Pulling apart the many chromosomes of Rhododendron to improve conservation

Polyploidy, a condition where organisms have more than two sets of chromosomes, is common in plants. These extra chromosomes can complicate taxonomy and make identifying a plant more difficult. Now, Hu and colleagues have undertaken a detailed exploration of the Rhododendron genus, of which a quarter of its over 1,300 species are considered at risk. Their comprehensive research has been published in the journal AoB PLANTS.

Hu and colleagues discovered that polyploidy, ranging from diploid (2x) to dodecaploid (12x), is most frequent in two subgenera of Rhododendron, namely, Pentanthera and Rhododendron. The new research study from Hu’s team has increased our understanding of polyploidy distribution across the genus and specifically in subsection Maddenia, a group within the genus.

The researchers used flow cytometry (a technique that measures the quantity of genetic material in a cell) and meiotic chromosome counts to estimate ploidy levels across 47 taxa within the Maddenia subsection. In their article Hu and colleagues write:

We used flow cytometry to estimate ploidy for 47 taxa in ss. Maddenia, including 12 taxa that had never been investigated in previous studies. In this subsection, polyploids have been definitively identified in only the R. maddenii complex, where its two subspecies exhibit ploidy series consisting of diploids and various polyploidy levels.

Hu et al. 2023

The high ploidy variation in the Rhododendron maddenii complex raises questions about whether the different ploidy levels result from intra-species variability or a mishmash of ploidy due to ”lumped” taxa — species that have been grouped together due to their similarities. Hu and colleagues write:

The R. maddenii complex shows a similar pattern of ploidy variation and geographic distribution as the genus Buddleja L. (Scrophulariaceae) which is also from the Sino-Himalayan region (Cullen 1980; Chen et al. 2007). One hypothesis for the ploidy differences between the two subspecies of R. maddenii may be the ongoing in situ speciation in this area (Hughes 2017). As one of the world’sworld’s youngest mountain ranges, with a high frequency of polyploidy in plants, the Sino-Himalayan region has been identified as a centre of species diversification that may be attributed to polyploidization (Irving and Hebda 1993; Schwery et al. 2015; Xing and Ree 2017; Shrestha et al. 2018; Rice et al. 2019; Xia et al. 2021). However, the Sino-Himalayan origin does not explain why polyploids should have continued to occur in R. maddenii, while other species in ss. Maddenia which are predominantly from the same region do not exhibit polyploids.

Hu et al. 2023

The team’s research highlights the necessity for further studies, particularly on wild populations, to better understand ploidy variation in the Rhododendron genus. These findings hold significant implications for biodiversity conservation and for untangling the taxonomy of polyploid complexes.

READ THE ARTICLE
Hu, L., Tate, J.A., Gardiner, S.E. and MacKay, M. (2023) “Ploidy variation in Rhododendron subsection Maddenia and its implications for conservation,” AoB PLANTS, 15(3), p. lad016. Available at: https://doi.org/10.1093/aobpla/plad016.