Universidad De Los Andes Facultad De Ciencias
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UNIVERSIDAD DE LOS ANDES FACULTAD DE CIENCIAS, DEPARTAMENTO DE CIENCIAS BIOLÓGICAS TRACING BACK THE POLLEN FOSSIL RECORD OF HEDYOSMUM (C HLORANTHACEAE ), A BASAL ANGIOSPERM CAMILA MARTÍNEZ AGUILLÓN Trabajo de grado para optar al título de Biólogo Director: Carlos Jaramillo, STRI Co-director: Santiago Madriñán, Universidad de los Andes Bogotá D.C., 2009 TRACING BACK THE POLLEN FOSSIL RECORD OF HEDYOSMUM (C HLORANTHACEAE ), A BASAL ANGIOSPERM ABSTRACT The Chloranthaceae family has been described as one of the oldest lineages within the angiosperms. Hedyosmum is the only Neotropical genus and is composed of approximately 45 species. The fossil record shows that the first apparition of the genus was in the Early Cretaceous (~120 Ma), followed by a time gap of 70 Ma during the Paleogene, where no fossil records of Hedyosmum were found. It is only until the Early Miocene when a new record associated with Hedyosmum: Clavainaperturites microclavatus is found. The association was established using only transmitted light microscopy. The aim of this study was to determinate the relationship between the fossil Clavainaperturites microclavatus from the Miocene and the extant genus Hedyosmum using transmitted light microscopy, scanning and transmitted electron microscopy. The quantitative characters were evaluated with a non-metric multidimensional scaling. Given the high morphological affinity of the pollen fossil with the extant Hedyosmum it is concluded that C. microclavatus belongs to Hedyosmum, and that the radiation of the genus could have occurred in Central and South America in the Early Miocene, before to the emergence of the Panamanian Isthmus and the rising of the Andean cordillera. Two hypotheses were suggested to explain the gap in the fossil record of the genus, a change in the pollination syndrome or/and a dramatic population decrease after the K/T event. Finally as a consequence of the last hypothesis, the divergence point of the Hedyosmum could have occurred ~ 30 Ma, instead of ~ 45 Ma as was suggested before. INTRODUCTION Evidence from the fossil record indicates that angiosperms could have appeared first approximately 140 Ma in the Early Cretaceous (Brenner 1996, Wills & MacElwain 2002, Zavada 2007). Some of the earliest known fossil flowers were found approximately 127– 120 Ma in deposits from Portugal, Australia and China (Taylor & Hickey 1990, Friis et al. 1999, Sun et al. 2002). Chloranthaceae has been suggested as one of the extant families that can be related to these flowers (Walker & Walker 1984, Crane et al. 1989, Herendeen et al. 1993, Eklund et al. 1997, 2004, Friis et al. 1999:). Additionally, Clavatipollenites , a fossil-pollen genus from the early Cretaceous has close similarity to pollen of Chloranthaceae, specifically the genus Ascarina (Doyle 1969, Chapman 1987, Pedersen et al. 1991, Traverse 2007). Chloranthaceae has 75 extant species with a disjunct Old and New World tropical distribution. It is composed by four genera: Sarcandra, Chloranthus, Ascarina and Hedyosmum (Todzia 1988). The phylogenetic position of Chloranthaceae within the angiosperms is uncertain. Early studies related the family to Piperales and Laurales (Takhtajan 1980), and then separated it on a monotypic order called Chloranthales (Dahlgren, 1983). Recent analyses based on morphological homologies and molecular data, suggest that Chloranthaceae could have diverged in different positions together with Ceratophyllum, the eudicots and monocots, above the ANITA grade, and below the eumagnoliids (Graham & Olmstead 2000, Mathews & Donoghue 2000, Soltis et al. 2000, Zanis et al. 2002, Eklund et al. 2004, Qiu et al. 2006). Chloranthaceae is a monophyletic family. Molecular and morphological studies show the genus Hedyosmum to be monophyletic and sister to other Chloranthaceae, and Ascarina to be sister to Sarcandra and Chloranthus (Ekund et al. 2004) . Todzia (1988) divided the genus Hedyosmum in two subgenera: Hedyosmum Solms–Laubach and Tafalla (Ruiz & Pavón) Solms–Laubach, and five sections. New molecular studies indicate that the infrageneric classification must be recircumscribed in order to become monophyletic (Antonelli 2008). Hedyosmum comprises 44 species of shrubs and small trees (Todzia 1993) . Most species have a fairly broad distribution range, only 14 species are local endemics, and more than 50% of the species occur in the northern Andes (Todzia 1988). Hedyosmum ranges from central Mexico to central Bolivia, east to Guyana, the West Indies and has a single species in Southeast Asia ( H. orientale ). The genus is mainly located in wet habitats of cool montane cloud forest between 600 and 3,000 m (Todzia 1988). The vast majority of pollen grains of Chloranthaceae have a relatively constant reticulate sculpture, whereas, the aperture configuration displays considerable variation (Eklund et al. 2004). Hedyosmum has a star-shaped monosulcate aperture with four to six branches (Eklund et al. 2004). Ascarina has a monosulcate aperture with trichotomosulcate variants (Eklund et al. 2004). Chloranthus has polycolpate and polyforate pollen grains, and Sarcandra has polyforate pollen with scattered pores (Eklund et al. 2004). Asteropollis, a pollen genus from the Early Cretaceous was related to Hedyosmum because it was found attached to Hedyosmum -like flower from the Portuguese Flora (Friis 1994, 1999). Asteropollis is a reticulate pollen grain, with a star-shaped aperture, that was originally described in Oklahoma and was later found in central Atlantic North America, Portugal and Australia (Walker & Walker 1984, Friis 1999). After the Campanian (~73 Ma), Asteropollis has not been reported. It is only until the Miocene, Pliocene and Quaternary that fossil pollen grains referred to Clavainaperturites microclavatus are assumed to be related to extant Hedyosmum, they are reported in South America and Panama (e.g. Hoorn 1994, van der Hammen & Hooghimiestra 2000). Clavainaperturites microclavatus was described by Hoorn (1994) as a pollen grain with polar, asymmetric, inaperturate, microclavate, with medium size and subespheroidal shape. Nevertheless, the description was done using only transmitted light microscopy. A deeper evaluation of pollen characters requires scanning electron and transmitted electron microscopes to reveal minute characters and firmly establish affinities with extant species (Ferguson et al. 2007). Li-Bing Zhang and Susanne Renner (2003) used the extensive fossil record of Chloranthaceae to determinate multiple calibration points of genetic distances for the tree of the family. Hedyosmum -like flower bearing Asteropollis pollen from the Barremian- Aptian (120 Ma) and Chloranthus -like androecia from the Turonian (90 Ma) were the fossils used as alternative calibration points to reconstruct the phylogeny. The ages of the topology of the tree vary greatly depending on the calibration point used. The midpoint of these ages indicates that the initial divergence among species in the crown group of Hedyosmum started 45 Ma, whereas the crown group of Chloranthus diverged 17 Ma, and the crown group of Ascarina had its initial divergence 14 Ma. The pollination syndrome of Hedyosmum has been another matter of discussion. Given the stamen and floral morphology of the Hedyosmum -like flower from the Barremian– Aptian, wind pollination has been suggested (Friis et al. 2000). Furthermore, Asteropollis pollen occurs abundantly in dispersed palynofloras suggesting high pollen production and high dispersal pollen potential of the plant, this is often seen in wind-pollinated taxa. Nectaries or specialized food bodies were not observed in this flower (Friis et al. 2000). Extant Hedyosmum also has been described as wind pollinated (Todzia 1988) although fieldwork to test this hypothesis has not been carried out. The temporal and spatial pattern of distribution of Hedyosmum suggest that the genus could have a Laurasian and Gondwanan origin in the Early Cretaceous (Eklund et al. 2004), then spreading to North America and finally to South America in the Late Neogene (Raven & Axelrod 1974, Todzia 1988). Its radiation in South America could have been a consequence of Panama’s land bridge and/or the uplift of the Andes cordillera (Todzia 1988). The Chloranthaceae fossil record indicates that today’s genera represent the survivors of a group that during the Early Cretaceous was much more widespread and diverse in both Laurasia and Gondwana (Todzia 1988, Doyle & Donogue 1993, Kong et al. 2002, Zhang & Renner 2003, Eklund et al. 2004). This may have contributed to the incredible morphological diversity among the four extant genera, and may have made difficult to resolve the phylogenetic placement of Chloranthaceae (Zhang & Renner 200, Hansen et al. 2007). The main goal in this study is to test if Clavainaperturites microclavatus is related to extant Hedyosmum. Six extant species of Hedyosmum will be compared to the grains from Clavainaperturites microclavatus using SEM, TEM and light microscopy. Another 22 species of Hedyosmum will be analyzed using only light microscopy. The morphological variation within and among species will be quantified to assess the degree of similarity of the fossil taxa versus the putative living relatives. METHODS Pollen of 47 individuals of 28 extant species of Hedyosmum was examined for the study. A pollen-fossil, Clavainaperturites microclavatus, from three different locations was analyzed (Table 1). All the specimens were observed first using transmitted light microscopy (LM); around 30 pollen grains were measured