http://web.stanford.edu/group/petrov/pdfs/33.pdf

The large genome constraint hypothesis: evolution, ecology and phenotype.

Summary
The gross form of the nuclear genome varies greatly among plant species in both anatomy and genetic organization. Chromosome number (ft) ranges from 2 to over 600, and ploidy from 1 to over 20. The amount of DNA in the unreplicated haplophase genome (the 1C value) differs by more than 2500-fold among angiosperms. Although it has been questioned since the 1930s whether such variation is of adaptive significance and whether it is related, perhaps causally, with environmental factors, no direct or causal links have yet been found. However, variation in DNA C-value has far-reaching biological consequences and can be of considerable adaptive and hence ecological significance. Strikingly precise interspecific relationships exist between DNA C-value and many diverse phenotypic characters at the cellular level, and DNA can affect the phenotype in two ways, firstly by expression of its genie content and, secondly, by the biophysical effects of its mass and volume, the latter defined as nucleotypic effects. Nucleotypic variation in DNA C-value sets absolute limits to both the minimum size and mass of the basic unit of plant anatomy (i.e. the cell) and the minumum time needed to produce a similar cell with newly synthesized organic molecules. Moreover, in complex multicellular vascular plants, such effects at successive cell cycles are additive, so that DNA C-value influences many characters, including growth rate, seed weight, minimum generation time and type of life-cycle. Thus, the nucleotype profoundly affects where, when and how plants grow. Selection for a particular genomic form acting on its spatial or temporal consequences may occur at various levels ranging from the cell to the whole organism and may operate throughout the life-cycle or at just one stage. DNA C-value is often indirectly related to environmental factors which determine time-limited environments via selection acting on the temporal phenotypic consequences of nucleotypic variation. However, in the case of radio-sensitivity, selection for a low DNA C-value may act directly on the nucleotype itself, as the size of the nuclear DNA target directly affects the ability of the plant to survive.

Variation in genomic form in plants and its ecological implications

Publisher Summary This chapter presents a comprehensive review of the published information on the cytology, genetics, and evolution of Gossypium . In addition, it presents recent data and information on genome organization with which a hypothesis is proposed for the origin of the allotetraploid species that is different from that generally assumed. The genus Gossypium consists of 35 diploid species that are divided into seven genome groups and six allotetraploid species, each with the same two subgenomes. The genome relationships are also discussed in the chapter. Moreover, with the advent of the new technology of genetic engineering and its potential for improving the commercial cottons by inter- and intra- genomic transfer of desirable genetic segments, the basic genetic analyses should have even greater application in the future. The successful application of genetic engineering is greatly enhanced by the availability of fundamental knowledge of the genetic organization of the chromosomes gained through the classical genetic and cytogenetic approaches. Thus, to utilize the full potential of the new technology, it is of utmost importance that the classical approaches to the genetic analysis of the chromosomes of cotton be augmented.

Genetics, cytology and evolution of Gossypium.

Allotetraploid cotton species (Gossypium) belong to a 1-2 million year old lineage that reunited diploid genomes that diverged from each other 5-10 million years ago. To characterize genome evolution in the diploids and allotetraploids, comparative RFLP mapping was used to construct genetic maps for the allotetraploids (AD genome; n = 26) and diploids (A and D genomes; n = 13). Comparisons among the 13 suites of homoeologous linkage groups permitted comparisons of synteny and gene order. Two reciprocal translocations were confirmed involving four allotetraploid At genome chromosomes, as was a translocation between the two extant A genome diploids. Nineteen locus order differences were detected among the two diploid and two allotetraploid genomes. Conservation of colinear linkage groups among the four genomes indicates that allopolyploidy in Gossypium was not accompanied by extensive chromosomal rearrangement. Many inversions include duplicated loci, suggesting that the processes that gave rise to inversio...

Comparative genetic mapping of allotetraploid cotton and its diploid progenitors

Polyploid formation has played a major role in the evolution of many plant and animal genomes; however, surprisingly little is known regarding the subsequent evolution of DNA sequences that become newly united in a common nucleus. Of particular interest is the repetitive DNA fraction, which accounts for most nuclear DNA in higher plants and animals and which can be remarkably different, even in closely related taxa. In one recently formed polyploid, cotton (Gossypium barbadense L.; AD genome), 83 non-cross-hybridizing DNA clones contain dispersed repeats that are estimated to comprise about 24% of the nuclear DNA. Among these, 64 (77%) are largely restricted to diploid taxa containing the larger A genome and collectively account for about half of the difference in DNA content between Old World (A) and New World (D) diploid ancestors of cultivated AD tetraploid cotton. In tetraploid cotton, FISH analysis showed that some A-genome dispersed repeats appear to have spread to D-genome chromosomes. Such spread may also account for the finding that one, and only one, D-genome diploid cotton, Gossypium gossypioides, contains moderate levels of (otherwise) A-genome-specific repeats in addition to normal levels of D-genome repeats. The discovery of A-genome repeats in G. gossypioides adds genome-wide support to a suggestion previously based on evidence from only a single genetic locus that this species may be either the closest living descendant of the New World cotton ancestor, or an adulterated relic of polyploid formation. Spread of dispersed repeats in the early stages of polyploid formation may provide a tag to identify diploid progenitors of a polyploid. Although most repetitive clones do not correspond to known DNA sequences, 4 correspond to known transposons, most contain internal subrepeats, and at least 12 (including 2 of the possible transposons) hybridize to mRNAs expressed at readily discernible levels in cotton seedlings, implicating transposition as one possible mechanism of spread. Integration of molecular, phylogenetic, and cytogenetic analysis of dispersed repetitive DNA may shed new light on evolution of other polyploid genomes, as well as providing valuable landmarks for many aspects of genome analysis.

/pdf/dispersed-repetitive-dna-has-spread-to-new-genomes-since-2sx6qtuj86.pdf

Dispersed Repetitive DNA Has Spread to New Genomes Since Polyploid Formation in Cotton

Cell and nuclear volumes were determined for 17 species and plant types in the genus Gossypium. All species and types, except one, showed a significant positive association between nuclear and cell volume. The overall species r-value (0.694) for cell and nuclear volume is highly significant and positive. For the 17 plant forms, the r-values for association of DNA content and nuclear volume and for association of DNA content and cell volume are 0.625 and 0.655, respectively, and both are highly significant.

G. A. Edwards

Papers

Cell size, nuclear size and dna content relationships in gossypium