Classification of grass pollen through the quantitative analysis of surface ornamentation and texture

Abstract

Taxonomic identification of pollen and spores uses inherently qualitative descriptions of morphology. Consequently, identifications are restricted to categories that can be reliably classified by multiple analysts, resulting in the coarse taxonomic resolution of the pollen and spore record. Grass pollen represents an archetypal example; it is not routinely identified below family level. To address this issue, we developed quantitative morphometric methods to characterize surface ornamentation and classify grass pollen grains. This produces a means of quantifying morphological features that are traditionally described qualitatively. We used scanning electron microscopy to image 240 specimens of pollen from 12 species within the grass family (Poaceae). We classified these species by developing algorithmic features that quantify the size and density of sculptural elements on the pollen surface, and measure the complexity of the ornamentation they form. These features yielded a classification accuracy of 77.5%. In comparison, a texture descriptor based on modelling the statistical distribution of brightness values in image patches yielded a classification accuracy of 85.8%, and seven human subjects achieved accuracies between 68.33 and 81.67%. The algorithmic features we developed directly relate to biologically meaningful features of grass pollen morphology, and could facilitate direct interpretation of unsupervised classification results from fossil material.

Pollen-vegetation richness and diversity relationships in the tropics

“…The loss of diversity information in the fossil record demonstrates the need for the development of improved taxonomic tools if we are going to be able to use fossil pollen to provide an accurate window into past diversity change, e.g. Mander et al (2013) and Julier et al (2016).…”

On the Taxonomic Resolution of Pollen and Spore Records of Earth’s Vegetation

“…This "diffraction limit" prevents conventional optical microscopes from imaging morphological features of sporomorphs that are less than 200-250 nm in size, and this is critical in the case of grass pollen. This is demonstrated by SEM images of grass pollen, which reveal that pollen morphology across Poaceae is characterized by a diverse suite of surface ornamentation patterns (Andersen and Bertelsen 1972;Page 1978;Peltre et al 1987;Chaturvedi et al 1998;Mander et al 2013;fig. 6).…”

“…This idea is highlighted by the example of grass pollen, which is a classic case of low taxonomic resolution (Mander et al 2013). Grass pollen is spheroidal and monoporate and when viewed with bright-field microscopy possesses a faint scabrate surface ornamentation (Wodehouse 1935;Faegri et al 1992;Beug 2004).…”

“…Low taxonomic resolution, the classification of sporomorphs above the rank of species, remains a key obstacle in both Quaternary and pre-Quaternary palynology (Birks and Birks 2000;Jackson and Booth 2007;Mander 2011;Punyasena et al 2011Punyasena et al , 2012May and Lacourse 2012;Mander et al 2013). In this review, we aim to examine the factors that can reduce taxonomic resolution and to appraise existing ways in which researchers have approached the problem of low taxonomic resolution.…”

“…Quaternary pollen diagrams typically report fluctuations in the relative abundance of these three taxa through time rather than fluctuations in the relative abundance of their constituent species. Morphological similarity between taxa either can result from a lack of morphological information on which to base a classification, such as Poaceae (Wodehouse 1935;Beug 2004;Mander et al 2013), or can occur when the natural range of morphological variation within two or more species overlaps considerably, such as the pollen of Palaua (Malvaceae), a genus of plants endemic to the coastal deserts of Chile and Peru (Schneider et al 2009). …”