Reticulate evolution and tobacco

The utility of Nicotiana tabacum as a model biological system for scientific studies is extremely broadly based. For example, this species has provided the biological material for testing hypotheses concerning molecular and genomic evolution, gene function, the timing/form of speciation, and the processes leading to the origin of cultivars (Volkov et al. 1999; Lim et al. 2000, 2004, 2007; Kitamura et al. 2001; Ren and Timko 2001; Fulnecek et al. 2002; Matyasek et al. 2002; Skalicka et al. 2003, 2005; Clarkson et al. 2004, 2005; Dadejova et al. 2007; Petit et al. 2007). However, the obvious notoriety of this plant centers on its use as one of H. sapiens' longstanding drugs of choice. The health risks of cigarette smoking, in particular, have been highlighted by the Centers for Disease Control and Prevention in the following way:

Smoking harms nearly every organ of the body; causing many diseases and reducing the health of smokers in general. The adverse health effects from cigarette smoking account for an estimated 438,000 deaths, or nearly 1 of every 5 deaths, each year in the United States. More deaths are caused each year by tobacco use than by all deaths from human immunodeficiency virus (HIV), illegal drug use, alcohol use, motor vehicle injuries, suicides, and murders combined. (http://www.cdc. gov/tobacco/data_statistics/Factsheets/health_ effects.htm)

Notwithstanding such dire statistics, ~15% of male and 9% of female citizens of the United States consumed >25 cigarettes per day in 2004 (http:// adult/table_4.htm).

I have pointed out previously that there is no comparison between the individual artisan's skill needed to construct high-end, hand-rolled, long-leaf cigars, and the mechanization with which a packet of cigarettes or a can of snuff is produced (Arnold 2006, p. 176). However, the common elements used to produce each of these diverse products are the leaves from N. tabacum. Furthermore, the origin of this widely applied drug involved reticulation. To understand the origin and evolutionary trajectory of the tobacco-producing lineage, it is necessary to review the history of use of Nicotiana species as sources for nicotine. An examination across geographic settings and cultures reveals that N. tabacum has not been the sole source for nicotine. N. tabacum was indeed utilized by Central and South American indigenous groups. In contrast, inhabitants of western North America consumed Nicotiana bigelovii, Nicotiana attenuata, and Nicotiana trigonophylla, while eastern North American, northern Mexican, and West Indian peoples used Nicotiana rustica. Finally, Nicotiana benthamiana was the source of the tobacco ingested by native Australians (Gerstel and Sisson 1995).

The origin of the tobacco industry in the New World (specifically, in the state of Virginia) involved the species used by the local Native Americans, N. rustica (Gerstel and Sisson 1995). However, this species was quickly supplanted by the better-flavored N. tabacum (Gerstel and Sisson 1995). Significantly, the origin of N. tabacum involved allopolyploid speciation. Indeed, DNA sequence information has identified the maternal and paternal progenitors of the tobacco cultivar to be the diploid species, Nicotiana sylvestris and Nicotiana tomentosiformis, respectively (Lim et al. 2000; Kitamura et al. 2001; Ren and Timko 2001; Fulnecek et al. 2002). The timing of N. tabacum's origin has been estimated at c.5-6 mya (see discussions by Fulncek et al. 2002; Clarkson et al. 2005). As with many examples of genetic exchange, the combination of the divergent, N. sylvestris and N. tomentosiformis parental genomes into a single nucleus and cytoplasm gave rise to a derivative lineage with novel adaptations. In this case, the adaptations were reflected in its utility as a source of the drug, nicotine. Gerstel and Sisson (1995) highlighted the novel set of adaptations by stating that "the wild parents of N. tabacum . . . possess a dominant 'converter' gene which . . . demethyl-ates nicotine into undesirable nornicotine in the leaves. For this reason these species may have been useless." Like all the examples used in this section, reticulate processes (in this case occurring millions of years before the origin of the genus Homo) resulted in the biological starting material for a drug-producing, agricultural industry.

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