Intraspecific variation in genome size: A critical reassessment

Genome size differences between angiosperm species can be very large, up to about three orders of magnitude. However, the occurrence and extent of genome size variation below the species level is, according to the case, controversial. It is obvious that some variation due to common chromosome polymorphisms and spontaneous aberrations ought to occur. Non-recognized taxonomic heterogeneity of the material may also be a cause of 'intraspecific' variation. Such cases of variation are well understood or at least plausible and may be termed 'orthodox'. The 'unorthodox' types of variation are those that require ad hoc assumptions to explain them, e.g. rapid transposable element spreads or amplification/diminution events in the course of development or reproduction (plastic genome), or are simply non-explainable in the context of the current taxonomy or of the breeding system of the organism (unorthodox intraspecific genome size variation). At present, many students of the topic seem to believe that such unorthodox variation is common. This review attempts to discuss some of the cases of unorthodox variation in angiosperms with emphasis on those that have been re-investigated by the present author. Upon re analysis, technical shortcomings (e.g. suboptimal performance of the Feulgen reaction or insufficient standardization) are often shown to be the probable cause of unorthodox variation. The 'plastic genome' seems to be an idea rather than a defendable scientific hypothesis; intraspecific variation is less frequent than presently thought. Unorthodox genome size variation should be treated with more scepticism. (C) 1998 Annals of Botany Company.