Multiple Origins of the Allotetraploid Tragopogon mirus (Compositae): rDNA Evidence

Tragopogon mirus is one of only a few polyploids whose time of origin is known with a high degree of certainty. Previous studies of morphological, karyotypic, and electrophoretic variation suggested multiple origins of allotetraploidy in T. mirus from the Palouse region of eastern Washington and adjacent Idaho. Restriction site analysis of the 17S-25S ribosomal RNA genes (rDNA) was used to test for multiple origins of T. mirus. The rDNA restriction profiles of six populations of T. mirus were compared with those of eight populations of T. dubius and six of T. porrifolius, the diploid progenitors. Populations of T. mirus combine the rDNAs of T. dubius and T. porrifolius, confirming the proposed origin of the tetraploid. Furthermore, the incorporation of restriction site polymorphisms of T. porrifolius into different populations of T. mirus demonstrates at least two independent origins of T. mirus. Although T. mirus often occurs with its diploid progenitors, these mixed populations do not necessarily reflect sites of origin of the tetraploid. In the three mixed populations examined, the diploids could not have produced the tetraploid plants found at the same site. Extensive dispersal of the tetraploid and fluctuating population sizes coupled with local extinctions and subsequent recolonization could explain the discordance between rDNA genotypes and their distributions. Thus, population dynamics in Tragopogon may obscure the sites of independent origin of T. mirus. Finally, although multiple origins of polyploid species have been reported frequently, few studies have demonstrated multiple polyploidization events on a geographic scale of a few kilometers. Tragopogon mirus Ownbey and T. miscellus Ownbey are well-documented products of allopolyploid speciation (Brehm and Ownbey 1965; Ownbey 1950; Ownbey and McCollum 1953, 1954; Roose and Gottlieb 1976; Soltis and Soltis 1989). Both allotetraploids arose within the current century in the Palouse region of eastern Washington and adjacent Idaho following the introduction and naturalization of their diploid progenitors, all of which are native to the Old World. The progenitors of T. mirus are T. dubius Scop. and T. porrifolius L., and those of T. miscellus are T. dubius and T. pratensis L. (Ownbey 1950). Whereas T. porrifolius and T. pratensis are restricted to waste places and lawns in a few towns within the Palouse, T. dubius and the allotetraploids have much broader distributions. Tragopogon dubius occurs in disturbed sites in towns and agricultural fields, along roadsides, and in disturbed areas of native vegetation in much of North America. The ranges of both tetraploid species have increased substantially in the Palouse since their discovery 40 years ago (Ownbey 1950; Novak et al. 1991). In addition, both T. mirus and T. miscellus have been reported from Arizona (Brown and Schaak 1972), and T. miscellus also occurs in Montana and Wy-