A comparison of plant chromosome number variation among Corsica, Sardinia and Sicily, the three largest Mediterranean islands

Chromosome counts of the vascular floras of Corsica, Sardinia, and Sicily were compared quantitatively, based on data stored in the online database “Chrobase.it”, and analysed to determine any trend associated to the latitudinal range. Mean chromosome number (CN) ± SD, frequency of B-chromosomes occurrence (fB), frequency of odd CN, and ICNH (Index of Chromosome Number Heterogeneity) were calculated for each island’s flora. The relative frequency of putative even ploidy levels was also calculated for the most frequent base numbers (x = 7, 8, 9) as the percentage of cytotype frequency value relative to the total sum of cytotypes. Our results show that ICNH and mean CN are lower in Sicily and higher in Sardinia and Corsica; furthermore, the relative frequencies of cytotypes of putative even polyploid series decrease with increasing ploidy levels, which can be modelled by an exponential decay of the general form f(x) = e(ax+b), with different parameters in the different islands. Such results are in agreement with previous researches showing an increase of mean CNs across Italy and Europe and a similar trend, modelled by an exponential decay, as regards the ploidy levels of endemic taxa of the Italian flora