Abstract
Two birch species coexist in Iceland, dwarf birch Betula nana and tree birch B. pubescens. Both species are variablemorphologically, which has been shown to be due to introgressive hybridization via interspecific hybrids. The aim of this study was to examine
if the introgression could be related to genome size. We characterized 42 plants from Bifröst woodland morphologically and
cytogenetically. The population consisted of diploid B. nana (38%), tetraploid B. pubescens (55%), and triploid hybrids (7%).
Genome size wasmeasured from 12 plants, using Feulgen DNA image densitometry (FDM) on spring leaf buds and flow cytometry
(FCM) with dormant winter twigs. The use of winter twigs for FCM is novel. The average 1C-values for diploid, triploid, and
tetraploid plants were 448, 666, and 882 Mbp, respectively. Monoploid genome sizes were found to be statistically constant among
ploidy levels. This stability is in contrast to the different taxonomic positions of the di- and tetraploids and also contrasts with the
frequent occurrence of genome downsizing in polyploids.