Pollen morphology and its taxonomic significance in the genus Bomarea Mirb. (Alstroemeriaceae) - I. Subgenera Baccata, Sphaerine, and Wichuraea

Acta Botanica Brasilica 29(3): 425-432. 2015. doi: 10.1590/0102-33062015abb0077 Pollen morphology and its taxonomic significance in the genus Bomarea Mirb. (Alstroemeriaceae) – I. Subgenera Baccata, Sphaerine, and Wichuraea Abul Khayer Mohammad Golam Sarwar1, 2*, Yoichiro Hoshino1 and Hajime Araki1 Received: April 7, 2015. Accepted: May 19, 2015 ABSTRACT Pollen morphology of 24 of the 33 species of three Bomarea subgenera, Baccata, Sphaerine, and Wichuraea, was examined by light microscopy (LM) and scanning electron microscopy (SEM), or SEM alone. The studied species of Bomarea were stenopalynous, characterized by large, monosulcate monads with reticulate exine sculpture in most species. Opercula-like structures were present on the sulcus in B. huanuco and B. involucrosa. Differences in pollen size, exine thickness, and exine sculpture were observed. The studied taxa were divided into four major groups based on exine ornamentation observed under SEM: microreticulate, reticulate, coarsely rugulate, or finely rugulate-perforate. Pollen characters alone did not appear to correlate clearly with the current subgeneric classification of Bomarea, but they may have some taxonomic utility below the subgeneric level. The most reliable infrageneric classification of Bomarea can be achieved through combined analyses of morphological, palynological, and molecular data from larger samples of specimens of all the species. Keywords: Bomarea, exine sculpture, infrageneric classification, opercula-like structures, scanning electron microscopy Introduction Bomarea is the most diverse genus of Alstroemeriaceae, with 100–120 species (Neuendrof 1977; Alzate 2005) distributed primarily in the Andean and Austroamerican regions, but found from Mexico and the Caribbean to Chile and Argentina (Sanso & Xifreda 1995). The genus includes climbing, erect, or prostrate herbs with persistent, often resupinate leaves and rhizomes, umbellate inflorescences, free perianth segments, six stamens, an inferior ovary, and elongate and anacolpate pollen (Alzate 2007). The characteristic large inflorescences of Bomarea represent an important food source for birds and insects in the Andean highlands (Sanso & Xifreda 1995); the starchy storage roots of several species of Bomarea are also used as food (Bayer 1998). The circumscription and taxonomic relationships between the two largest and closely related genera of the family Alstroemeriaceae, Alstroemeria and Bomarea, have also been controversial, as some species have been published under both generic names with different epithets (Sanso & Xifreda 1995; Hofreiter & Tillich 2002). Along with other characters, the differences in pollen grain wall structure and exine ornamentation (sculpture) are found 1 to be valuable characteristics for distinguishing between Alstroemeria and Bomarea (Sanso & Xifreda 2001; Sarwar et al. 2010), and the close relationship between these two genera has also been confirmed by molecular data (Aagesen & Sanso 2003). Based on morphological features, the genus Bomarea is traditionally divided into three to four subgenera: Baccata (five spp.), Bomarea s.str. (c. 79 spp.), Sphaerine (12 spp.), and Wichuraea (18 spp.) (Hofreiter & Tillich 2002). Recently, the subgeneric circumscriptions of Baccata (Hofreiter 2008), Sphaerine (Hofreiter 2006), and Wichuraea (Hofreiter & Tillich 2003) have been revised. However, three clades within Bomarea have been identified by Alzate et al. (2008), using the genomic regions nrDNA ITS, psbA-trnH, rpoB-trnC, and matK, which conflicts with the traditional subgeneric classification of Bomarea based on morphology and biogeography. Erdtman (1952, p. 44) described the pollen grains of Bomarea as “1-sulcate, large (longest axis 75–100 μm), usually distinctly plane-convex with sulcus on the convex part of grains, sexine scrobiculoidate; sexine is thicker than nexine.” Pollen morphology of Bomarea has also been included in some taxonomic papers and/or as a part of regional flora (Elsik & Thanikaimoni 1970; Heusser 1971; Neuendorf Field Science Center for Northern Biosphere, Hokkaido University, Sapporo 060-0811, Japan Department of Crop Botany, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh * Corresponding author: 2 Abul Khayer Mohammad Golam Sarwar, Yoichiro Hoshino and Hajime Araki 1977; Schulze 1978; Kosenko 1994; Rudall et al. 2000; Rojas & Gutiérrez 2001; Sanso & Xifreda 2001; Hofreiter 2006; Alzate 2007). As a part of the comprehensive pollen morphological survey on the family Alstroemeriaceae, pollen morphology of the genus Alstroemeria has already been published (Sarwar et al. 2010). We present herein pollen morphological features of three subgenera, viz. Baccata, Sphaerine, and Wichuraea, of the genus Bomarea to search for new characters that could add information pertinent to infrageneric classification of this genus. The evolutionary trend in palynological features has also been discussed in light of the molecular phylogenetic relationships of Bomarea (Alzate et al. 2008). Materials and Methods Pollen morphology of a total of 24 species of three subgenera of the genus Bomarea, i.e., Baccata Hofreiter (three spp. out of five, Hofreiter 2008), Sphaerine (Herb.) Baker (nine spp. out of 12, Hofreiter 2006), and Wichuraea (M. Roemer) Baker (12 spp. out of 16, Hofreiter & Tillich 2003), was examined by means of light microscopy (LM) and scanning electron microscopy (SEM), or SEM alone (Tab. 1). Polliniferous materials used in this investigation were taken from dried specimens from the herbaria MO, USM, K, and MOL. Abbreviations of the herbarium names are according to the Index Herbariorum (Thiers 2007). The preparation of pollen grains for LM and SEM, and pollen parameters studied follow Sarwar et al. (2010). Pollen slides of all collection are deposited at the Hokkaido University Museum, Sapporo, Japan. The measurements are based on at least 30 randomly selected grains from each specimen (Tab.2). Pollen size and shape classes were made following Erdtman (1952) and descriptive terminology follows Punt et al. (2007). Results The pollen grains of Bomarea species investigated were monad, large, ellipsoid (boat-shaped), heteropolar; monosulcate, sulcus on the convex part of the grains, distinct, long, straight, wide at the equator, narrow near the poles, sometimes extended to the proximal pole (Fig. 1A); auricula-like structures at the end of sulcus were observed in B. brachysepala, B. bracteolata, B. glaucescens, and B. huanuco (Fig. 1B; Tab. 2), opercula-like structures were present on the sulcus in B. huanuco and B. involucrosa (Fig. 2A-B). Symmetry was bilateral. Sizes ranged from 27.88–52.70 μm (polar length P) × 44.46–79.19 μm (equatorial diameter E), P/E 0.51–0.68, oblate in shape, exine thickness 1.08–2.39 μm (Tab. 2). Table 1. List of Bomarea taxa used in this study along with their voucher specimens. No. Taxa Voucher specimens Bomarea subg. Baccata (5 spp.) Panama: Ridge (...truncated)