DNA AMOUNT, LATITUDE, AND CROP PLANT DISTRIBUTION

Avdulov and Stebbins noted a tendency for species with large chromosomes in several angiosperm groups or families (including the Gramineae, Commelinaceae, Liliales, Polemoniales and the Leguminosae) to be localized in distribution to temperate latitudes. As chromosome size and DNA content are closely correlated, the distribution of species with large DNA amounts per chromosome, or per diploid genome, might be expected to be similarly localized. This hypothesis was tested using samples of crop species with large ranges of DNA amounts from the Gramineae and the Leugiminosae. For instance, the mean DNA amount per chromosome for the sample of cereal grain species showed about a 36-fold range from 0.033 picograms (pg) in Eragrostis tef to 1.186 pg in Secale cereale, while for the sample of pulse crops the range was about 70-fold from 0.032 pg in Lablab niger to 2.225 pg in Vicia faba. The results for cereal grain crops, cultivated pasture grasses and pulse crops show that cultivation of species with high DNA amounts per diploid genome tends to be localized in temperate latitudes, or to seasons and regions at lower latitudes where the conditions approximate to those normally found in temperate latitudes. Moreover, man has shown a strong tendency to choose species for cultivation with increasingly lower DNA amounts at successively lower latitudes. Thus, there is a cline for DNA amount and latitude. This cline is exhibited independently by both C3 and C4 crop species, and by both annuals and perennials, and hence is independent of life cycle type and the taxonomic distribution of C3 and C4 photosynthesis. The cline is apparently a natural phenomenon which man has modified and exaggerated in agriculture. It is suggested, therefore, that interspecific variation in DNA amount between angiosperm species may have adaptive significance affecting the distribution of both crop and non-crop species. The cline might be caused either by variation in DNA amount, or by variation in some factor(s) correlated with DNA amount. The factor(s) causally responsible for the cline, and their mode of action should be investigated since they may have implications for agriculture and plant breeding.