Genome Size in the <i>Arenaria ciliata</i> Species Complex (Caryophyllaceae), with Special Focus on <i>A. ciliata</i> subsp. <i>bernensis</i>, a Narrow Endemic of the Swiss Northern Alps

The genus <i>Arenaria</i> (Caryophyllaceae) comprises approximately 300 species worldwide; however, to date, just six of these taxa have been investigated in terms of their genome size. The main subject of the present study is the <i>A. ciliata</i> species complex, with special focus on <i>A. ciliata</i> subsp. <i>bernensis</i>, an endemic plant occurring in the Swiss Northern Alps. Altogether, 16 populations and 77 individuals of the <i>A. ciliata</i> complex have been sampled and their genome sizes were estimated using flow cytometry, including <i>A. ciliata</i> subsp. <i>bernensis</i>, <i>A. ciliata</i> s.str., <i>A. multicaulis</i>, and <i>A. gothica</i>. The <i>Arenaria ciliata</i> subsp. <i>bernensis</i> shows the highest 2c-value of 6.91 pg of DNA, while <i>A. gothica</i> showed 2c = 3.69 pg, <i>A. ciliata</i> s.str. 2c = 1.71 pg, and <i>A. multicaulis</i> 2c = 1.57 pg. These results confirm the very high ploidy level of <i>A. ciliata</i> subsp. <i>bernensis</i> (2n = 20x = 200) compared to other taxa in the complex, as detected by our chromosome counting and previously documented by earlier work. The genome size and, thus, also the ploidy level, is stable across the whole distribution area of this taxon. The present study delivers additional support for the taxonomic distinctiveness of the high alpine endemic <i>A. ciliata</i> subsp. <i>bernensis</i>, which strongly aligns with other differences in morphology, phylogeny, phenology, ecology, and plant communities, described previously. In affirming these differences, further support now exists to re-consider the species status of this taxon. Upgrading to full species rank would significantly improve the conservation prospects for this taxon, as, because of its precise ecological adaptation to alpine summit habitats, the <i>A. ciliata</i> subsp. <i>bernensis</i> faces acute threats from accelerated climate warming.