Micromorphology of the upper anthecium in Mesosetum Steud. and related genera (Poaceae, Arthropogoninae) and its taxonomic applications

Abstract

Mesosetum is a Neotropical genus with 25 species, and 21 of them occur in Brazil. Mesosetum forms a clade closely related with Keratochlaena rigidifolia and Tatianyx arnacites. The objective of this work was to establish the taxonomic value of the upper anthecium micromorphology for the group. A total of 96 specimens, representing all Mesosetum species (except M. wrightii), K. rigidifolia and T. arnacites were examined under a scanning electron microscope (SEM). PCA and UPGMA analysis were applied using the micromorphologicalcharacters. Fifithteen useful taxonomic characters were identified. We highlight the following characters to distinguish the Mesosetum species: the presence, type and distribution of papillae on the abaxial surface of the upper lemma; the presence of unicellular macrohairs and bicellular microhairs; presence or absence of silica cells and the structure of the germination lid. The present work shows that the micromorphological characters identified are important taxonomically at the specific level, allowing the species identification in many cases.

Key words
Grass; Keratochlaena; Tatianyx; PCA; UPGMA

Introduction

Mesosetum Steud. belongs to the subfamily Panicoideae, tribe Paspaleae. It is inserted in the subtribe Arthropogoninae, along with 15 other American genera (Morrone et al. 2012). Within Arthropogoninae, Mesosetum forms a clade with Tatianyx Zuloaga & Soderstr. and Keratochlaena Morrone genera. as observed in many phylogenies (Giussani et al. 2001; Aliscioni et al. 2003; GPWG 2011; Morrone et al. 2012).

Swallen (1937) proposed 33 binomials for Mesosetum, but only 25 were recognized as valid taxa by Filgueiras (1989). The genus has a predominantly Neotropical distribution, and Brazil is the centre of diversity with 21 species, being 12 endemic to the country (Filgueiras et al. 2014). Most of these species occur in Central Brazil, where a total of 16 species occur, being seven endemic to this region (Filgueiras et al. 2014).

Mesosetum is characterized by raceme-like solitary terminal inflorescence, spikelets with the first glume adaxial to rachis, glume and lower lemma with tuffs of hairs and caryopsis with linear hilum (Filgueiras 1986, 1989). All these characters are homoplastic because they occur in other genera of the tribe.

There is controversy in the infrageneric classification of the genus Mesosetum. Chase (1911) was the first to establish a morphological delimitation of the genus and proposed two sections: Mesosetum sect. Mesosetum and .. sect. Bifaria (Hack.) Chase. Swallen (1937) revised the genus and recognized 33 species and added four new sections to those proposed by Chase (1911): .. sect. Annua Swallen, .. sect. Penicillata Swallen, .. sect. Loliiformia Swallen, .. sect. Schlerochloae Swallen and .. sect. Fulva Swallen (latter equivalent to M. sect. Mesosetum). Pilger (1940) partially accepted the work of Swallen, recognizing 30 species and only three sections: .. sect. Bifaria, M. sect. Eumesosetum Pilg. and .. sect. Schlerochloae. Filgueiras (1986, 1989) recognized 25 species distributed in five sections. This author included .. sect. Annua in .. sect. Loliiformia, and transferred .. sect. Fulva (containing the type species of the genus) to .. sect. Mesosetum.

Investigations on the ornamentation of the upper anthecium has revealed important taxonomic information in the Panicoideae (Mejia-Saules & Bisby 2003; Giraldo-Cañas 2004), specially in the delimitation of genera and species phylogenetically related to Mesosetum, e.g. Apochloa Zuloaga & Morrone, Canastra aristella (Döll) Zuloaga & Morrone (under Panicum aristellum Döll), Cyphonanthus Zuloaga & Morrone, Homolepis Chase, Keratochlaena (under Sclerochlamys, nom. illeg.), Oncorachis Morrone & Zuloaga, Stephostachys Zuloaga & Morrone, Streptostachys Desv., and Tatianyx (Zuloaga & Sordestrom 1985, Morrone & Zuloaga 1991; Morrone et al. 2007; Sede et al. 2008, 2009; Zuloaga et al. 2010). Nevertheless, there are no studies on the upper anthecium ornamentation using scanning electron microscopy (SEM) in Mesosetum species.

While Mesosetum is apparently well circumscribed from the taxonomic and nomenclatural viewpoint, the identification of specimens is difficult. The identification keys and descriptions of many species are based on a small number of specimens available at the time of the last two revisions (Swallen 1937; Filgueiras 1989), which does not represent the range of the morphological variability of the taxa. The present work analyses the upper anthecium micromorphological characters using SEM in Mesosetum and related species to verify their taxonomic value, if these characters support the consistency of the Mesosetum sections proposed by Filgueiras (1989), and if there are common character states between the clade MesosetumTatianyx - Keratochlaena.

Materials and Methods

This work is based on the upper anthecium micromorphology of 24 Mesosetum species, with a total of 93 specimens analyzed. plus one of Keratochlaena rigidifolia (Filg., Morrone & Zuloaga) Morrone & Zuloaga and two of Tatianyx arnacites (Trin.) Zuloaga & Soderstr. The samples were obtained from herbarium specimens. The specimens examined are listed in Table 1 and the herbaria acronyms are according to Thiers (2014, continuously updated). The dataset included all the Mesosetum species except M. wrightii Hitchc., endemic to Cuba, which had very limited collections available, and two other related genera: Tatianyx and Keratochlaena.

Table 1
Specimens examined for micromorphological characters from upper anthecium. * According to Filgueiras (1989)

Species	Section*	Collection Site	Voucher	Herbarium
Mesosetum agropyroides Mez	Penicillata	s.l. - GO	Glaziou s.n.	K
Mesosetum alatum Filg.	Bifaria	Água Boa - MT	R.C. Oliveira et al. 2765	UB
		Novo Santo Antonio - MT	H. Jancoski et al. 444	UB
		Xavantina - MT	H.S. Irwin et al. 16467	UB
Mesosetum annuum Swallen	Loliiformia	Flores de Goiás - GO	A.S. Silva et al. 269	UB
		Jerumenha - PI	A.S. Silva et al. 270	UB
		Campo Maior - PI	M.S.B. Nascimento 103	K
Mesosetum ansatum (Trin.) Kuhlm.	Bifaria	Poconé - MT	R.C. Oliveira et al. 2766	UB
Mesosetum arenarium Swallen	Loliiformia	Diamantina - MG	T.S. Filgueiras et al. 11321	IBGE
		Diamantina - MG	T.S. Filgueiras et al. 1101	IBGE
		Cristalina - GO	H.S. Irwin et al. 13337	IBGE
Mesosetum bifarium (Hack.) Chase	Bifaria	Teresina de Goiás - GO	A.S. Silva et al. 235	UB
Mesosetum blakei Swallen	Penicillata	Zelaya, Nicarágua	W.D. Stevens 8188-a	K
Mesosetum cayennense Steud.	Mesosetum	Alto Paraíso - GO	R.C. Oliveira et al. 2744	UB
		Cachoeira Paraíso - TO	R.C. Oliveira et al. 2848	UB
		Mineiros - MT	G.H. Rua 940	UB
		La Paz, Bolívia	R. Haase 630	K
		La Paz, Bolívia	C.E. White 2318	K
		Santa Cruz, Bolívia	B. Bruderreck 70	K
		Santa Cruz, Bolívia	J.R.I. Wood et al. 20791	K
		Santa Cruz, Bolívia	J.R.I. Wood et al. 20006	K
		Santa Cruz, Bolívia	J.R.I. Wood et al. 17248	K
		Santa Cruz, Bolívia	J.R.I. Wood et al. 25982	K
		Santa Cruz, Bolívia	J.R.I. Wood et al. 13516	K
		Santa Cruz, Bolívia	J.R.I. Wood et al. 26130	K
Mesosetum chaseae Luces	Bifaria	Poconé - MT	R.C. Oliveira et al. 2772	UB
		Corumbá - MS	A. Allem et al. 1152	CEN
		Corumbá - MS	A. Allem et al. 1397	CEN
		Corumbá - MS	A. Pott et al 1771	CEN
		Furnas - MG	S.C. Pereira et al. 7178	IBGE
		Boa Vista - RR	L. Coradin et al. 871	CEN
		Boa Vista - RR	L. Coradin et al. 909	CEN
		Bolivar, Venezuela	G. Davidse 22876	K
Mesosetum chlorostachyum (Döll)	Loliiformia	Rio Negro - AM	Spruce 1310	K
Chase		Rio Negro - AM	Spruce s.n.	K
		Atabapo, Venezuela	G. Davidse 16863	ESA
Mesosetum comatum Swallen	Loliiformia	Missiones, Argentina	M. Crovetto 8241	BAA
		Missiones, Argentina	M. Crovetto 9533	BAA
		Missiones, Argentina	M.E. Mulgura 3461	BAA
Mesosetum compressum Swallen	Bifaria	Pesqueira - PE	D. Andrade-Lima et al. 9438	UB
		Brejo da Madre de Deus - PE	L.M. Nascimento et al. 393	HUEFS
		Estreito, Marabá - PA	G.S. Pinheiro et al. 68	IAN
		Estreito, Marabá - PA	G.S. Pinheiro et al. 495	IAN
Mesosetum elytrochaetum (Hack.)	Bifaria	Alto Paraíso - GO	R. C. Oliveira et al. 2722	UB
Swallen		Alto Paraíso - GO	H.M. Longhi-Wagner 3934	IBGE
		Teresina de Goiás - GO	T. S. Filgueiras et al. 3229	SP
Mesosetum exaratum (Trin.) Chase	Penicillata	Jaguariaíva - PR	J.F.M. Valls et al. 11260	CEN
		Jaboticatubas - MG	T. Sendulski 443	CEN
		São José do Barreiro - SP	L. Freitas 753	CEN
Mesosetum ferrugineum (Trin.) Chase	Mesosetum	São João del Rei - MG	Glaziou 17385	P
Mesosetum filifolium F.T. Hubb.	Penicillata	Parauapebas, Pará	C.R. Martins 998	UB
		Atabapo, Venezuela	G. Davidse et al. 17209	K
		Trinidad, Belize	G. Davidse et al. 32780	K
		San Luis, Honduras	R.R. Innes 19	K
Mesosetum gibbosum Renvoize & Filg.	Loliiformia	Rio de Contas - BA	S. Atkins et al. 14783	ESA
		Lençóis - BA	J.R. Pirani et al. 7238	K
		Abaíra - BA	R.M. Harley et al. 50772	K
		Palmeiras - BA	VC. Souza et al. 5238	K
Mesosetum loliiforme (Hochst. ex Steud.) Chase	Loliiformia	Niquelândia - GO	A.S. Silva et al. 247	UB
		Brasília - DF	M.W.S. Souza 62	UB
		St. Ignatius, Guiana	R. Goodland 226	K
		Pakaraima, Guiana	T.W. Henkel 5820	K
		Kayserberg, Suriname	K.U. Kramer et al. 3003	K
Mesosetum longiaristatum Filg.	Bifaria	Teresina de Goiás - GO	R.C. Oliveira et al. 2816	UB
		São Domingos - GO	J.F.M. Valls et al. 15812	CEN
		São Domingos - GO	J.F.M. Valls et al. 15841	CEN
Mesosetum pappophorum (Nees)	Loliiformia	Flores de Goiás - GO	A.S. Silva et al. 267	UB
Kuhlm.		Flores de Goiás - GO	J.F.M. Valls et al. 15949	CEN
		Flores de Goiás - GO	J.F.M. Valls et al. 15953	CEN
		Serra Negra do Norte - RN	M.F. Simon 498	HUEFS
		Serra Negra do Norte - RN	C.G.T. Ferreira 254	UFRN
Mesosetum penicillatum Mez	Penicillata	Xavantina - MT	G. Eiten &L.T. Eiten 8541	K
		Xavantina - MT	G.D. Smith 81	K
		Beni, Bolívia	G. Beck 20682	K
		La Paz, Bolívia	R. Haase 122	K
Mesosetum pittieri Hitchc.	Sclerochloae	Chepo, Panamá	H. Pittier 4516	K
Mesosetum rottboellioides (Kunth)	Mesosetum	Grão Mogol - MG	P.A. Reis et al. 331	UB
Hitchc.		Cristália - MG	Hatschbach 55064	ICN
		Buíque - PE	D.N. Silva et al. 36	UB
		Serra Branca - PB	M.C. Pesssoa et al. 477	UB
		Atures, Venezuela	G. Davidse 15250	K
		Atures, Venezuela	G. Davidse 15428	K
		Atures, Venezuela	O. Hubber 2335	K
		Atures, Venezuela	O. Hubber 5207	K
		Atures, Venezuela	O. Hubber 5259	K
		Atabapo, Venezuela	J.J. Wurdack 43753	K
		Bolivar, Venezuela	J.J. Wurdack 39762	K
		Piar, Venezuela	O. Hubber 12154	K
		Santa Barbara, Venezuela	M.J. Eden 224	K
Mesosetum sclerochloa (Trin.)	Sclerochloae	Flores de Goiás - GO	A.S. Silva et al. 266	UB
Hitchc.		Flores de Goiás - GO	J.F.M. Valls et al. 15937	CEN
		Flores de Goiás - GO	J.F.M. Valls et al. 15947	CEN
		Flores de Goiás - GO	J.F.M. Valls et al. 15954	CEN
Keratochlaena rigidifolia (Filg., Morrone & Zuloaga) Morrone & Zuloaga	-	Loreto - MA	J.F.M. Valls et al. 8432	CEN
Tatianyx arnacites (Trin.) Zuloaga & Soderstr.	-	Abaíra - BA	W. Ganev 2728	K
		Guiratinga - MT	S.S. Silva 133	IBGE

The mature anthecia were mounted onto aluminum metal stubs using carbon double-stick tape and sputter-coated with platinum without pretreatment. The material was carried out on SEM (Jeol JSM 7001S) at 15 KV, at the Electronic Microscope Laboratories of the University of Brasilia, and “Embrapa Recursos Genéticos e Biotecnologia”. Deformed and flaccid immature anthecia were not considered. The terminology followed Ellis (1979), Sordestrom & Zuloaga (1989) and Giraldo-Cañas (2004).

Table 2 shows the descriptors used in the present work. Significant differences were not found between the lemma and palea, which presented similar ornamentation.

Table 2
Micromorphological characters and character states of the upper anthecium of Mesosetum, Keratochlaena, and Tatianyx species used in the PCA and UPGMA analyses

1. Papilla on the abaxial surface: absent(0), present(1)

2. Papilla on the germination lid surface: absent(0), present(1)

3. Papilla on the median surface: absent(0), present(1)

4. Papilla on the distal surface: absent(0), present(1)

5. Simple papilla: absent(0), present(1)

6. Compound papilla: absent(0), present(1)

7. Number of papilla per epidermical cell: one per cell(0), more than one per cell(1)

8. Coalescence between the larger papillas: absent(0), present(1)

9. Bicellular microhairs: absent(0), present(1)

10 Unicellular macrohairs: absent(0), present(1)

11. Unicellular macrohairs on the median surface: absent(0), present(1)

12. Unicellular macrohairs on the distal surface: absent(0), present(1)

13. Cruciform silica cells: absent(0), present(1)

14. Germination lid: bilobed(0), unilobed(1)

15. Germination lid lobe width/dorsal spikelet width: narrow(0), equal(1)

Each specimen measured was considered an operational taxonomic unit (OTU), following (Rohlf 2000). Cluster analysis was carried out to analyze the taxonomic value of the micromorphological characters in the genus. The similarity matrix was based on Jaccard´s coefficient and then unweighted pair group method with arithmetic averages (UPGMA) applied in the dendrogram. A cophenetic matrix was computed from the clustering matrix in order to assess the correlation between the matrix of similarity and the dendrogram. Principal component analysis (PCA) was used based on the correlation matrix (Tab. 3). All these analyses were undertaken using the software NTSYS–PC 2.0 (Rohlf 2000).

Table 3
Data matrix used in the cluster analysis of Mesosetum and related genera. Character states according to Table 2

Species/ Character states	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Mesosetum agropyroides Mez	1	0	1	1	1	1	1	0	0	1	0	1	0	1	0
Mesosetum alatum Filg.	1	0	0	1	1	0	0	0	1	1	1	1	1	1	1
Mesosetum annuum Swallen	1	1	1	1	1	1	1	1	0	0	0	0	0	0	0
Mesosetum ansatum (Trin.) Kuhlm.	1	0	0	1	1	1	1	1	1	1	0	1	0	0	0
Mesosetum arenarium Swallen	1	1	1	1	1	1	1	1	0	1	0	1	0	0	0
Mesosetum bifarium (Hack.) Chase	1	0	0	1	1	0	1	0	1	1	1	1	1	0	0
Mesosetum blakei Swallen	1	1	1	1	1	0	1	0	0	1	0	1	1	0	1
Mesosetum cayennense Steud.	1	1	1	1	0	1	0	0	1	1	1	1	0	0	0
Mesosetum chaseae Luces	1	0	1	1	1	1	1	1	1	1	0	1	0	0	0
Mesosetum chlorostachyum (Doll) Chase	1	1	1	1	1	1	1	1	0	0	0	0	0	0	0
Mesosetum compressum Swallen	1	0	0	0	1	0	0	0	1	1	1	1	1	0	0
Mesosetum elytrochaetum (Hack.) Swallen	1	0	0	1	1	1	1	0	1	1	1	1	1	0	0
Mesosetum exaratum (Trin.) Chase	0	0	0	0	0	0	0	0	0	0	0	0	0	1	1
Mesosetum ferrugineum (Trin.) Chase	1	1	1	1	0	1	0	0	1	1	1	1	0	1	1
Mesosetum filifolium F.T. Hubb.	0	0	0	0	0	0	0	0	1	0	0	1	1	1	0
Mesosetum gibbosum Renvoize & Filg.	1	0	1	1	1	1	1	0	0	1	0	1	0	1	1
Mesosetum loliiforme (Hochst. ex Steud.) Chase	1	1	1	1	1	1	1	1	0	0	0	0	0	0	0
Mesosetum longiaristatum Filg.	1	0	0	1	1	1	1	0	1	1	1	1	1	0	0
Mesosetum pappophorum (Nees) Kuhlm.	0	0	0	0	0	0	0	0	0	0	0	0	0	1	1
Mesosetum penicillatum Mez	1	1	1	1	0	1	0	0	0	1	0	1	0	1	0
Mesosetum pittieri Hitchc.	1	1	1	1	1	1	0	0	0	1	0	1	0	1	0
Mesosetum rottboellioides (Kunth) Hitchc.	1	1	1	1	0	1	0	0	1	1	1	1	0	0	0
Mesosetum sclerochloa (Trin.) Hitchc.	1	1	1	1	1	1	0	0	0	1	0	1	0	1	0
Mesosetum comatum Swallen	1	1	1	1	1	1	1	1	0	0	0	0	0	0	0
Keratochlaena rigidifolia (Filg., Morrone & ;Zuloaga) Morrone & Zuloaga	1	1	1	1	0	1	0	0	0	0	0	1	0	0	0
																Tatianyx arnacites (Trin.) Zuloaga & Soderstr.	1	0	1	1	1	1	1	0	0	0	0	0	0	0	0

Results

Micromorphology Characterization

Our results are new to Mesosetum species. The anthecia can be elliptic, lanceolate or ovate (M. pappophorum (Nees) Kuhlm.) and the apex acute to acuminate (M. sect Sclerochloae). The anthecia vary from glabrous to hairy, with bicellular microhair and unicellular macrohair short or long. The surface of anthecia is quite variable, can be totally smooth (M. pappophorum) to completely covered by papillae and silica cells. The epidermal cells vary into shape and size. Stomata (Fig. 1a) were seen in 16 species, however difficulties in the technique could hide their presence in the other species, hence they were not analyzed.

Figure 1 –
a-g. Detail of the upper anthecium in some of the species analyzed. a. Mesosetum cayennense (R. Haase 630) - stomata (arrow). b. M. filifolium (R.R. Innes 19) - silica cells. c. M. pittieri (H. Pittier 4516) - papillae. d. M. filifolium (C.R. Martins 998) - bicellular microhairs (arrow). e. M. annuum (A.S. Silva et al. 270) - papillae. f. Tatianyx arnacites (W. Ganev 2728) - papillae. g. M. chaseae (A. Pott et al 1771) - papillae. h. M. alatum (H. Jancoski et al. 444) - unilobed germination lid. i. M. elytrochaetum (R. C. Oliveira et al. 2722) - bilobed germination lid. j. upper anthecium of M. loliiforme (T.W. Henkel 5820).

Simple and compound papillae (Fig. 1a,c,e-g) were found on short and long cells, generally with 1–3 papillae per cell. In M. loliiforme (Hochst. ex Steud.) Chase, M. annuum Swallen, M. arenarium Swallen, M. chlorostachyum (Döll) Chase, M. chaseae Luces, and M. ansatum (Trin.) Kuhlm., coalescence between the larger papillae occurs in the basal and distal portions of the anthecium, forming cells with two to six coalescent papillae (Fig. 1e, g).

The silica cells were found in the cross form (Fig. 1b). They were registered in M. alatum Filg., M. bifarium (Hack.) Chase,

M. compressum Swallen, M. elytrochaetum (Hack.) Swallen, M. exaratum (Trin.) Chase, M. longiaristatum Filg., M. blakei Swallen, and M. filifolium F.T. Hubb.

The germination lid in the anthecium basal portion is very conspicuous in all species, and differences were observed in these structures (Fig. 1h, i). We identified two new character states. Unilobed germination lid is present in Mesosetum alatum (Fig. 1h), M. agropyroides Mez, M. chlorostachyum, M. ferrugineum (Trin.) Chase, M. filifolium, M. gibbosum Renvoize & Filg., and M. pappophorum. This is the typical form found in Paniceae (Rost & Simper 1975; Johnston & Watson 1981; Giraldo-Cañas 2004), but bilobed germination lid was observed in the other 17 species (e.g. M. elytrochaetum, Fig. 1i).

In the third distal portion, the presence or absence of long macrohairs aided in the distinction of the Mesosetum species. Only five species analyzed did not show a conspicuous hairy anthecium apex. This character contributed to distinguish M. exaratum from M. filifolium.

Bicellular microhairs were found in M. cayennense Steud., M. rottboellioides (Kunth) Hitchc., M. filifolium and M. sect. Bifaria. Only the panicoid type was registered (Zuloaga & Soderstrom 1985), with the basal cell smaller than the lanceolate apical cell with an acute apex (Fig. 1d, arrow).

Numerical Analysis

Figures 2 and 3 show the relationship between the analyzed species and the micromorphological characters of the upper anthecium. The UPGMA dendrogram shows three clearly separated groups (Fig. 2). A high correlation of 0.91 was found between cophenetic matrix and similarity matrix. These major groups have no correspondence to taxonomic clades, nor genera or sections of Mesosetum.

Figure 2 –
UPGMA dendrogram based on Jaccard’s similarity using micromorphological characters of the upper anthecium for Mesosetum, Tatianyx, and Keratochlaena species. *Bi. Mesosetum sect Bifaria. *Lo. Mesosetum sect Loliiformia. *Me. Mesosetum sect Mesosetum. *Pe. Mesosetum sect Penicillata. *Sc. Mesosetum sect Sclerochloae. *The classification of sections follows Filgueiras (1989). ●. Group 1. ▲. Group 2. ■. Group 3. ★. Subgroups corresponding exactly to sections proposed by Filgueiras (1989).

Figure 3 –
PCA using micromorphological characters of the upper anthecium for Mesosetum, Tatianyx, and Keratochlaena species. ●. Group 1. ▲. Group 2. ■. Group 3. ♦. Species not grouped.

The first three axes of PCA (Fig. 3) explained 71.74% of the total variation (50.95% for axis 1, 12.60% for axis 2 and 8.17% for axis 3). The characters that contributed more to the formation of the four groups were: presence or absence of papillae, quantity of papillae per epidermal cell, presence of hairs on the apex and presence of silica cells.

The absence of papillae was the most important character to distinguish M. pappophorum, M. exaratum, and M. filifolium from the other species (Fig. 2, 3). However, these species did not group.

Group 1 was composed by species of M. sect. Bifaria (Fig. 2, 3): M. alatum, M. bifarium, M. compressum, M. elytrochaetum, and M. longiaristatum. This group shares many characters, e.g. absence of papillae on the median surface, unicellular macrohairs on the median and distal surface, bicellular microhairs and silica cells.

Group 2 is comprised of M. arenarium, M. annuum, M. chlorostachyum, M. comatum Swallen. M. loliiforme, M. ansatum, M. chaseae, and Tatianyx arnacites (Fig. 2, 3). It has more than one papilla per cell, compound papillae, coalescence between the larger papillae, and hairy anthecium apex.

Group 3 is characterized by one papilla per epidermal cell and is composed of M. cayennense, M. ferrugineum, M. rottboellioides, M. agropyroides, M. gibbosum, M. penicillatum Mez, M. pittieri Hitchc.. M. sclerochloa (Trin.) Hitchc., and Keratochlaena rigidifolia (Fig. 2, 3).

Discussion

Micromorphological characters of the upper anthecium have recognized taxonomic value and are widely used in the taxonomy of the Poaceae (Thompson & Estes 1986; Filgueiras 2001; Giraldo-Cañas 2001; Oliveira et al. 2008). Our results reinforce the importance of such characters, but after analyzing several specimens some of the characters are more variable than previously expected and should be used carefully.

Sede et al. (2009) segregated the genera Oncorachis and Keratochlaena (under Sclerochlamys nom. illeg.) based on molecular data, and anatomical, morphological, and micromorphological characters. In their study, Mesosetum was reported to have a smooth anthecium, i.e. without papillae and open at the apex, based on M. cayennense and M. loliiforme. However, our results differ significantly. We analyzed populations of each of these Mesosetum species, from distinct localities (Tab. 1) and the upper anthecia was closed at the apex and covered by abundant verrucose papillae (Fig. 1j). Unfortunately, Sede et al. (2009) did not cite vouchers.

Sede et al. (2009) also reported the apex of the upper anthecium as glabrous in Keratochlaena rigidifolia (under Sclerochlamys) supporting the segregation of this species into a new genus Keratochlaena. However, we recorded prickly hairs on the upper anthecium apex in one voucher (Fig. 4a). On the other hand, a voucher of Tatianyx arnacites analyzed by Zuloaga & Sorderstrom (1985) had prickly hairs on the upper anthecium, differing from the two specimens analyzed in this current study which had a glabrous upper anthecium apex (Fig. 4d).

Figure 4 –
Apex of the upper anthecium in some of the species analyzed. a. Keratochlaena rigidifolia (J.F.M. Valls et al. 8432). b. Mesosetum annuum (A.S. Silva et al. 270). c. M. pittieri (H. Pittier 4516). d. Tatianyx arnacites (W. Ganev 2728). e. M. pappophorum (A.S. Silva et al. 267). f. M. cayennense (R. Haase 630).

Another major divergence was found in our study with that of Zuloaga & Sorderstrom (1985). In our study, T. arnacites has simple papillae, and they are localized in depressions on the epidermis surface of the anthecium (Fig. 1f). In Zuloaga & Soderstrom (1985) the papillae are compound and not included in depressions. We have observed a wide morphological variation in Tatianyx and found distinct micromorphological characters of the upper anthecium, requiring reevaluation of this species.

In Arthropogoninae, the bicellular microhairs were observed only in species of Altoparadisium Filg., Davidse, Zuloaga & Morrone, Canastra Morrone, Zuloaga, Davidse & Filg. and Homolepis (Zuloaga & Soderstrom 1985; Filgueiras et al. 2001; Sede et al. 2008). Silica cells in Mesosetum have the form of crosses, with a minimum of two grooves (Fig. 1b), differing from those encountered in other members of Arthropogoninae whose silica cells has a dumbbell (halteriform) shape (Zuloaga & Sorderstrom 1985; Filgueiras et al. 2001).

The germination lid is reported as little evident or conspicuous in Paniceae (Giraldo-Cañas 2004; Filgueiras et al. 2001). In Mesosetum all the species had a conspicuous germination lid, and we proposed two new character states for this structure, i.e. unilobed or bilobed.

According to Zucol (1998, 2000), the form of silica cells can show a minimum of two grooves, but it is more like lobes than grooves. They were mainly found in the distal portion of the abaxial surface of fertile lemma, like in the majority of Paniceae (Hsu 1935; Zuloaga & Soderstrom 1985; Giraldo-Cañas 2004).

The SEM analyses helped to distinguish between the closely related taxa. Mesosetum annuum and M. pappophorum, were previously distinguished by the length of spikelet hairs, according to Swallen (1937) and Filgueiras (1989). However, this character is variable between populations, and sometimes the overlapping of the hair length brings doubts if the species are indeed distinct. The micromorphology of the upper anthecium showed in the present study, supports the separation of these species with more certainty. Mesosetum annuum has papillae on all surfaces and a few hairs at the apex (Fig. 4b), while M. pappophorum shows a totally smooth and glabrous upper anthecium (Fig. 4e).

The character used by Filgueiras (1989) to separate M. alatum from M. bifarium (upper glume with tufts of hairs) is also inconsistent (Ribeiro 2013), when more samples are analyzed. Our results show that the best character is the germination lid. Mesosetum alatum is the only species in .. sect. Bifaria that has a unilobed germination lid and the width of this structure is equal to the dorsal spikelets, while in M. bifarium it is narrower. In addition, the anthecium has one papilla per cell in M. alatum, while it has at least three or more papillae per cell in M. bifarium (Tab. 2).

Another case where the morphological distinction was also supported by unstable characters is that of M. exaratum and M. filifolium. Swallen (1937) used the pilosity of the second glume to support their distinction, while Filgueiras (1989) put more value on the plant height and leaf blade length. Mesosetum filifolium shows silica cells and bicellular microhairs all over the upper anthecium surface, and a few macrohairs on the distal portion, while M. exaratum is glabrous and showed one silica cell in the three examined specimens.

There are two subgroups of species within the major groups of the dendrogram (Fig. 2, star) that coincide with the sections suggested by Filgueiras (1989). The species that compose the section Mesosetum adopted by this author include M. cayennense, M. rottboellioides, and M. ferrugineum and it corresponds to the subgroup identified by one compound papillae per cell. This section is characterized by spikelets with ferruginous indumentum, which is an exclusive character (Filgueiras 1989). Species of this same group are associated with Keratochlaena rigidifolia, that is the sister group of Mesosetum genus. Similarities in chromosome size and number of species from M. sect. Mesosetum were also found by Sousa (2014).

Another subgroup found in the dendrogram using the upper anthecium micromorphological characters (Fig. 2, star) corresponds to the very homogenous .. sect. Loliiformia: M. annuum, M. arenarium, M. chlorostachyum, M. loliiforme and M. comatum. This section is characterized by spikelets with an inflated asymmetric lower glume and the second glume triangular (Filgueiras 1989). The upper anthecium of this group has more than one papilla per cell and coalescent papillae.

This study endorses the importance of the upper anthecium micromorphological characters for delimitation of Mesosetum species, but the taxonomic value of these characters are reduced for higher hierarchical levels. Also a new character is described, the germination lid bilobed, which has never before been mentioned. The lack of standard descriptive terms in Poaceae and the lack of correspondence between vouchers examined and the description morphological in SEM studies are still problematic, whereas it brings difficulties to compare species and their structures. More studies that include SEM additional character are needed to achieve a stronger systematic study of this group, accurate species identification, and more comprehensive biogeographical relationships.

Acknowledgments

This study was supported in part by Protax (562340/2010-6), Reflora and CNPq Edital Universal (474880/2012-5) and Capes for the doctorate grants to the first two authors. We are grateful to Sueli Maria Gomes for her assistance with the anatomy descriptions and help during revision of this paper.

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Publication Dates

• Publication in this collection
  Apr-Jun 2015

History

• Received
  18 Sept 2014
• Accepted
  27 Mar 2015