Diploid and tetraploid cytotypes of the flagship Cape species Dicerothamnus rhinocerotis (Asteraceae): variation in distribution, ecological niche, morphology and genetics

Abstract

Background and Aims The Greater Cape Floristic Region is one of the world's biodiversity hotspots and is considered poor in polyploids. To test this assumption, ploidy variation was investigated in a widespread Cape shrub Dicerothamnus rhinocerotis (renosterbos, Asteraceae). The aim is to elucidate the cytotype distribution and population composition across the species range, and to assess differences in morphology, environmental niches, and genetics. Methods Ploidy level and genome size were determined via flow cytometry, cytotype assignment was confirmed by chromosome counting. RADseq analyses were used to infer genetic relationships. Cytotype climatic and environmental niches were compared using a range of environmental layers and a soil model, while morphological differences were examined using multivariate methods. Key Results The survey of 171 populations and 2370 individuals showed that the species comprises diploid and tetraploid cytotypes, no intermediates and only 16.8 % of mixed populations. Mean 2C-values are 1.80–2.06 pg for diploids and 3.48–3.80 pg for tetraploids, with very similar monoploid genome sizes. Intra-cytotype variation showed a significant positive correlation with altitude and longitude in both cytotypes and with latitude in diploids. Although niches of both cytotypes are highly equivalent and similar, their optima and breadth are shifted due to differences mainly in isothermality and available water capacity. Morphometric analyses showed significant differences in the leaves and corolla traits, in the number of florets per capitulum, and cypsela dimensions between the two cytotypes. Genetic analyses revealed four groups, three of them including both cytotypes. Conclusions Dicerothamnus rhinocerotis includes two distinct cytotypes that are genetically similar. While tetraploids arise several times independently within different genetic groups, morphological and ecological differences are evident between cytotypes. Our results open up new avenues for questions regarding the importance of ploidy in the megadiverse Cape flora, and exemplify the need for population-based studies focused on ploidy variation.

Morphological and habitat differentiation between diploids and tetraploids of a Drakensberg near‐endemic taxon, Rhodohypoxis baurii var. platypetala (Hypoxidaceae)

“…For instance, in North America, while climatic niche differences have been found between diploids and polyploids and they were best explained by a shift in precipitation availability in some species, with diploids occurring in drier conditions than polyploids (e.g., Tolmeia (Saxifragaceae), Visger et al., 2016), in other species, this pattern was reversed as polyploid populations occupied drier habitats than diploid populations (e.g., Chamerion angustifolium (Onagraceae), Thompson et al., 2014). In the Cape Floristic Region of South Africa, the distribution of the different ploidy levels of Oxalis obtusa (Oxalidaceae) showed evidence for ploidy co‐occurrence (Krejčíková et al., 2013), and in Dicerothamnus rhinocerotis (Asteraceae), high niche equivalency and niche similarity was found for diploids and tetraploids (Chumová et al., 2023), suggesting that a prevalent pattern of niche shifts may not always exist between diploids and polyploids (Glennon et al., 2014).…”

“…For example, polyploids may be more successful than diploids in drier conditions (Maherali et al., 2009) and under intense solar radiation (Zozomová‐Lihová et al., 2015). There is a limited understanding of polyploidy in the southern hemisphere mountain grasslands, especially in the Drakensberg grassland ecosystem (but see work on the geographically close but floristically dissimilar Cape Floristic Region such as Chumová et al., 2023; Elliot et al., 2023; Krejčíková et al., 2013; Veselý et al., 2020, where genome size and ploidy levels have been associated with plant phenotypes and environmental correlates). The intraspecific ploidy variation in Rhodohypoxis baurii var.…”