Genetic structure of the invasive Chromolaena odorata in China WHYE,HPMU,HLCAO&XJGE South China Institute of Botany, The Chinese Academy of Sciences, Guangzhou 510650, China Received 10 July 2003 Revised version accepted 23 December 2003 Summary Chromolaenaodorata is one of the world’s worst tropical weeds, and it can be found in Guangdong, Guangxi, Hainan and Yunnan provinces in China. Genetic variation of 27 C. odorata populations sampled from across its invasive range in China was investigated using inter-simple sequence repeats (ISSR) analysis. All pop- ulations exhibited low levels of genetic variation. On average, 2.35% of the ISSR loci were polymorphic, total genetic diversity (H T ) was 0.0406 and Shannon’s infor- mation index (H sp ) was 0.0623. High genetic identities (I) between populations (0.9687 ± 0.01204) indicated low genetic differentiation. A frequent founder effect was interpreted as the main cause of the genetic structure observed in C. odorata. Keywords: biocontrol, Chromolaena odorata, genetic variation, inter-simple sequence repeats, invasive species, weed. Introduction Chromolaena odorata (L.) RM King and H. Robinson (Siam weed) is a perennial member of the Compositae family. This species is a native of Central and South America. Its range extends from the south-east USA and Mexico, through the Caribbean islands to South Amer- ica, and reaches as far as northern Argentina (von Senger et al., 2002). In recent decades, this species has become a serious pest in the humid tropics of South East Asia, Africa and Pacific Islands. In China, C. odorata can be found in Guangdong, Guangxi, Hainan and Yunnan provinces (Xie et al., 2000). This species is considered as one of the world’s worst tropical weeds. It spreads rapidly in lands used for forestry, pasture and plantation crops such as rubber, coffee, coconut, cocoa and cashew (McFadyen, 1989). The history of the spread of C. odorata throughout Asia is well established. In the 1840s, C. odorata was introduced as an ornamental to Calcutta, from where all or most of the SE Asian and Pacific infestations are thought to have arisen (McFadyen, 1989). Chromolaena odorata has efficient short- and long-distance dispersal abilities. As a diploid species with 60 chromosomes (2n ¼ 60) (YL Feng, YH Wang, YY Liu and KY Ding, pers. comm.), C. odorata produces masses of ÔfluffyÕ pale pink to white flowers that result in enormous quantities of wind-dispersed achenes (Gautier, 1993). In order to constrain the expansion of C. odorata, biological control, such as introduction of its natural enemies (Actinote anteas Doubleday and Hewitson; Cecidochares connexa Macquart; Pareucha- etes pseudoinsulata Rego Barros), has been practised in many countries, such as Indonesia and Malaysia (Azmi, 2002; Wilson & Widayanto, 2002). In spite of its serious invasiveness, surprisingly little is known about the population genetic structure of C. odorata. Detailed studies of the patterns of genetic variation within invasive weed populations is important from both pure scientific and applied standpoints as the amount, kinds and organization of genetic diversity in populations will largely determine their capacity to respond to the local selection pressures imposed by the physical and biotic environment (Barrett, 1992). An understanding of how the diversity is distributed will help predict the potential for populations of invasive species to evolve in response to management practices (Sakai et al., 2001). Recent developments in molecular techniques provide the weed scientist with a larger array of genetic tools for studying genetic diversity of popu- lations than previously available (O’Hanlon et al., 2000). In the past decade, inter-simple sequence repeat Correspondence: X J Ge, South China Institute of Botany, The Chinese Academy of Sciences, Guangzhou 510650, China. Tel: (+86) 20 3725 2551; Fax: (+86) 20 3725 2831; E-mail: xjge@scib.ac.cn; xjge@yahoo.com Ó European Weed Research Society Weed Research 2004 44, 129–135