Guard cell sizes and ploidy levels in Polylepis (Rosaceae)

Abstract

The Andean tree genus Polylepis (Rosaceae) has recently been recognized to include polyploid species, but their occurrence within the genus is still incompletely known, especially in light of a forthcoming taxonomic treatment based on a narrow species concept including morphological, climatic and biogeographic distinctness that recognizes 45 species. We obtained guard cell measurements as proxies of ploidy level from 114 individuals of 33 species of Polylepis, including all species for which no previous measurements were available. In combination with previously published data, also on nucleus mass and chromosome counts, we infer that on current knowledge 19 (42%) species are probably purely diploid, 15 (33%) purely tetraploid, and one (2%) purely octoploid. The remaining eight (18%) species have mixed ploidy levels, with three (7%) being di-and tetraploid, two (4%) di-and hexaploid, and one each tetra-and hexaploid, tetra-and octoploid, and di-, tri-, tetra-and hexaploid. Based on our understanding of the evolutionary relationships in Polylepis, it would appear that polyploidy has originated at least about eight times independently in the genus, sometimes as autopolyploidy, sometimes as a result of interspecific hybridization, and sometimes in relation to cultivation. The taxonomic implications of the ploidy levels are complex, in some cases supporting species-level distinction and in others posing the question whether different ploidy levels within a species should better be treated as distinct species. Ploidy level needs to be taken into account for the conservation of the genus, as for example if different populations of a species have different ploidy levels, mixing these origins in reforestation schemes may lead to the formation of sterile hybrids. Guard cell measurement is a low cost and simple technique that can be readily used on both live and dried plant material for such applications, but it has limitations and further data on chromosome counts and nucleus mass are also needed to fully understand the evolution of ploidy levels in Polylepis and its implications.

An Affordable Image-Analysis Platform to Accelerate Stomatal Phenotyping During Microscopic Observation

“…The mean SS, or the range of SS, is a species-specific trait, and useful for taxonomic classification in a case where no macroscopic trait is available to classify the different species clearly (Aryavand et al, 2003;Boza Espinoza et al, 2020). It is known that SSs correlate well with genome size in Triticum and other plants (Masterson, 1994;Beaulieu et al, 2008).…”

A monograph of the genus Polylepis (Rosaceae)

“…The following account of ploidy levels in Polylepis is based on Boza Espinoza et al (2020b). Ploidy levels in Polylepis have been studied by direct chromosome counts, nucleus size measurements via flow cytometry and guard cell measurements (Table 2).…”

“…Guard cell size is well known to be correlated to ploidy level in angiosperms in general (Masterson 1994;Sugimoto-Shirasu and Roberts 2003;Beaulieu et al 2008) and Rosaceae in particular (Joly and Bruneau 2007) and has been shown to be correlated to genome size in Polylepis (Schmidt-Lebuhn et al 2010). Still, there is notable variation of guard cell sizes within species, even when only a single ploidy level is known in the species (Schmidt-Lebuhn et al 2010;Boza Espinoza et al 2020b). This variation may be due to anatomical plasticity of a species depending on growth conditions, differences in measurement methods or aneuploidy.…”

“…Based on this approach, for guard cell length, based on comparison with chromosome counts and genome size measurements, Boza Espinoza et al (2020b) proposed that diploidy is related to guard cell lengths of 9-15 µm, tetraploidy to 14-20 µm and higher ploidy levels to 18-23 µm, showing that some overlap occurs that may make it difficult to assign a specific measure to a ploidy level. For genome size, the diploid condition is related to 2C values around 1.4-1.7 pg, triploidy to 2.0-2.3, tetraploidy to 2.6-3.4 pg, hexaploidy to 4.6-4.9 pg and octoploidy to 5.7-5.8 pg.…”

Geometry morphometrics of plant structures as a phenotypic tool to differentiate Polylepis incana Kunth. and Polylepis racemosa Ruiz & Pav. reforested jointly in Ecuador

“…Several studies have been reported involving stomata in epidermal studies to delimit species [59,60]. A recent study indicates that the length of the guardian cells allows to differentiate species in Polylepis [61], but there are no studies that use geometry morphometric as a tool to differentiate species in the Rosaceae family.…”