Several alternative interpretations of the pollen records of lake sediments in the Amazonia may arise due to a lack of understanding of the modern environments, mainly related to pollen–vegetation relationship. Therefore, in this work, we studied the modern pollen rain in sediments of the Amendoim Lake, plateaus of the Serra Sul dos Carajás, southeastern Amazonia. This study indicates the predominance of pollen grains from forest formation (ombrophilous forests and capão florestal) over montane savanna in the lake sediments, despite the fact that montane savanna occupies ~90% of the drainage basin. Additionally, these sediments reflect stronger signal of pollen from ombrophilous forests that occur in the slopes of the plateau. Important components of montane savanna such as Vellozia and Cereus were not observed in the modern pollen rain, which may be related to their shorter flowering periods restricted to the beginning of wetter conditions. The absence of Sacoglottis, Trichilia, Clusia, Emmotum, Guapira and Simarouba in the pollen rain is likely associated with different pollination strategy. The occurrence of windblown pollen of Alchornea, Pseudolmedia, Ilex and Cecropia, which are found from low to highlands of the Carajás region, reinforces a regional vegetation signal in sediments. Several plants from the studied site have been improperly described, according to taxonomy and ecology. For example, the current occurrence of Ilex and Styrax in the study site clearly suggests that they cannot be used as indicators of colder palaeoclimate conditions, as previously described. In addition, Poaceae can be found in drier to wetter substrate conditions, and abundance of their pollen grains in lakes can significantly vary according to relief morphology and cannot indicate palaeovegetation openness. Therefore, this work is a good background for further palaeobotany studies for this region.
Isoetes are ancient quillworts members of the only genus of the order Isoetales. The genus is slow evolving but is resilient, and widespread worldwide. Two recently described species occur in the Eastern Brazilian Amazon, Isoetes serracarajensis and Isoetes cangae. They are found in the ironstone grasslands known as Canga. While I. serracarajensis is present mostly in seasonal water bodies, I. cangae is known to occur in a single permanent lake at the South mountain range. In this work, we undertake an extensive morphological, physiological and genetic characterization of both species to establish species boundaries and better understand the morphological and genetic features of these two species. Our results indicate that the morphological differentiation of the species is subtle and requires a quantitative assessment of morphological elements of the megaspore for diagnosis. We did not detect differences in microspore output, but morphological peculiarities may establish a reproductive barrier. Additionally, genetic analysis using DNA barcodes and whole chloroplast genomes indicate that although the plants are genetically very similar both approaches provide diagnostic characters. There was no indication of population structuring I. serracarajensis. These results set the basis for a deeper understanding of the evolution of the Isoetes genus.
The variety, relative importance and eco-evolutionary stability of reproductive barriers are critical to understanding the processes of speciation and species persistence. Here we evaluated the strength of the biotic prezygotic and postzygotic isolation barriers between closely related morning glory species from Amazon canga savannahs. The flower geometry and flower visitor assemblage analyses supported pollination by the bees in lavender-flowered Ipomoea marabaensis and recruitment of hummingbirds as pollinators in red-flowered Ipomoea cavalcantei. Nevertheless, native bee species and alien honeybees foraged on flowers of both species. Real-time interspecific hybridization underscored functionality of the overlap in flower visitor assemblages, questioning the strength of prezygotic isolation underpinned by diversification in flower colour and geometry. Interspecific hybrids were fertile and produced offspring in nature. No significant asymmetry in interspecific hybridization and hybrid incompatibilities among offspring were found, indicating weak postmating and postzygotic isolation. The results suggested that despite floral diversification, the insular-type geographic isolation remains a major barrier to gene flow. Findings set a framework for the future analysis of contemporary evolution of plant-pollinator networks at the population, community, and ecosystem levels in tropical ecosystems that are known to be distinct from the more familiar temperate climate models.
For accurate interpretations of palaeopalynological data, it is important to understand the influence of the current vegetation composition and characteristics on the formation of the palynological assemblage recorded in artificial or natural pollen traps. Therefore, in this study, modern pollen rain was characterized using artificial pollen traps under different ecological conditions and climatic seasons in Trilha da Mata Lake, Carajás North Ridge, southeastern Amazonia. The collection of pollen rain data comprises the total period from September 2015 to August 2016. There were two periods of low rainfall (September to December 2015 and May to August 2016) and one period of high rainfall (January to April 2016). The first two periods were represented by a predominance of canga vegetation pollen relative to forest pollen. Under dry conditions, the associations among the taxa Aparisthmium/Alchornea, Myrcia and Bellucia dichotoma were considered important for forest ecosystems, while those among Pleroma, Hyptis parkeri, Borreria and Perama carajensis were considered important for canga ecosystems. Under wet conditions, the forests were well represented by Schefflera, Anthurium lindmanianum, Pseudopiptadenia suaveolens and Glycydendron, and the most represented canga vegetation in the pollen rain were Poaceae undif., Miconia, M. acutistipula var. ferrea and Psychotria. Aparisthmium/Alchornea and Poaceae undif. Aparisthmium/Alchornea was related to periods of low water availability, and Poaceae undif. was associated with marshy or flooded environments. Thus, an increase in the influx of Poaceae undif. pollen grains was determined by variations in the extension of flooded areas, indicating an increase in the amount of rainfall and not the opposite case. Thus, future studies on vegetation reconstruction must consider modern pollen assemblages to precisely determine paleoenvironmental and paleoclimate conditions during sediment deposition.
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