Intraspecific Genome Size Variation Research Articles

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 hocassumptions 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.

Karyotypes, chromosome bands and genome size variation in New Zealand endemic gymnosperms

Karyotype studies on 20 taxa of gymnosperms endemic to New Zealand show a wide diversity of chromosome number and form. Fluorochrome banding with DAPI and CMA reveals a depauperate pattern of bands with CMA and no reliable banding with DAPI. Characteristically one pair of chromosomes shows a prominent CMA band, which may or may not be associated with a secondary constriction. A band size polymorphism was observed in all plants ofDacrycarpus dacrydioides, irrespective of the sex of the plant. Measurements of genome size by flow cytometry show a range of values from 12.3 pg to 40.0 pg DNA per 2C nucleus. Intraspecific variation in genome size was observed inManoao colensoi.

Nuclear DNA Amounts in Angiosperms

Bennett and Smith (Philosophical Transactions of the Royal Society of LondonB274:227-274; B334: 309-345) and Bennett, Smith and Heslop-Harrison (Proceedings of the Royal Society of London, B216: 179-199) published lists of nuclear DNA amounts estimated for 1612 angiosperm species collected from 163 sources dated between 1951 and 1986. Subsequently, interest in genome size in angiosperms and its significance has continued, and many new DNA estimates were published during 1986-1994. Their inaccessibility, and the flow of enquiries for such information, shows that a further compilation is needed. This paper presents a supplementary list of nuclear DNA C-values for 105 sources for 899 angiosperm species not listed in the above-mentioned compilations, plus 284 additional estimates for 208 species already listed by them. The data are assembled primarily for reference purposes, with species listed in alphabetical order, rather than by any taxonomic scheme. Some advantages and limitations of flow cytometry, now increasingly used to quantify DNA C-values in plants, are reviewed. Recent reports regarding the occurrence and extent of intraspecific variation in genome size are also discussed. While some examples are real, others reflect technical shortcomings. Work has begun to combine the genome size data compiled in this and the above-mentioned papers into a unified data base, and to present the information in separate lists, with species in alphabetical and systematic orders, respectively. DNA C-values are now known for 1% of the world's angiosperm flora, but improved representation of taxonomic groups, geographical regions and plant life forms is urgently needed.

Genome Size in Beaufort Sea Coastal Assemblages of Arctic Ciscoes

Intraspecific variation in nuclear DNA content (genome size) was measured for the Arctic cisco Coregonus autumnalis by fluorescence flow cytometry with the fluorochrome 4′,6-diamidino-2-phenylindole (DAPI). Frozen erythrocytes from the domestic chicken Gallus domesticus were analyzed simultaneously with Arctic cisco samples, providing an internal reference standard. Mean DNA content for 140 Arctic ciscoes from seven sites along the Beaufort Sea coast of Alaska and Canada was 5.99 pg/cell. Sample variances and mean DNA content were significantly heterogeneous among coastal sites. A significant increasing linear trend in mean DNA content was found on a west-to-east cline along the Beaufort Sea coast. Intraspecific variation in genome size observed in this study was substantial, which indicates that genome size characterizations of species based on small samples from a single population may be grossly inadequate.

Intra- and interspecific genome-size variation in the Salmonidae.

The nuclear DNA contents of 22 salmonid taxa, estimated primarily by flow-cytometric analysis relative to a chicken internal standard, were evaluated to compare intra- and interspecific variation in DNA content within this fish family. The average variability within taxa exceeded that among taxa. Intraspecific genome-size variation was substantial and, in some cases, exceeded the mean genome size for the species. The flow-cytometric method used here allows for rapid and reliable comparison of nuclear DNA contents within and among individuals from natural animal populations.

Genome Size Variation in Maize Populations Selected for Cold Tolerance

AbstractPrevious studies have indicated a relationship between genome size and cold tolerance in plants. Many species adapted to growth in cool environments have large genome sizes. These studies are based on interspecific DNA content variation. In this study, the nuclear DNA content of eight maize populations was determined. These populations were obtained from the University of Nebraska and represent populations selected for cold tolerance and their respective unselected original populations. Intraspecific DNA content variation was observed between the selected and unselected populations. Upon assessing the data based solely on cold tolerance, no clear relationship between genome size and cold tolerance was apparent. When both freeze tolerance and cold tolerance were considered, populations which were cold tolerant and exhibited a certain degree of freeze tolerance were observed to have significantly larger genomes relative to the unselected populations. Thus, it appears that the relationship between intraspecific genome size variation and cold tolerance is similar to the relationship between interspecific genome size variation and growth at cooler temperatures in plants.