A study of the chromosome number and genome size of the rare species Rhododendron keiskei var. hypoglaucum in Korea

Abstract

Rhododendron keiskei var. hypoglaucum (Ericaceae) was recently reported in Korea, with a disjunct distribution on the southern islands of the Korean Peninsula. Although chromosome numbers and ploidy variations are important traits in angiosperms, gaining a clear understanding the cytological features of Rhododendron has been hampered by the small size of its chromosomes. We herein report the chromosome number, karyotype structure, and genome size of R. keiskei var. hypoglaucum for the first time. The chromosome number of the investigated plants was 2n = 26 with x = 13 as the base chromosome number, which is the one of the frequently encountered base chromosome numbers in Rhododendron. The karyotype of R. keiskei var. hypoglaucum is composed of metacentric and submetacentric chromosomes similar in length, which ranged from 1.39 to 2.40 μm. The DNA 1C-value in all examined accessions was small, ranging from 0.63 to 0.65 pg, further supporting the stable genome size in Rhododendron. These comprehensive cytological results provide a framework for detailed molecular, cytogenetic, and phylogenomic analyses that can be used to interpret the slow species diversification rate in Rhododendron.

Ploidy variation in Rhododendron subsection Maddenia and its implications for conservation

“…Differing values of DNA contents from different standards slightly change the calculated ploidy ratio, particularly for polyploids. Our results for R. fortunei and R. parryae fall within the range of genome sizes of reported Rhododendron diploids ( Bou Dagher-Kharrat et al 2013 ; Khan et al 2021 ; Choi et al 2022 ). However, the larger genome size of R. parryae likely caused the interpreted ploidy for the same accessions being one level lower than using R. fortunei as the standard ( Table 4 and [see Supporting Information—Table S4 ]).…”

“…Chromosome data were compiled from the Chromosome Counts Database (CCDB, Rice et al 2015 ), Index to Plant Chromosome Numbers (IPCN, Goldblatt and Johnson 1979 ) and literature not covered by these two databases ( Sax 1930 ; Ammal et al 1950 ; Li 1957 ; Cubey 2003 ; Contreras et al 2007 ). FCM ploidy data were collected from previous reports ( De Schepper et al 2001 ; Jones et al 2007 ; De et al 2010 ; Khan et al 2021 ; Choi et al 2022 ). Taxonomic classification (subgenus, section, subsection) followed Chamberlain et al (1996) , except that the vireya species were treated as subgenus Vireya following Argent (2015) (subgenus Vireya may be treated as the broad section Schistanthe under subgenus Rhododendron as in recent studies).…”

“…In contrast, flow cytometry (FCM) saves time by enabling rapid determination of nuclear DNA content (genome size) for a large number of samples ( Doležel et al 1998 ; De et al 2010 ; Zaytseva et al 2018 ). Apart from the measurement of genome size ( Bou Dagher-Kharrat et al 2013 ; Khan et al 2021 ; Choi et al 2022 ), FCM has been applied to estimate Rhododendron ploidy levels in several studies ( Doležel et al 1998 ; De Schepper et al 2001 ; Jones et al 2007 ; Zhou et al 2008 ; Zaytseva et al 2018 ; Khan et al 2021 ). Cytological studies and FCM generally require access to living material.…”