MULTIPLE INDEPENDENT FORMATIONS OF TRAGOPOGON TETRAPLOIDS (ASTERACEAE) - EVIDENCE FROM RAPD MARKERS

Polyploidy is widely recognized as a significant force leading to the formation of new plant species. Estimates of the number of angiosperm species with polyploid origins are as high as approximate to 50%; howe ver, in spite of this prevalence, many aspects of polyploid evolution remain poorly understood. Recent studies have suggested that recurrent origins of polyploid species are the rule rather than the exception. The present study is one of only a few designed to quantify the number of independent origins of a polyploid species. The two tetraploid spe cies Tragopogon mints and T. miscellus (Asteraceae) arose within the p ast 50 years in the Palouse region of eastern Washington and adjacent northern Idaho. Previous work using morphology, cpDNA and rDNA restric tion site analyses, allozymes, cytology, and flavonoid chemistry estab lished that T. mirus had arisen at least five times, and T. miscellus at least twice, on the Palouse. To assess the frequency of multiple or igins of these species more rigorously, seven populations of T. mirus and three populations of T. miscellus that were indistinguishable base d on previous markers were surveyed using random amplified polymorphic DNA (RAPD) markers; populations of the diploid progenitor species fro m the same sites were also analysed. Each tetraploid population had a unique RAPD marker profile, suggesting that each population surveyed o riginated independently of the other populations in the region. Only t wo of the tetraploid populations combined the RAPD marker profiles of the diploid progenitors occurring at the same site. Both polyploid spe cies, whose ranges and numbers have greatly increased since their form ation in the early part of the twentieth century, have formed repeated ly on a local geographical scale and during a short time frame. Furthe rmore, each tetraploid species is spreading not primarily by dispersal of propagules from a single population of origin, but through repeate d, independent polyploidization events that recreate the polyploid tax a.