA/ED C scanning Electron Microscope Photograph (6000x); Exine Ornamentation, Spinule length (SL), Spinule base (SB), Perforation diameter (PD).
Eleven morphological continuous pollen characters of the studied species were measured, including 1) equatorial diameter in polar view, 2) apocolpium (area delimited by lines connecting the apices of the colpi at the pole of the pollen grain), 3) mesocolpium (area delimited by lines between the apices of adjacent colpi), 4) polar axis in equatorial view, and 5) equatorial diameter in equatorial view. They were measured for a maximum of 25 pollen grains from at least three preparations of a single specimen for each species, with 800× optical zoom under a Carl Zeiss Fomi III Optical Microscope, equipped with a Cannon Power Shot G9 digital camera. Additionally, under 1250× optical zoom, 10 pollen grains were observed to measure 6) exine thickness (Fig.
A principal component analysis (PCA) was performed using the packages Factoextra and FactoMine in R (R Development Core Team 2018) to reduce the dimensionality of phenotypic variation and summarize the variables that are correlated. PCA was carried out to identify the characters that explained the greatest proportion of the variability and to identify pollen grains occupying different spaces.
The taxa studied in tribe Hylocereeae are stenopalynous, i.e. there is slight variation in pollen grains. They are isopolar and radially symmetrical monads, mostly tricolpate, with the exception of Kimnachia, Pseudoripsalis and Weberocereus, whose pollen grains are pantocolpate, with 12 to 15 colpi.
In the following paragraphs detailed descriptions of the pollen grains are provided.
trizonocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: varies from subprolate to oblate-spheroidal (P/E=0.97–1.29). Apertures: 3, colpate, large; polar area of medium size (PAI=0.31–0.40). Measurements: pollen grains of large size (49.02)56.81(68.53) × (49.89)63.62(79.4) μm in equatorial view; exine thickness (2.41)3.29(4.19) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (1.05)1.43(1.76) μm length × (1.08)1.37(1.83) μm diameter in base; perforations (0.17)0.25(0.33) μm in diameter (Figure
Pollen grains of tribe Hylocereeae (Scanning Electron Microscope photographs). Left polar view, right equatorial view A Acanthocereus tetragonus (C. Ruiz et al. 576) B Disocactus ackermanii (R. Torres et al. 309) C Epiphyllum oxypetalum (C. Ruiz et al. 640) D Selenicereus costaricensis (C. Ruiz et al. 555) E S. escuintlensis (C. Ruiz et al. 635) F S. guatemalensis (M. Véliz et al. 20227) G S. minutiflorus (C. Ruiz et al. 627) H S. ocamponis (M. Cházaro 7334) I S. polyrhizus (C. Ruiz et al. 566) J S. sp. (C. Ruiz et al. 608).
Acanthocereus tetragonus (L.) Hummelinck (Colombia, Valle del Cauca. C. Ruiz et al. 576 CUVC); Acanthocereus chiapensis Bravo (México, Chiapas. C. Gómez-Hinostrosa et al. 2325 MEXU).
trizonocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: subprolate (P/E=1.15). Apertures: 3, colpate, large; polar area of medium size (PAI=0.36). Measurements: pollen grains large to very large, (75.98)93.62(110.47) × (80.52)106.91(117.69) μm in equatorial view; exine thickness (3.16)3.66(4.34) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (1.31)1.64(1.93) μm length × (1.18)1.53(2.10) μm diameter in base; perforations (0.14) 0.23(0.34) μm in diameter.
Aporocactus martianus (Zucc.) Britton & Rose. (México, Veracruz. H. Narave et al. 308 XAL).
trizonocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: varies from subprolate to prolate-spheroidal (P/E=1.13–1.18). Apertures: 3, colpate, large; polar area of medium size (PAI=0.33–0.34). Measurements: pollen grains large to very large, (80.36)97.51(116.99) × (99.25)112.18(125.04) μm in equatorial view; exine thickness (2.24)3.27(3.99) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (1.22)1.63(2.50) μm length × (1.07)1.45(2.06) μm diameter in base; perforations (0.30)0.61(0.98) μm in diameter (Figure
Disocactus ackermanii (Haw.) Ralf Bauer (México, Oaxaca. R. Torres et al. 309 MEXU); Disocactus speciosus (Cav.) Barthlott (México, Edo de México. J. Canek Ledesma 2211 MEXU).
trizonocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: varies from oblate-spheroidal to prolate-spheroidal (P/E=0.90–1.06). Apertures: 3, colpate, large; polar area of medium size (PAI=0.32–0.46). Measurements: pollen grains large, (69.33)88.76(111.93) × (74.43)86.49(105.34) μm in equatorial view; exine thickness (2.24)2.73(3.67) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (1.22)1.69(2.02) μm length × (0.97)1.26(1.57) μm diameter in base; perforations (0.32)0.51(0.65) μm in diameter (Figure
Epiphyllum oxypetalum (DC.) Haw. (Guatemala, Sacatepéquez. C. Ruiz et al. 640 BIGU); Epiphyllum thomasianum (K. Schum.) Britton & Rose (Guatemala, Sacatepéquez. C.K. Horich 572922 MEXU).
pantocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: prolate-spheroidal (P/E=1.07). Apertures: 15, brevicolpate, very large; polar area small (PAI=0.05). Measurements: pollen grains medium-sized to large, (43.95)49.14(55.73) × (48.04)52.45(58.32) μm in equatorial view; exine thickness (3.10)3.84(4.74) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (0.19)0.33(0.43) μm length × (0.34)0.45(0.55) μm diameter in base; perforations (0.09)0.18(0.35) μm in diameter (Figure
Pollen grains of tribe Hylocereeae (Scanning Electron Microscope photographs). A–D: Left polar view, right equatorial view A S. undatus (C. Ruiz et al. 560) B Weberocereus tunilla (C.K. Horich BGA 58344) C S. setaceus (H. Bravo et al. 2755). D–J: left polar view, left details of spinules and perforations D S. megalanthus (C. Ruiz et al. 563) E S. stenopterus (C.K. Horich s.n.) F S. triangularis (C. Gómez-Hinostrosa 2110) G Kimnachia ramulosa (L. Velásquez et al. 4884) H Pseudorhipsalis amazonica (E. Gudiño 145) I S. alliodorus (A. Ruiz Velazco et al. 86) J S. glaber (C.K. Horich BGA 57239).
Kimnachia ramulosa (Salm-Dyck) S. Arias & N. Korotkova (Guatemala, Suchitepéquez. L. Velásquez et al. 4884 BIGU).
pantocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: oblate-spheroidal (P/E=0.99). Apertures: 12, brevicolpate, small; polar area large (PAI=0.62). Measurements: pollen grains large, (73.59)81.71(93.64) × (76.00)81.23(86.72) μm in equatorial view; exine thickness (2.85)3.19(3.48) μm. Exine: marginated. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (0.64)0.78(0.99) μm length × (0.39)0.48(0.54) μm diameter in base; perforations (0.05)0.11(0.19) μm in diameter (Figure
Pseudorhipsalis amazonica (K. Schum.) Ralf Bauer (Ecuador, Napo. E. Gudiño 145 MEXU).
trizonocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: prolate-spheroidal (P/E=1.06–1.07). Apertures: 3, colpate, large; polar area medium-sized (PAI=0.36–0.37). Measurements: pollen grains large to very large, (66.74)87.12(110.39) × (70.79)90.62(119.77) μm in equatorial view; exine thickness (2.19)2.68(3.39) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (1.09)1.58(2.07) μm length × (0.92)1.30(1.86) μm diameter in base; perforations (0.17)0.30(0.56) μm in diameter.
Selenicereus grandiflorus (L.) Britton & Rose (México, Veracruz. Rivera-Alarcón et al. 37 XAL); Selenicereus hamatus (Scheidw.) Britton & Rose (México, Veracruz. D. Jimeno-Sevilla 1079 XAL).
trizonocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: varies from suboblate to subprolate (P/E=0.86–1.18). Apertures: 3, colpate (brevicolpate in S. minutiflorus), large (small in S. minutiflorus); polar area medium-sized to large (PAI=0.30–0.59). Measurements: pollen grains of medium to very large size, (54.52)78.67(97.83) × (45.94)83.68(102.53) μm in equatorial view; exine thickness (1.66)2.95(4.10) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules (S. minutiflorus and S. stenopterus without spinules) of (1.06)1.47(2.13) μm length × (0.72)1.23(1.53) μm diameter in base; perforations (0.20)0.39(0.89) μm in diameter (Figures
Selenicereus costaricensis (F.A.C. Weber) Britton & Rose (Colombia, Valle del Cauca. C. Ruiz et al. 555 CUVC); S. escuintlensis Kimnach (Guatemala, Escuintla. C. Ruiz et al. 635 BIGU); S. guatemalensis (Eichlam ex Weing.) Britton & Rose (Guatemala, El Progreso. M. Véliz et al. 20227 BIGU); S. minutiflorus Britton & Rose (Guatemala, Izabal. C. Ruiz et al. 627 BIGU); S. monacanthus (Lemaire) Britton & Rose (Honduras, Francisco Morazán. C. Ruiz et al. 493 TEFH); S. polyrhizus (F.A.C. Weber) Britton & Rose (Colombia, Valle del Cauca. C. Ruiz et al. 566 CUVC); Selenicereus sp. (México, Oaxaca. C. Ruiz et al. 608 XAL); S. stenopterus (F.A.C. Weber) Britton & Rose (Costa Rica, Alajuela. C.K. Horich s.n. MEXU); S. triangularis (L.) Britton & Rose (México, Yucatán. C. Gómez-Hinostrosa 2110 MEXU); S. undatus (Haworth) Britton & Rose (Colombia, Valle del Cauca. C. Ruiz et al. 560 CUVC).
trizonocolpate, radially symmetrical, isopolar with circular contour in polar view. Shape: Prolate-spheroidal to subprolate (P/E=1.01–1.16). Apertures: 3, colpate, large; polar area medium-sized (PAI=0.30–0.40). Measurements: pollen grains large to very large, (81.09)89.11(127.4) × (83.13)98.23(129.26) μm in equatorial view; exine thickness (1.94)2.92(4.56) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (1.17)1.61(2.07) μm length × (0.81)1.20(1.53) μm diameter in base; perforations (0.23)0.46(0.70) μm in diameter (Figure
S. megalanthus (K. Schumann ex Vaupel) Ralf Bauer (Colombia, Valle del Cauca. C. Ruiz et al. 563 CUVC); S. ocamponis (Salm-Dyck) Britton & Rose (México, Michoacán. M. Cházaro 7334 MEXU); S. setaceus (Salm-Dyck ex DC) Ralf Bauer (Brazil, Rio de Janeiro. H. Bravo et al. 2755 MEXU).
trizonocolpate to pantocolpate, radially symmetrical, isopolar with circular or convex-quadrangular contour in polar view. Shape: prolate-spheroidal (P/E=1.04–1.12). Apertures: 3 or 12, brevicolpate to colpate; polar area medium-sized to large (PAI=0.41–0.60). Measurements: pollen grains large to very large, (50.9)77.50(100.2) × (66.79)82.21(100.86) μm in equatorial view; exine thickness (2.29)2.96(3.77) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (1.12)1.48(1.82) μm length × (1.13)1.44(1.91) μm diameter in base; perforations (0.17)0.29(0.50) μm in diameter (Figure
Selenicereus alliodorus (México, Oaxaca. Gómez-Hinostroza & H. M. Hernández) S. Arias & N. Korotkova (A. Ruiz Velazco et al. 86 MEXU); S. glaber (Eichlam) G.D. Rowley (Guatemala, Sacatepéquez. C.K. Horich BGA 57239 MEXU).
pantocolpate, radially symmetrical, isopolar with convex-cuadrangular contour in polar view. Shape: prolate-spheroidal (P/E=1.07). Apertures: 12–15, brevicolpate, small; polar area large (PAI=0.56). Measurements: pollen grains large, (77.07)82.94(89.61) × (82.67)88.73(98.94) μm in equatorial view; exine thickness (1.96)2.43(2.85) μm. Ornamentation: smooth surface with tectum perforate, ornated with spinules of (1.24)1.41(1.58) μm length × (1.28)1.65(1.91) μm diameter in base; perforations (0.17)0.28(0.45) μm in diameter (Figure
Weberocereus tunilla (F.A.C. Weber) Britton & Rose (Costa Rica, Cartago. C.K. Horich BGA 58344 MEXU).
Of the fourteen characters examined, five were identified as variable: Amb (the outline of a pollen grain seen in polar view), colpi number, aperture type (colpate or brevicolpate pollen), marginate exine (an area of the exine around an ectocolpous that is differentiated from the remainder of the exine by difference in thickness and the presence of spinules). The other qualitative characters were not variable (Table
The size of pollen grains for the 27 taxa examined varies from 55.47 to 154.42 μm in polar axis, and this is large according to
Box plots of two coded pollen characters, which are discriminant for genus in tribe Hylocereeae. A Spinule length B perforation diameter. Boxes represent the first and third percentiles and black lines indicate median values. Character states are indicated (0, 1 and 2) for each character and measurement range is indicated. Measurements are in µm.
Qualitative and quantitative morphological characters of pollen for the representative species in the tribe Hylocereeae. Spinule length and perforation diameter are coded based on the simple gap method by
Species | PE ratio | Shape class | PAI ratio | PA type | Amb | Pollen type | Aperture | Margo | No of colpus | Spninules | Spinule length | Perforation diameter |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Acanthocereus chiapensis | 1.30 | Subprolate | 0.32 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Acanthocereus tetragonus | 0.97 | Oblate-spheroidal | 0.41 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Aporocactus martianus | 1.15 | Subprolate | 0.36 | Medium | Circular | Very Large | Colpate | A | 3 | P | 2 | 1 |
Disocactus ackermanii | 1.19 | Subprolate | 0.34 | Medium | Circular | Very Large | Colpate | A | 3 | P | 2 | 1 |
Disocactus speciosus | 1.13 | Prolate-spheroidal | 0.35 | Medium | Circular | Very Large | Colpate | A | 3 | P | 2 | 1 |
Epiphyllum oxypetalum | 1.07 | Prolate-spheroidal | 0.32 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Epiphyllum thomasianum | 0.90 | Oblate-spheroidal | 0.47 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Kimnachia ramulosa | 1.07 | Prolate-spheroidal | 0.05 | Small | Circular | Large | Colpate | A | 15 | P | 1 | 0 |
Pseudorhipsalis amazonica | 0.99 | Oblate-spheroidal | 0.63 | Large | Circular | Large | Colpate | P | 12 | P | 1 | 0 |
Selenicereus alliodorus | 1.12 | Prolate-spheroidal | 0.41 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus costaricencis | 1.18 | Subprolate | 0.38 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus escuintlensis | 0.97 | Oblate-spheroidal | 0.37 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus glaber | 1.05 | Prolate-spheroidal | 0.61 | Large | Convex quadrangular | Large | Colpate | A | 12 | P | 2 | 1 |
Selenicereus grandiflorus | 1.07 | Prolate-spheroidal | 0.37 | Medium | Circular | Very Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus guatemalensis | 1.04 | Prolate-spheroidal | 0.31 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus hamatus | 1.06 | Prolate-spheroidal | 0.37 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus megalanthus | 1.01 | Prolate-spheroidal | 0.40 | Medium | Circular | Very Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus minutiflorus | 0.87 | Suboblate | 0.60 | Large | Circular | Large | Colpate | A | 3 | A | 0 | 1 |
Selenicereus monacanthus | 1.11 | Prolate-spheroidal | 0.34 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus ocamponis | 1.17 | Subprolate | 0.31 | Medium | Circular | Very Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus polyrhizus | 1.07 | Prolate-spheroidal | 0.34 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus setaceus | 1.10 | Prolate-spheroidal | 0.34 | Medium | Circular | Very Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus sp. | 1.11 | Prolate-spheroidal | 0.35 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus stenopterus | 1.17 | Subprolate | 0.38 | Medium | Circular | Large | Colpate | A | 3 | A | 0 | 1 |
Selenicereus triangularis | 0.91 | Oblate-spheroidal | 0.45 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Selenicereus undatus | 1.04 | Prolate-spheroidal | 0.40 | Medium | Circular | Large | Colpate | A | 3 | P | 2 | 1 |
Weberocereus tunilla | 1.07 | Prolate-spheroidal | 0.57 | Large | Convex quadrangular | Large | Colpate | A | 12, 15 | P | 2 | 1 |
The PCA graph displays projections of pollen characters in a multidimensional space in which the first two components explained 62.4% of the observed variance. PC 1 explains 44% of the variance and is associated with size (equatorial diameter, polar axis, and spinule dimensions), while PC 2, which explains 18.4% of the variance, is associated with proportions (PAI and PE ratios) (Fig.
Principal Component Analysis graph. A Scatterplot displaying projections in a multidimensional space of the pollen grain characters: Apocolpium (A), Mesocolpium (M), Equatorial diameter in polar view (EDP) Exine length (EL), Polar Area Index (PAI), Equatorial diameter (ED), Polar axis (PA), Shape class (PE), Spinule length (SL), Spinule base (SB), Perforation diameter (PD). PC1 explains 44% of the variance and PC2 explains 18.4% of the variance B Sorting of the 27 species of tribe Hylocereeae, in relation to the morphometric variables of pollen grains.
Pollen grains of the representative species of the genera of tribe Hylocereeae studied here share the pollen type common to Caryophyllales: tricolpate to pantocolpate with the exine spinulose and perforate (
From the 25 pollen characters analyzed, only seven attributes (five qualitative and two continuous) exhibited useful taxonomic variation. Four genera in Hylocereeae: Epiphyllum, Acanthocereus, Disocactus, Selenicereus (comprising Hylocereus and three species of Weberocereus), and Aporocactus have pollen grains with essentially similar morphology. That being said, Disocactus and Epiphyllum form part of the Phyllocactoid clade while Selenicereus and Weberocereus form part of the Hylocereoid clade in the molecular phylogeny constructed by
Despite the fact that the majority of taxa studied here share many pollen attributes, certain characters were common to limited groups of species. By way of example, Kimnachia ramulosa and Pseudorhipsalis amazonica are the only two species included in our study that share the attributes of brevicolpate pollen grains with small apertures. Kimnachia is a recently described genus whose sole species was previously included in Pseudorhipsalis (
Two species in Selenicereus (S. minutiflorus and S. stenopterus) stand out for lacking spinules in the exine in tribe Hylocereeae. They were retrieved in the Hylocereus clade in the plastid phylogeny of
The Salmdyckia group, including S. ocamponis, S. setaceus and S. megalanthus, possesses the largest pollen grains in the genus. Of these three species, Selenicereus megalanthus had the largest pollen grains, with a pollen grain size that could be correlated with polyploidy, a process that can produce large to very large pollen grains (
Furthermore, multivariate analyses corroborated the results of discrete and qualitative characters, displaying species such as Selenicereus megalanthus, S. stenopterus, S. multiflorus and Kimnachia ramulosa as outliers in the multidimensional space. Variation in these analyses was found to be mainly associated with size (equatorial diameter, polar axis, and spinule dimensions).
Of the genera in Hylocereeae, Selenicereus in its current concept including the species previously considered in Hylocereus and three species formerly classified in Weberocereus, is the taxon with the greatest variation in pollen grains. For instance, it includes species with and without spinules in the exine, variable shape (subprolate to oblate-spheroidal), and polar area index is either small, medium or large. Moreover, the generic limits of Hylocereus and Selenicereus have changed over time (
Pollen research that concentrates on finding crucial taxonomical characters in Cactaceae has been scarce. Nevertheless, current studies in other plant groups have demonstrated their utility and that of other data sources (e.g.
The pollen attributes identified here and that are shared by a number of species belonging to different genera that have recently been segregated or grouped together, suggests that additional evidence should be gathered and new phylogenetic analyses performed to clarify boundaries. Circumscription of the genera in tribe Hylocereeae has only been carried out based on a set of molecular or morphological characters. Our project on the Hylocereus clade will include the palynological characters determined here, along with other sources of attributes such as their morphological, ecological and molecular traits.
The authors thank the two anonymous reviewers for comments that greatly improved this paper. We also thank the curators of the MEXU, XAL and BIGU herbaria for kindly allowing us to sample pollen. We are grateful to Diana Isabel Vergara and Sonia Galicia for their invaluable help in the anatomical lab and with optical microscopy; to Tiburcio Laez for SEM photographs; Carlos Gómez-Hinostrosa and María del Carmen Mandujano for their review of the manuscript; Mario Véliz, Germán Sandoval, Lilian Ferrufino, Guillermo Reina, Isabel Nicholls, Favio González, and Diego Angulo for help during field trips. We thank Bianca Delfosse for her editorial work on our English. This research was supported by CONACYT, Mexico to V. S. (PNDP2015/1023). C.R-D. was supported by a Doctorate in Science scholarship from CONACyT, Mexico (300231).
Table S1
Data type: species data
Explanation note: Qualitative and quantitative morphological characters of pollen for the representative species in the tribe Hylocereeae.