Wendlandiella, Dammer, 1905

Stauffer, Fred W., Eychenne, Joan, Ortega, Nesly & Balslev, Henrik, 2019, Structural biology and evolution in the monotypic Amazonian palm Wendlandiella (Arecoideae: Chamaedoreeae), Candollea 74 (1), pp. 15-30 : 24-26

Main morphological features of Wendlandiella

Here we discuss the main morphological and anatomical features of Wendlandiella in the context of its ecology and evolution within the subfamily Arecoideae . The terrestrial root system of Wendlandiella displays a relatively simple architecture resembling other small palms thriving in rain forest understory conditions, either in other members of Chamaedoreeae (e.g. Chamaedorea ), as described by SEUBERT (1996), or as in Geonoma Willd. ( Geonomateae Luerss. ), a more recently diversified genus of Arecoideae (A. Briceño pers. comm.). Wendlandiella shares with most Chamaedoreeae the presence of hairy velamen, the lack of aerenchyma and the occurrence of an outer zone of the inner cortex having thin cell walls. Roots of Wendlandiella differ from other Chamaedoreeae in having a 1-layered exodermis and lacking cork and stone cells. However, additional observations are needed to confirm these apparently unique features of the genus as not all the currently recognized varieties (as defined by EYCHENNE et al., 2018) were included in Seubert’s root anatomical assessment or our study. The adventitious roots emerging from the basal region of the stems of Wendlandiella are relatively few in number and extremely thin, whereas those emerging from aerial rings on the stem are much more robust. In particular old stems of this palm may produce new plantlets directly from their aerial part, resembling the vegetative propagation by stem leaning and aerial layering described by GRANVILLE (1977) for the Amazonian palm Geonoma baculifera (Poit.) Kunth , which leads to a reiteration process that may strongly shape the spatial distribution of populations. Vegetative propagation along the stem also occurs in Hyospathe elegans Mart. ( SKOV & BALSLEV, 1989). The recent study of EDELMAN & RICHARDS (2018) included the aerial vegetative branching observed in Wendlandiella in the lateral axillary category (aerial suckering type), in which the lateral axillary branch is not limited to basal internodes. Our research did not attempt to determine the exact origin of the aerial branching in Wendlandiella (i.e. single at each node, multiple directly from the node or a developing fascicle of branches from a single nodal branch); however, this would be an interesting subject of study given the evident facilities to access the responsible vegetative meristems in these small palms.

Our study of wild populations of Wendlandiella suggests that vegetative multiplication of these palms by developing rhizomes and plantlets directly on upper regions of the stem in the mother plant may be extremely important as a vegetative mechanism of reproduction. Wendlandiella stems still attached to their parent plants may form abundant roots, without or in direct contact with the ground. This method of vegetative propagation promotes a high success rate and may benefit from the high environmental humidity predominant in the forests where the palms thrive. Indeed, the locally dense populations often observed for Wendlandiella could be at least partially explained by prolific clonal reproduction, highly contrasting with a restricted sexual multiplication in which only a relatively low number of fruits is produced in each infructescence ( LISTABARTH, 1992; C. Listabarth, pers. comm.). The prevailing vegetative multiplication observed in Wendlandiella represents a challenging case for evolutionary studies, in which intra- and inter-population genetic observations could reveal low genetic diversity as expected for predominantly clonal reproducing species such as reported in the early divergent monospecific palm subfamily Nypoideae ( JIAN et al., 2010; SUGAI et al., 2015). A better understanding of the genetic diversity is important for the development of effective conservation management programs ( ELLSTRAND & ELAM, 1993). In the case of Wendlandiella it would be particularly interesting to test current theories about whether geographically marginal populations have lower genetic diversity and higher genetic differentiation than geographically central populations.

The leaf blade morphology of Wendlandiella displays all possible transitional stages from the entire-bifid pattern found in many members of other Amazon palm genera such as Bactris Jacq. ex Scop. , Geonoma Willd. and Hyospathe Mart. , often associated with scant light conditions characterizing the lower strata of the forest, to the completely and regularly pinnate pattern observed in sun-exposed palms. Such large diversity has also been documented in the small Amazon palm Bactris simplicifrons Mart. ( STAUFFER & BRICEÑO, 2000), with which Wendlandiella grows sympatrically in some regions of the Amazon basin. Understanding the micro-ecological conditions responsible for such impressive leaf morphological diversity and its underlying genetic basis would be critical to assess the real taxonomic value currently attributed to this character in the recognition of several infraspecific taxa within a polymorphic single species. The leaf anatomy of Wendlandiella is relatively simple and fits within the patterns already proposed by TOMLINSON et al. (2011), resembling in particular the genus Synechanthus H. Wendl. , with which it seems to share an extremely simplified mesophyll, restricted to 3–4, highly chlorophyllous cell layers.

The peduncular bract is widely recognized as one of the most important protective structures of the palms inflorescence, in particular during its early stages of development. The apparently low protection offered by the single peduncular bract observed in Wendlandiella may be efficiently complemented by the protective leaf sheaths under which the inflorescence remains during early stages of development. Our study of Wendlandiella confirms the presence of only one peduncular bract in both male and female inflorescences, reinforcing the importance of this organ as a constant herbarium and field character that separates Wendlandiella from all other genera of Chamaedoreeae . According to DRANSFIELD et al. (2008) multiple peduncular bracts is the most common condition in this tribe ( Hyophorbe : 4–9, Synechanthus : 4–5, Gaussia : 4–7, and Chamaedorea : 2-several), and multiple peduncular bracts are also observed in all five genera of Iriarteeae which have 3–12 peduncular bracts. Our study of Wendlandiella confirms previous research ( UHL & MOORE, 1978; ORTEGA-CHÁVEZ & STAUFFER, 2011) describing female flowers that are solitary or arranged in pairs, whereas the male flowers are arranged in either acervuli of two alternating rows or in an unordered floral complex.

Structural biology and pollination ecology

The small size of Chamaedoreeae palms that flower not more than a few meters above the ground make them easy subjects for observing pollination, but still only a few species have been studied. Wind pollination has been reported in 18% of the world’s angiosperm families and it is the dominant pollination syndrome in some groups ( ACKERMAN, 2000). In the palm family 7% of the studied species are wind pollinated ( BARFOD et al., 2011), and animal-mediated pollination is dominant. Wind may play an important role in the reproduction of a few palm taxa (i.e. READ, 1975; HERRERA, 1989), or at least act as a supporting pollination agent ( ANDERSON et al., 1988; SCARIOT et al., 1991; BARFOD et al., 2011; RIOS et al., 2014). LISTABARTH (1992) studied the reproductive biology in populations of Wendlandiella gracilis and no visiting insects were observed. Our structural studies could not detect any secretory histological features associated with the gynoecium or the pistillode, offering further support to suggest that Wendlandiella is a wind pollinated palm. The genus Chamaedorea , also often thriving in understory conditions, on the contrary, displays a wide array of pollination syndromes, ranging from primarily wind-pollinated ( OTERO-ARNAIZ & OYAMA, 2001; BERRY & GORCHOV, 2004), to insect-pollinated ( HODEL, 1992; MORGAN, 2007). Chamaedorea pinnatifrons (Jacq.) Oerst. has insect induced wind pollination, in which insects visiting male flowers trigger pollen release in small puffs ( LISTABARTH, 1992). In more derived groups of Arecoideae wind pollination has been reported in two sympatric species of Howea Becc. , both of which are monoecious and endemic to Lord Howe Island ( SAVOLAINEN et al., 2006). Only a few studies have addressed the role of habitat or habitat fragmentation in the wind pollination mechanisms, but it appears to be more common at higher latitudes and elevations, and in open-structured and floristically less diverse vegetation ( WHITEHEAD, 1969; CULLEY et al., 2002). In Attalea phalerata Mart. ex Spreng. ( Arecoideae , Attaleinae ) in Brazil wind pollination is important when it grows in open pastures ( ANDERSON et al., 1988). Apart from the studies of LISTABARTH (1992), knowledge of wind-pollination in understory Amazon palms remains very limited. The understory conditions prevailing in rainforest may a priori be considered unsuitable for anemophily because low wind velocities caused by dense evergreen foliage, frequent rainfall and high humidity reduce pollen transport (FRIEDMAN & BARRET, 2009). However, studies in other angiosperm families (e.g. Piperaceae , Rubiaceae ) show that wind pollination in understory dioecious plants can produce high fertilization rates (MERRET & ROBERTSON, 2012). The low success of sexual reproduction in Wendlandiella may also be due to the disadvantageous interfoliar position of the inflorescences, and also its habitat in the deepest layer of the understory, where even some grasses have turned to entomophily ( LISTABARTH, 1992). The compact populations of these palms, in which dense clumps grow side by side, suggests that even weak air currents controlled primarily by microclimate factors may efficiently contribute to the pollen transport in the understory of the lowland forest.