Genome size stability among five subspecies of Pinus nigra Arnold s.l

Despite the fact that genome size should be constant at species level, many reports of intraspecific variations exist. Thus, we carried out an analysis to determine the possible existence of nuclear DNA content variation in European black pine (Pinus nigra s.l.), a good model for such a study given its karyological uniformity, morphological polymorphism, broad geographical distribution, ecological plasticity and taxonomic heterogeneity. The panel comprised 20 populations across the natural range of P. nigra from Europe, Northwest Africa and Asia Minor including five subspecies: subsp. nigra, salzmanni, dalmatica, pallasiana and mauretanica. Mean 1C DNA content of the species was 23.62 pg (±0.209) assessed by flow cytometry. This converts to 23.1 G base pairs. The coefficients of variation within and between populations did not exceed 2.6%. Although we had already reported the existence of significant differences for three Black pine populations in our previous work on five Pinus spp. [Bogunic, F., Muratovic, E., Brown, S.C., Siljak-Yakovlev, S., 2003. Genome size of five Pinus from Balkan region. Plant Cell Rep. 22, 59-63], intraspecific variation was not confirmed in the present study dealing with many more populations. Subspecific divisions of Black pine were characterised with following mean values: subsp. pallasiana--23.80 pg, dalmatica--23.79 pg, nigra--23.65 pg, salzmanni--23.55 pg, and mauretanica--23.24 pg. A positive relationship between genome size and longitude was observed (r = 0.44, p < 0.05). We conclude that the diversification of populations of P. nigra has occurred without significant genome size changes throughout its wide geographical range from ecologically contrasting habitats. A clinal mode of genome size variation is present, in line with hypothesis of P. nigra spreading from south-western Asia towards European habitats.