| Parthenium hysterophorus , a Parthenium species of Compositae, is a herb of fast-maturing annual. The pollen is mainly spread by wind and vehicles. The original habitat is in the north of Mexico to the southeastern of the United States, and now widely distributed in the tropical regions of global. In China it was found in Yunnan, Guizhou, Guangxi, Guangdong, Hainan, Hong Kong, Fujian and Shandong Province. It is regarded as one of the worst weeds. Some people suffer severe allergic reactions to the plant or its pollen. It has allelopathy to other plants and threatens biodiversity. In a good season , four or five generations may emerge. It does not usually become established in undisturbed vegetation or vigorous pastures. It has caused large economic and environment impacts because of its invasiveness and potential for spread. In this study, inter-simple sequence repeat (ISSR) method was applied to detect genetic variation of Parthenium hysterophorus from 90 accessions, and genetic structure between and within the six populations. It will lay the foundation for its control strategy, and will have some reference for other invasive plants. The main results were as follows:1. A total of 8 ISSR primers were employed on 90 individuals of Parthenium hysterophorus from 69 primers, and 46 scorable amplified polymorphic were detected, of which 38 (PPL =82.61%) were polymorphic loci. Genetic diversity analysis showed that Nei's gene diversity (He) was 0.2887 and Shannon's genetic diversity index (I) was 0.4327 in species. Parthenium hysterophorus had a high level of genetic diversity. This result was closely linked with the characteristics of the non-native plants of Compositae. These characteristics made it spread rapidly and multiply. The cross-pollination provided opportunities for gene recombination. All of them enhanced the genetic diversity. With the right conditions, Parthenium hysterophorus can grow at any where. The potential of invasion and distribution are extremely strong.2. The total genetic diversity (Ht) was 0.2887 and within populations (Hs)was 0.2365. Genetic differentiation mainly occurred within populations (81.91%). This result was consistent with the study that the genetic variation between populations in species with wind pollination was low. The gene flows between populations were high (Nm=2.2641), which indicated that the genes between populations exchanged frequently. The breeding characteristics of Parthenium hysterophorus were advantageous to its genetic variation to generate and maintain, and promoted genes between populations exchanges frequently, thereby limited the genetic differentiation. UPGMA was conducted for the six populations, and the results of UPGMA according to this study.3. Although the samples in this study was small, it showed that the genetic differentiation is very high. The capacity of adaptation to the environment is enhanced. The high genetic differentiation indicated that these populations have the properties of spreading to the surrounding areas. In order to control its further spread, we should intensify efforts. Not only to remove before flowering, but also to strengthen researches in chemical and biological method. Besides the introduction of foreign advanced technologies, we should study the chemical and biological control methods which will suit for China.
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