Chromosome numbers of populations of three varieties of Bidens pilosa in Taiwan

BACKGROUND: Hairy beggar-ticks (Bidens pilosa L.) is a common invasive plant in tropical and subtropical regions. The Flora of Taiwan listed three varieties of B. pilosa in Taiwan, var. minor, var. pilosa and var. radiata. Among the three varieties, var. radiata was the most recently, in 1970s, introduced into Taiwan. However, after its introduction into Taiwan, var. radiata has become dominant over the other two varieties and is considered a serious invasive plant in lowland of Taiwan. Our previous study showed that var. radiata is self-incompatible and the other two varieties are self-fertile. Could it be possible that different chromosome numbers contribute to the different breeding systems of these three varieties? In addition, the heterogeneities of traits of var. radiata were found higher than those of var. minor and var. pilosa. Is the phenomenon resulting from the hybridization between var. radiata with other varieties? We counted chromosome numbers of populations of these three varieties distributed in Taiwan and conducted hand pollination treatment between var. radiata (as pollen receiver) and var. minor or var. pilosa (as pollen donor) to provide answer for the aforementioned questions. RESULTS: No difference was found in chromosome numbers among populations of the same variety. Forty-eight chromosomes (2n = 48) were counted for var. radiata while 72 (2n = 72) chromosomes for var. minor and var. pilosa. Therefore, var. radiata is tetraploid and var. minor and var. pilosa are hexaploid. No successful hybridization was found between var. radiata and var. minor or between var. radiata and var. pilosa. CONCLUSIONS: This study provided the evidence that the invasive plant (B. pilosa var. radiata) has different chromosome numbers from the other two varieties and is unlikely to hybridize with the other two varieties.