A genetic appraisal of a new synthetic nicotiana tabacum

Amanda Ratliff

Bio Lab 1615

Journal review

A GENETIC APPRAISAL OF A NEW SYNTHETIC NICOTIANA TABACUM

Thirty to eighty percent of plants evolution-driving force is from its polyploidy, polyploidy means it has more than two paired chromosomes. With the importance of polyploidy in plant evolutions is has become a great interest in regards to the early events associated with the establishment of populations of polyploidy individuals. For further exploration, in this article, synthetic polyploidy that mimics natural species have been constructed and early genetic events were recorded. These analyses give some reason as to why polyploid species are so successful and why polyploidy is a recurrent rarity in seed producing plant evolution.

Presented in this article are genetic analyses of three artificial allotetraploid lines that closely resemble Nicotiana tabacum (tobacco). The materials studied are a tobacco line (TH7) made by Kostoff (1938), which has not been investigated in depth. The other species of allopolyploids have been synthesized for this article. Allopolyploids are polyploids with chromosomes derived from different species. The information gathered is compared to the data of other synthesized allopolyploids.

It was found that genetic analyses of allopolyploids have yielded some surprising and often contradictory data. Studies point to accelerated genetic change in early allopolyploid generations. There is now a need to understand whether a reported change is stochastic, which means it is of random sequences, or whether it is directed. Such changes can be studied in similar, but independent synthetic allopolyploids, as done in this report. The hybrids and derived synthetic tobacco lines reported here uses a N. tomentosiformis (paternally derived genome) cultivar that is very similar to the T-genome found in naturally occurring tobacco. An absence of genetic change in the new hybrids and synthetic allopolyploids was shown by comparing the independently synthesized tobacco lines, attempts to discriminate between those events associated with polyploidy that appear directed and those events that are stochastic, or non-deterministic.
The F1 hybrids and synthetic tobacco were produced by crossing the female genome N. sylvestris with the male genome N. tomentosiformis by pollination with a paintbrush, ninety five percent of the seeds were fertile and placed into pots to be grown. The results established were a cross of 122 hybrid female N. Sylvestris crossed with the male N. tomentosiformis plants, and the three allotetraploid plants were generated for this article. The three plants all had twenty-four T-genome chromosomes and eight rDNA sites as expected from the parents. No evidence was found for intergenomic translocations in either the diploid or haloid hybrids. When Kostoff’s TH7 line was studied more in depth, there were at least three tanslocations, the rDNA units were similar to the hybrids and natural tobacco lines but there were qualitative changes.

The data from the synthetic allopolyploids did not reveal any common trends in the genetic changes. Only in synthetic allopolyploids of Triticum are there reports of reproducible and nonrandom genetic change associated with allopolyploidy. Comparing allopolyploids of different genera reveals no common patterns that could be used to predict outcome. It is thought that perhaps the genetic events that do occur are simply stochastic, or are possible outcomes that occur over widely different time frames so that many generations are needed before common trends become obvious. Whichever argument may be true, some allopolyploids can show drastic genetic changes in just a few generations. It is thought that one of the reasons the allopolyploids are so successful in nature is that the allelic diversity in the gene pools of both parents, if there are multiple recurrent allopolyploid origins, can show to be beneficial to the offspring. Therefore, if allopolyploidy also stimulates genetic change, then this could also be a major source of variation of which natural selection can act.

The three allotetraploid plants generated for this study, appeared sterile, although there was some pollen germination and the generative and vegetative nuclei looked normal, it has been proposed that the generation of fertile diploid hybrids requires recombination between homologous chromosome sets, it was proposed that there were adverse interactions between the paternal genome and maternal cytoplasm in newly formed allopolyploid. It was suggested that the sterility is overcome by species-specific translocations. These translocations can potentially influence gene expression through altered gene activity.
It was also discovered that Kostoff’s TH7 is not a synthetic allopolyploid as reported but instead may have been created by crossing synthetic allotetraploid or hybrid with normal tobacco, evidence for this hypothesis is in collected data. However there is also an idea that a new family of rDNA unit evolved and amplifie TH7, if this hypothesis is correct this new family is the same size as in natural tobacco, support for this hypothesis is also found in some collected data.

All the allopolyploid material had the expected number of rDNA loci on the same homologues sites. The patters found were in respect to the parental contributions and similar results were found in natural tobacco, these studies involved allotetraploid plants of the second and fourth generations. It is possible that rearrangements of rDNA loci occur after the first meiosis and in following generations. Attempts will be made to establish future generations from the synthetic allotetraploids. Tobacco is a particularly useful plant for biopharmacy including the manufacture of pharmaceutical drugs and industrial chemicals, and the infertility can act as a safety net for future research by prohibiting the chances of the synthetic tobacco cross hybridization to wild or cultivated species.