Relationship between genome size and maturity group in soybean

Genome size, chromosome number variation and its correlation with stomatal characters for assessment of ploidy levels in a core subset of mulberry (Morus spp.) germplasm

The large size of the germplasm collection along with scanty information on their cytological and genome constitution have hindered well-planned breeding schemes in mulberry. To address the issue, a study was undertaken to investigate the variability in DNA content and genome size, chromosome number, ploidy and its relation with important stomatal characteristics among 162 mulberry germplasm. These germplasm comprise a core subset of 150 collections along with a representative collection of different mulberry species including the wild. Among the germplasm belonging to 16 species, we identified 122 diploids (2n = 28), 4 aneuploids (2n = 30), 13 triploids (2n = 42), 15 tetraploids (2n = 56), 7 hexaploids (2n = 84) and 1 dodecosaploid (2n = 308) based on the chromosome count. Most of the cultivated mulberries are found to be diploids. The mean nuclear 2C DNA content estimated by Flow cytometry, varied from 0.723±0.006 pg (M. australis, 2n = 2x) to 7.732 pg (M. nigra, 2n = 22x). The 2C DNA content positively correlated with the ploidy status and stomatal length (r = 0.814, p<0.001). Based on the 1Cx value, the study also suggests that the majority of the polyploid species have experienced genome downsizing in relation to their diploid progenitors. This study provides the most essential information on chromosome number, ploidy and DNA content to facilitate the utilization of a core subset of germplasm in the mulberry breeding program.

Determination of genome size variations among different date palm cultivars (Phoenix dactylifera L.) by flow cytometry

Date palm is an important fruit crop and member of palm family, reported with varied ploidy levels, i.e., 14 (2n = 28) to 18 (2n = 36) pairs of chromosomes and genomic size due to the limited work done on its cytological aspect. The amount of nuclear DNA content is extremely important to understand the hereditary constituent of any species. Hence, the present study was conducted with the aim to estimate the ploidy level and especially the genomic size (C value) of date palm by studying fifty date palm cultivars with flow cytometry using propidium iodide (PI) as the fluorescent dye. The maize genome size (5.14 pg) was used as the internal reference. The results obtained regarding genomic size of date palm cultivars significantly varied and ranged from 1.59 to 1.84 pg 2C^-1 or 780.11 to 903.06 Mbp when converted into base pairs of DNA. The average genome size of studied cultivars was observed to be 1.726 2C^-1, while, that of nuclei significantly varied from cultivar to cultivar and appeared independent to the genomic size of the studied cultivars. Intraspecific variations were not detected among five exotic cultivars (Amber, Sugae, Medjool, Safawi and Ajwa) grown in Pakistan in relation to their place of origin. The cluster analysis exhibited two main groups of cultivars, the first group comprised 65.3% (33 cultivars) and the second group contained 35.7% (17 cultivars). In addition, no ploidy was observed among all the studied cultivars. The findings regarding the estimated genomic sizes of studied cultivars may be helpful in understanding date palm genetics, breeding and genome sequencing programs further.

Estimation of nuclear DNA content and its variation among Indian Tea accessions by flow cytometry

Nuclear DNA content and genome size variation among 36 Indian tea accessions were analyzed by flow cytometry. Initial standardization of protocols for isolation of nuclei, DNA staining and selection of an internal standard for tea accessions which have significantly high amount of phenolic secondary metabolites in their cytosol was carried out. Results obtained revealed that 2C DNA content of Indian tea is 7.46 pg which corresponds to 1C genome size of 3673 Mb. Inter accession variation in 2C DNA content was also observed among 35 diploid taxa ranging from 7.23 to 7.73 pg which was significant at 1% probability level. The 2C DNA content of triploid (UPASI 3) was observed to be 11.47 pg which is concurrent with the expected value. Results obtained showed that Assam and Cambod type tea accession have higher 2C DNA content of 7.73 pg whereas Assam Cambod hybrids and Assam China hybrids have reduction in DNA content with 2C amounts, 7.23 and 7.32 pg DNA respectively. The present study suggests that the species involved in origin of Indian tea must have differed in their genome sizes owing to significant inter accession variation in nuclear DNA content.

Genome size and genome evolution in diploid Triticeae species

One of the intriguing issues concerning the dynamics of plant genomes is the occurrence of intraspecific variation in nuclear DNA amount. The aim of this work was to assess the ranges of intraspecific, interspecific, and intergeneric variation in nuclear DNA content of diploid species of the tribe Triticeae (Poaceae) and to examine the relation between life form or habitat and genome size. Altogether, 438 plants representing 272 lines that belong to 22 species were analyzed. Nuclear DNA content was estimated by flow cytometry. Very small intraspecific variation in DNA amount was found between lines of Triticeae diploid species collected from different habitats or between different morphs. In contrast to the constancy in nuclear DNA amount at the intraspecific level, there are significant differences in genome size between the various diploid species. Within the genus Aegilops, the 1C DNA amount ranged from 4.84 pg in A. caudata to 7.52 pg in A. sharonensis; among genera, the 1C DNA amount ranged from 4.18 pg in Heteranthelium piliferum to 9.45 pg in Secale montanum. No evidence was found for a smaller genome size in annual, self-pollinating species relative to perennial, cross-pollinating ones. Diploids that grow in the southern part of the group's distribution have larger genomes than those growing in other parts of the distribution. The contrast between the low variation at the intraspecific level and the high variation at the interspecific one suggests that changes in genome size originated in close temporal proximity to the speciation event, i.e., before, during, or immediately after it. The possible effects of sudden changes in genome size on speciation processes are discussed.

Phenotypic impacts of repetitive DNA in flowering plants

The discovery that nuclear DNA content varies widely among species, and even within species, was unexpected because it was thought that the number of genes required for an organism should be common across taxa. We now know that the bulk of nuclear DNA content variation is caused by repetitive DNA sequences characterized according to the nature of repeat (tandem vs dispersed) or chromosomal location/mechanism of replication (pericentromeric, telomeric or subtelomeric, microsatellites, minisatellites, satellites, transposable elements, retroelements). Variation in repetitive DNA, manifested as variation in nuclear DNA content, has been shown to have broad ecological and life-history consequences. For example, large genome size appears to limit fitness in extreme environmental conditions. Within species, variation in DNA content has been coupled to growth and development, such as maturation time in crop species. In Silene latifolia, DNA content is negatively correlated with flower size, a character that, in turn, has well documented ecological significance. These intraspecific studies suggest a connection between repetitive DNA and quantitative genetic determination of continuous characters. Novel insights into mechanisms by which repetitive DNA influences phenotype will lead to models of evolutionary change that extend well beyond the conventional view of evolution by allelic substitution.

Regulating transgenic crops sensibly: lessons from plant breeding, biotechnology and genomics

The costs of meeting regulatory requirements and market restrictions guided by regulatory criteria are substantial impediments to the commercialization of transgenic crops. Although a cautious approach may have been prudent initially, we argue that some regulatory requirements can now be modified to reduce costs and uncertainty without compromising safety. Long-accepted plant breeding methods for incorporating new diversity into crop varieties, experience from two decades of research on and commercialization of transgenic crops, and expanding knowledge of plant genome structure and dynamics all indicate that if a gene or trait is safe, the genetic engineering process itself presents little potential for unexpected consequences that would not be identified or eliminated in the variety development process before commercialization. We propose that as in conventional breeding, regulatory emphasis should be on phenotypic rather than genomic characteristics once a gene or trait has been shown to be safe.