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Island hopping with a flow cytometer
Angiosperms as a group have a very large range in genomic DNA content (i.e. their C-values), amounting to a 1000-fold difference between the largest and smallest genomes. Measurement of C-values can give insights into the evolution and ecology of plant groups. In general, genomes have tended to increase in size during evolution so large genomes are regarded as more ‘advanced’ than smaller genomes; we note here that monocots generally have larger genomes than dicots. In plant development, increasing amounts of DNA are correlated with larger nuclei and longer cell division cycles. In any one taxon, the lowest C-values are often found in the species with the shortest life-cycle, such as ruderals. There are also correlations between larger genome size and lower temperature so that within any one group there are latitudinal or/and altitudinal gradients in C-value. This brings us to the paper by Suda et al. (Prague, pp. 153-164). They have measured C-values of angiosperms (in fact, mainly dicots) in Macronesia, those Atlantic islands, including the Canaries, the Azores and Madeira that lie between 15 and 40 degrees N. The islands are very rich floristically, with a high proportion of shrubs and other woody forms. Nuclear DNA contents were measured in 104 species in all, revealing, very surprisingly, that C-values are in general very low, falling into the lowest third of the known range and, for nearly all species, lower than the C-values found in members of the same families growing in mainland Europe or Africa. It is an interesting picture: large shrubs with genomes as small as those of some cool-temperate ruderals. Overall, there was no obvious correlation between C-values and environmental factors, although some correlations were observed within particular groups, e.g with altitude. However the general picture that Macronesian plants have small genomes remains a puzzle and surely shows the way for further research.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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