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Genetic diversity of alligator weed ecotypes is not the reason for their different responses to biological control
Aquatic Botany 85 (2006) 155–158 www.elsevier.com/locate/aquabot
Short communication
Genetic diversity of alligator weed ecotypes is not the reason for their different responses to biological control Jing Li, Wan-Hui Ye * South China Botanical Garden, the Chinese Academy of Sciences, Guangzhou 510650, China Received 26 May 2005; received in revised form 13 December 2005; accepted 22 February 2006
Abstract Biological control of alligator weed Alternanthera philoxeroides (Mart.) Griseb. using Agasicles hygrophila, a Chrysomelid beetle, has been successful in limiting growth in water, but not on land. In order to understand a possible genetic basis of this difference, technique using intersimple sequence repeats (ISSR) markers was applied to analyse genetic diversity of this invasive weed. No genetic variation was detected not only within or between populations growing in the same habitats, but also between land- and water-grown populations. Thus we consider that the genetic variation is not the baseline factor resulting in the biological control difference in China. The differential success of pupation by the beetle may be related to the phenotypic plasticity of the plant stem diameter, rather than to genotypic factors. # 2006 Elsevier B.V. All rights reserved. Keywords: Alligator weed; Alternanthera philoxeroides; Biological control; Agasicles hygrophila; ISSR; Genetic diversity; Phenotypic plasticity
1. Introduction The invasion of nonindigenous species is recognized as a major threat to global biodiversity, second only to habitat loss (McNeely, 1997). Understanding fundamental aspects of invasive populations, such as reproductive characteristics, genetic structure within and among populations, pathways of introduction and bottlenecking factors, is important to the development of management strategies (Carter and Sytsma, 2001). For example, knowledge of the extent and patterns of genetic variation can enable the detection of ecotypes or races which may differ considerably in their susceptibility to predators, parasites, or herbicides (Wain et al., 1985) and in their ecological preferences (Stebbins, 1942). Alligator weed, Alternanthera philoxeroides (Mart.) Grisb. (Amaranthaceae) originated in the Parana River region of South America (Vogt et al., 1979). It has spread to other areas of South America and to the continents of North America, Asia and Australia and some of the adjacent island countries. At present the weed has not been recorded in Africa; however, all African wetlands should be considered at risk (Sainty et al., 1998).
* Corresponding author. Tel.: +86 20 3725 2996; fax: +86 20 3725 2831. E-mail address: [email protected] (W.-H. Ye). 0304-3770/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2006.02.006
A. philoxeroides is a stoloniferous and rhizomatous perennial that will grow rapidly in both terrestrial and aquatic habitats. In some areas alligator weed chokes waterways while in others it invades pastoral and agricultural land (Coulson, 1977; Julien and Broadbent, 1980). Growth in aquatic areas always exceeded terrestrial growth. The aquatic plants have larger hollow stems, providing buoyancy; while on land, the stems are smaller and solid to slightly hollow (Julien et al., 1992). The weed is a well adapted species which is able to reproduce both sexually and asexually through stolons rhizomes or whatever, although seeds are often inviable (Julien, 1995). Thus, the asexual reproduction is by far a more important strategy for the build up of a large population within a comparatively short time. Classical biological weed control uses the planned relocation of natural enemies of introduced weeds from their native habitats onto weeds in their naturalized habitats. Agasicles hygrophila Selman and Vogt, a flea beetle, originating from Argentina in South America, is the specialist enemy of alligator weed. American entomologist Vogt found this insect in Argentina and introduced it in the United States in 1964 (Spencer and Coulson, 1976). Later, the Argentinian beetle was released in Australia (Coulson, 1977) and New Zealand (Stewart et al., 1999), while China introduced beetles from Florida in 1986 (Wang, 1990). The introduced flea beetles reduced the large floating mats of alligator weed, but had little impact on terrestrial plants (Coulson, 1977; Buckingham et al., 1983).
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J. Li, W.-H. Ye / Aquatic Botany 85 (2006) 155–158
The reasons of the variable nature of biological weed control have been debated but not explained. One important factor could be the genetic diversity in the target weed species (Lym et al., 1996). Moreover, plant genotype differences can affect the success of classical biological control and can be used as an indicator of the suitability of the target weed for this approach (Nissen et al., 1995). For example, when Puccinia chrondillina Bubak and Syd. was introduced as a biological control agent for Chondrilla juncea L. (skeleton weed), the introduced isolate attacked only the narrow-leaved form of the weed (Hasan and Jenkins, 1972). In the United States, two alligator weed biotypes are genetically distinct and differ in their susceptibility and response to control agents (Kay and Haller, 1982; Wain et al., 1985). Some previous studies (Xu et al., 2003; Ye et al., 2003; Wang et al., 2005) have demonstrated that there was extremely low genetic diversity or even no genetic variation among the studied samples of A. philoxeroides in China, but studies did not compare terrestrial with aquatic plants. In this study, our purpose is two-fold: to use ISSR technology which has been shown to generate sufficient markers to readily identify genotypic diversity in invasive plants (Ye et al., 2003; Chapman et al., 2004), and to detect if there is genetic difference between land- and water-grown populations of this weed in China. 2. Materials and methods 2.1. Sample collection Samples of A. philoxeroides were collected from 10 populations from 5 provinces in southern China. Each province included a terrestrial population and an aquatic population. Leaf samples were dried in silica gel until DNA extraction. 2.2. PCR reaction and ISSR analysis Leaf tissues were grounded into powder in liquid nitrogen. Genomic DNA was extracted following CTAB procedure (Doyle, 1991). DNA quality and quantity were checked on 1% agarose gels. One hundred ISSR primers from the University of British Columbia set no. 9 were initially screened for a few DNA samples from each population. PCR amplification was carried out in 20 mL of reaction mixture consisting of 20 ng of template DNA, 10 mM Tris–HCl (pH 9.0), 50 mM KCl, 0.1% Triton X-100, 2.7 mM MgCl2, 0.1 mM dNTPs, 2% formamide, 200 nM primer, 1.5 units of Taq polymerase and doubledistilled water. PCR was performed using an MJ Research (Waltham, MA, USA) 96-well thermal cycler with hot bonnet following the conditions of Ge and Sun (1999). Amplification products were resolved by electrophoresis on 1.5% agarose gels buffered with 0.5 TBE. A 100-bp DNA Ladder (New England Biolabs, Beverly, MA, USA) was used as a size marker (100– 1000 bp). DNA fragments were visualized by image analysis software for gel documentation (LabWorks Software Version 3.0; UVP, Upland, CA, USA) following staining with ethidium bromide. Only those bands that showed consistent amplification were considered. Smeared and weak bands were ignored.
Table 1 Sample size and location of populations of Alternanthera philoxeroides Population
Province
Type of habitat
Sample size
Longitude (E)
Latitude (N)
Shanghai
Shanghai
Aa Tb
18 18
1218280
318130
Nanjing
Jiangsu
A T
18 18
1188460
328240
Wuhan
Hubei
A T
18 18
1148190
308210
Kunming
Yunnan
A T
18 18
1028430
258240
Xiangtan
Hunan
A T
18 18
1128540
278470
a b
Aquatic habitat. Terrestrial habitat.
3. Results One hundred and eighty samples were collected totally. The collection data is shown in Table 1. From the pre-screening, 11 primers (Table 2) that gave clear and reproducible bands were used. A total of 86 clear and reproductive bands were amplified by the 11 primers with an average frequency of 7.8 bands per primer, but no polymorphic bands were detected. The ISSR markers not only revealed no genetic diversity within and among the populations growing in same habitats (Figs. 1 and 2), like previous studies, but also no difference in genetic diversity between populations in different type of habitats (Fig. 2). 4. Discussion Extensive clonal growth has also been detected in other species using molecular genetic techniques such as Fallopia japonica (Hollingsworth and Bayley, 2000). The favourable biological features of alligator weed for its invasiveness, for example its acclimation to varying climatic conditions (Julien et al., 1995), abilities to grow on a wide range of substrata (Sainty et al., 1998) and of fast vegetative propagation, and its ability of long distance dispersal of shoots, overcome the Table 2 ISSR primers used to amplify DNA samples of A. philoxeroides and generated PCR product bands Primer
Sequence
T (8C)
Number of bands
808 809 835 840 864 880 886 888 889 890 891
(AG)8C (AG)8G (AG)8 (GA)8CT (ATG)5 G(GA)8 (CTC)5 (CA)7 TGT(AC)7 CAC(GT)7 (TG)7
50 50 50 49 50 49 50 50 50 48 51
5 7 6 5 7 10 11 7 8 12 8
J. Li, W.-H. Ye / Aquatic Botany 85 (2006) 155–158
157
Fig. 1. ISSR bands amplified by the primer 840. The samples were arranged from left to right in the following sequence: (1) molecular weight marker, (2–16) Shanghai aquatic population.
Fig. 2. ISSR bands amplified by the primer 891. The samples were arranged from left to right in the following sequence: (1 and 2) Shanghai aquatic population, (3 and 4) Shanghai terrestrial population, (5 and 6) Nanjing aquatic population, (7 and 8) Nanjing terrestrial population, (9 and 10) Wuhan aquatic population, (11) molecular weight marker (the size of the marker is as Fig. 1.), (12 and 13) Wuhan terrestrial population, (14 and 15) Kunming aquatic population, (16 and 17) Kunming terrestrial population, (18 and 19) Xiangtan aquatic population, (20 and 21) Xiangtan terrestrial population.
disadvantage of its limited genetic variation and make it a successful invader in China. Alligator weed may have a ‘‘general-purpose genotype’’ facilitating its invasion. In 1965 Herbert Baker coined the term ‘‘general-purpose genotype’’ to describe colonizing species that thrive in a wide range of environmental conditions through phenotypic or developmental plasticity (Baker, 1965). A general-purpose genotype allows for the success of populations founded by small numbers of individuals through reproductive systems such as autogamy that do not promote genetic exchange but do provide reproductive assurance. In the United States, the two distinct biotypes (broadstemmed and narrow-stemmed) are genetically different and represent separate introductions of previously differentiated populations into the United States from origins in South America (Kay and Haller, 1982; Wain et al., 1984). In China, contrarily, the two biotypes appear to be same in genetic composition and the morphological variations may be assigned to environmental induced changes. It has been known that alligator weed exhibits considerable morphological variation, much of which is directly attributable to environmental conditions (Wain et al., 1985). Our study suggests that the genetic diversity between land- and water-grown populations of alligator weed is not the reason for the differing effect of A. hygrophila as a biological control agent in the two ecological conditions in China. Another difference from the United States is that alligator weed was introduced to China in the 1930s from
Japan (Zhang et al., 2001), and the populations in China may contain a mere subset of the genetic diversity represented in A. philoxeroides Japanese populations. The biological control mechanism of A. hygrophila on alligator weed can be explained as follows. The larvae and adult flea beetles chew the upper leaves and tender stems of the weed; on the other hand, the 3-year larvae pupate in stems, prohibit the growth of internodes and secrete toxins that may inhibit growth and eventually kill the plants (Coulson, 1977; Stewart et al., 2000). Pupation is the key phase for the beetle to complete its life cycle and control alligator weed effectively. Vogt et al. (1979) noted that small stem diameters restricted the population of the flea beetle even though prepupae could enlarge the stem cavity somewhat in the more slender stems and pupate (Kay and Haller, 1982). And from our observation, we also agree with the previous conclusion that the stem diameter factor, as larger pupation chambers may favour insect survival. Another possible reason may be the phytochemical basis of agent– weed interaction which needs further research. Acknowledgements We are grateful to Dr. Zhang-Ming Wang and Dr. Xue-Jun Ge for giving advice. We thank G. Bowes and two anonymous reviewers for helpful comments on the manuscript. This work was supported by the State Key Basic Research and Development Plan (G2000046803).
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References Baker, H.G., 1965. Characteristic and modes of origin of weeds. In: Baker, H.G., Stebbins, G.L. (Eds.), The Genetics of Colonizing Species. Academic Press, New York, pp. 147–168. Buckingham, G.R., Boucias, D., Theriot, R.F., 1983. Reintroduction of the alligatorweed flea beetle (Agasicles hygrophila Selman and Vogt) into the United States from Argentina. J. Aquat. Plant Manage. 21, 101–102. Carter, M.C., Sytsma, M.D., 2001. Comparison of the genetic structure of North and South American populations of a clonal aquatic plant. Biol. Invasions 3, 113–118. Chapman, H.M., Robson, B., Pearson, M.L., 2004. Population genetic structure of a colonising, triploid weed, Hieracium lepidulum. Heredity 92, 182–188. Coulson, J.R., 1977. Biological control of alligatorweed, 1959–1972: a review and evaluation. U.S. Department of Agriculture Technical Bulletin No. 1547, p. 98. Doyle, J., 1991. DNA protocols for plants—CTAB total DNA isolation. In: Hewitt, G.M., Johnston, A. (Eds.), Molecular Techniques in Taxonomy. Springer, Berlin, Germany, pp. 283–293. Ge, X.J., Sun, M., 1999. Reproductive biology and genetic diversity of a cryptoviviparous mangrove Aegiceras corniculatum (Myrsinaceae) using allozyme and intersimple sequence repeat (ISSR) analysis. Mol. Ecol. 8, 443–451. Hasan, S., Jenkins, P.T., 1972. The effect of some climatic factors on infectivity of the skeleton weed rust, Puccinia chondrillina. Plant Dis. Rep. 56, 858–860. Hollingsworth, M.L., Bayley, J.P., 2000. Evidence for massive clonal growth in the invasive weed Fallopia japonica (Japanase knotweed). Bot. J. Linn. Soc. 133, 463–472. Julien, M.H., 1995. Alternanthera philoxeroides (Mart.) Griseb. In: Groves, R.H., Shepherd, R.C.H., Richardson, R.C. (Eds.), The Biology of Australian Weeds. R.G. and F.J. Richardson, Frankston, Victoria, pp. 1–12. Julien, M.H., Broadbent, J.E., 1980. The biology of Australian weeds 3, Alternanthera philoxeroides (Mart.) Griseb. J. Aust. Inst. Agric. Sci. 46, 150–155. Julien, M.H., Chan, R.R., Low, V., 1992. Growth of the weed Alternanthera Philoxeroides (Martius) Grisebach, (alligator weed) in aquatic and terrestrial habitats in Australia. Plant Prot. Q. 7, 1–7. Julien, M.H., Skarratt, B., Maywald, G.F., 1995. Potential geographical distribution of alligator weed and its biological control by Agasicles hygrophila. J. Aquat. Plant Manage. 33, 55–60. Kay, S.H., Haller, W.T., 1982. Evidence for the existence of distinct alligatorweed biotypes. J. Aquat. Plant Manage. 20, 37–41. Lym, R.G., Nissen, S.J., Rowe, M.L., Lee, D.J., Masters, R.A., 1996. Leafy spurge (Euphorbia esula) genotype affects gall midge (Spurgia esulae) establishment. Weed Sci. 44, 629–633.
McNeely, J., 1997. IUCN (International Union for Conservation of Nature and Natural Resources) alien invasive species: a framework for action. In: Rubec, C.D.A., Lee, G.O. (Eds.), Conserving Vitality and Diversity: Proceedings of the World Conservation Congress Workshop on Alien Invasive Species, IUCN and Environment Canada, Ottawa, pp. 3–9. Nissen, S.J., Masters, R.A., Lee, D.J., Rowe, M.L., 1995. DNA based marker systems to determine genetic diversity of weedy species and their application to biocontrol. Weed Sci. 42, 504–513. Sainty, G.R., MuCorkelle, G., Julien, M., 1998. Control and spread of alligator weed Alternanthera philoxeroides (Mart.) Griseb., in Australia: lessons for other regions. Wetlands Ecol. Manage. 5, 195–201. Spencer, N.R., Coulson, J.R., 1976. The biological control of alligator weed, Alternanthera philoxeroides, in the United States of America. Aquat. Bot. 2, 177–190. Stebbins, G., 1942. The genetic approach to problems of rare and endemic species. Madrono 6, 242–272. Stewart, C.A., Chapman, R.B., Emberson, R.M., Syrett, P., Frampton, C.M.A., 1999. The effect of temperature on the development and survival of Agasicles hygrophila, a biological agent for alligatorweed (A. philoxeroides) NZ J. Zool. 26, 11–20. Stewart, C.A., Chapman, R.B., Frampton, C.M.A., 2000. Growth of alligator weed (Alternanthera philoxeroides) and population development of Agasicles hygrophila in northern New Zealand. Plant Prot. Q. 15, 95– 101. Vogt, G.B., McGurie Jr., J.U., Cushman, A.D., 1979. Probable evolution and morphological variation in South American Disonychine flea beetles (Coleoptera: Chrysomelidae) and their Amaranthaceous hosts. USDA Technical Bulletin. 1593, p. 148. Wain, R.P., Haller, W.T., Martin, D.F., 1984. Genetic relationship among two forms of alligatorweed. J. Aquat. Plant Manage. 22, 104–105. Wain, R.P., Haller, W.T., Martin, D.F., 1985. Isozymes in studies of aquatic plants. J. Aquat. Plant Manage. 23, 42–45. Wang, B.R., Li, W.G., Wang, J.B., 2005. Genetic diversity of Alternanthera philoxeroides in China. Aquat. Bot. 81 (3), 277–283. Wang, R., 1990. Biological control of weeds in China: a status report. In: Proceedings of the Eighth International Symposium on Biological Control of Weeds. Rome, Italy. Istituto Sperimentale per la Patologia Vegetale, Ministero dell’Agricoltura e delle Foreste. Xu, C.Y., Zhang, W.J., Fu, C.Z., Lu, B.R., 2003. Genetic diversity of alligator weed in China by RAPD analysis. Biodivers. Conserv. 12, 637–645. Ye, W.H., Li, J., Cao, H.L., Ge, X.J., 2003. Genetic uniformity of Alternanthera philoxeroides in South China. Weed Res. 43, 297–302. Zhang, B., Jin, Y.G., Huai, H.G., Shi, H.Y., 2001. The comparative leaf anatomy of alligator weed under two ecological conditions. Weed Sci. 4, 6–8.
Short communication
Genetic diversity of alligator weed ecotypes is not the reason for their different responses to biological control Jing Li, Wan-Hui Ye * South China Botanical Garden, the Chinese Academy of Sciences, Guangzhou 510650, China Received 26 May 2005; received in revised form 13 December 2005; accepted 22 February 2006
Abstract Biological control of alligator weed Alternanthera philoxeroides (Mart.) Griseb. using Agasicles hygrophila, a Chrysomelid beetle, has been successful in limiting growth in water, but not on land. In order to understand a possible genetic basis of this difference, technique using intersimple sequence repeats (ISSR) markers was applied to analyse genetic diversity of this invasive weed. No genetic variation was detected not only within or between populations growing in the same habitats, but also between land- and water-grown populations. Thus we consider that the genetic variation is not the baseline factor resulting in the biological control difference in China. The differential success of pupation by the beetle may be related to the phenotypic plasticity of the plant stem diameter, rather than to genotypic factors. # 2006 Elsevier B.V. All rights reserved. Keywords: Alligator weed; Alternanthera philoxeroides; Biological control; Agasicles hygrophila; ISSR; Genetic diversity; Phenotypic plasticity
1. Introduction The invasion of nonindigenous species is recognized as a major threat to global biodiversity, second only to habitat loss (McNeely, 1997). Understanding fundamental aspects of invasive populations, such as reproductive characteristics, genetic structure within and among populations, pathways of introduction and bottlenecking factors, is important to the development of management strategies (Carter and Sytsma, 2001). For example, knowledge of the extent and patterns of genetic variation can enable the detection of ecotypes or races which may differ considerably in their susceptibility to predators, parasites, or herbicides (Wain et al., 1985) and in their ecological preferences (Stebbins, 1942). Alligator weed, Alternanthera philoxeroides (Mart.) Grisb. (Amaranthaceae) originated in the Parana River region of South America (Vogt et al., 1979). It has spread to other areas of South America and to the continents of North America, Asia and Australia and some of the adjacent island countries. At present the weed has not been recorded in Africa; however, all African wetlands should be considered at risk (Sainty et al., 1998).
* Corresponding author. Tel.: +86 20 3725 2996; fax: +86 20 3725 2831. E-mail address: [email protected] (W.-H. Ye). 0304-3770/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2006.02.006
A. philoxeroides is a stoloniferous and rhizomatous perennial that will grow rapidly in both terrestrial and aquatic habitats. In some areas alligator weed chokes waterways while in others it invades pastoral and agricultural land (Coulson, 1977; Julien and Broadbent, 1980). Growth in aquatic areas always exceeded terrestrial growth. The aquatic plants have larger hollow stems, providing buoyancy; while on land, the stems are smaller and solid to slightly hollow (Julien et al., 1992). The weed is a well adapted species which is able to reproduce both sexually and asexually through stolons rhizomes or whatever, although seeds are often inviable (Julien, 1995). Thus, the asexual reproduction is by far a more important strategy for the build up of a large population within a comparatively short time. Classical biological weed control uses the planned relocation of natural enemies of introduced weeds from their native habitats onto weeds in their naturalized habitats. Agasicles hygrophila Selman and Vogt, a flea beetle, originating from Argentina in South America, is the specialist enemy of alligator weed. American entomologist Vogt found this insect in Argentina and introduced it in the United States in 1964 (Spencer and Coulson, 1976). Later, the Argentinian beetle was released in Australia (Coulson, 1977) and New Zealand (Stewart et al., 1999), while China introduced beetles from Florida in 1986 (Wang, 1990). The introduced flea beetles reduced the large floating mats of alligator weed, but had little impact on terrestrial plants (Coulson, 1977; Buckingham et al., 1983).
156
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The reasons of the variable nature of biological weed control have been debated but not explained. One important factor could be the genetic diversity in the target weed species (Lym et al., 1996). Moreover, plant genotype differences can affect the success of classical biological control and can be used as an indicator of the suitability of the target weed for this approach (Nissen et al., 1995). For example, when Puccinia chrondillina Bubak and Syd. was introduced as a biological control agent for Chondrilla juncea L. (skeleton weed), the introduced isolate attacked only the narrow-leaved form of the weed (Hasan and Jenkins, 1972). In the United States, two alligator weed biotypes are genetically distinct and differ in their susceptibility and response to control agents (Kay and Haller, 1982; Wain et al., 1985). Some previous studies (Xu et al., 2003; Ye et al., 2003; Wang et al., 2005) have demonstrated that there was extremely low genetic diversity or even no genetic variation among the studied samples of A. philoxeroides in China, but studies did not compare terrestrial with aquatic plants. In this study, our purpose is two-fold: to use ISSR technology which has been shown to generate sufficient markers to readily identify genotypic diversity in invasive plants (Ye et al., 2003; Chapman et al., 2004), and to detect if there is genetic difference between land- and water-grown populations of this weed in China. 2. Materials and methods 2.1. Sample collection Samples of A. philoxeroides were collected from 10 populations from 5 provinces in southern China. Each province included a terrestrial population and an aquatic population. Leaf samples were dried in silica gel until DNA extraction. 2.2. PCR reaction and ISSR analysis Leaf tissues were grounded into powder in liquid nitrogen. Genomic DNA was extracted following CTAB procedure (Doyle, 1991). DNA quality and quantity were checked on 1% agarose gels. One hundred ISSR primers from the University of British Columbia set no. 9 were initially screened for a few DNA samples from each population. PCR amplification was carried out in 20 mL of reaction mixture consisting of 20 ng of template DNA, 10 mM Tris–HCl (pH 9.0), 50 mM KCl, 0.1% Triton X-100, 2.7 mM MgCl2, 0.1 mM dNTPs, 2% formamide, 200 nM primer, 1.5 units of Taq polymerase and doubledistilled water. PCR was performed using an MJ Research (Waltham, MA, USA) 96-well thermal cycler with hot bonnet following the conditions of Ge and Sun (1999). Amplification products were resolved by electrophoresis on 1.5% agarose gels buffered with 0.5 TBE. A 100-bp DNA Ladder (New England Biolabs, Beverly, MA, USA) was used as a size marker (100– 1000 bp). DNA fragments were visualized by image analysis software for gel documentation (LabWorks Software Version 3.0; UVP, Upland, CA, USA) following staining with ethidium bromide. Only those bands that showed consistent amplification were considered. Smeared and weak bands were ignored.
Table 1 Sample size and location of populations of Alternanthera philoxeroides Population
Province
Type of habitat
Sample size
Longitude (E)
Latitude (N)
Shanghai
Shanghai
Aa Tb
18 18
1218280
318130
Nanjing
Jiangsu
A T
18 18
1188460
328240
Wuhan
Hubei
A T
18 18
1148190
308210
Kunming
Yunnan
A T
18 18
1028430
258240
Xiangtan
Hunan
A T
18 18
1128540
278470
a b
Aquatic habitat. Terrestrial habitat.
3. Results One hundred and eighty samples were collected totally. The collection data is shown in Table 1. From the pre-screening, 11 primers (Table 2) that gave clear and reproducible bands were used. A total of 86 clear and reproductive bands were amplified by the 11 primers with an average frequency of 7.8 bands per primer, but no polymorphic bands were detected. The ISSR markers not only revealed no genetic diversity within and among the populations growing in same habitats (Figs. 1 and 2), like previous studies, but also no difference in genetic diversity between populations in different type of habitats (Fig. 2). 4. Discussion Extensive clonal growth has also been detected in other species using molecular genetic techniques such as Fallopia japonica (Hollingsworth and Bayley, 2000). The favourable biological features of alligator weed for its invasiveness, for example its acclimation to varying climatic conditions (Julien et al., 1995), abilities to grow on a wide range of substrata (Sainty et al., 1998) and of fast vegetative propagation, and its ability of long distance dispersal of shoots, overcome the Table 2 ISSR primers used to amplify DNA samples of A. philoxeroides and generated PCR product bands Primer
Sequence
T (8C)
Number of bands
808 809 835 840 864 880 886 888 889 890 891
(AG)8C (AG)8G (AG)8 (GA)8CT (ATG)5 G(GA)8 (CTC)5 (CA)7 TGT(AC)7 CAC(GT)7 (TG)7
50 50 50 49 50 49 50 50 50 48 51
5 7 6 5 7 10 11 7 8 12 8
J. Li, W.-H. Ye / Aquatic Botany 85 (2006) 155–158
157
Fig. 1. ISSR bands amplified by the primer 840. The samples were arranged from left to right in the following sequence: (1) molecular weight marker, (2–16) Shanghai aquatic population.
Fig. 2. ISSR bands amplified by the primer 891. The samples were arranged from left to right in the following sequence: (1 and 2) Shanghai aquatic population, (3 and 4) Shanghai terrestrial population, (5 and 6) Nanjing aquatic population, (7 and 8) Nanjing terrestrial population, (9 and 10) Wuhan aquatic population, (11) molecular weight marker (the size of the marker is as Fig. 1.), (12 and 13) Wuhan terrestrial population, (14 and 15) Kunming aquatic population, (16 and 17) Kunming terrestrial population, (18 and 19) Xiangtan aquatic population, (20 and 21) Xiangtan terrestrial population.
disadvantage of its limited genetic variation and make it a successful invader in China. Alligator weed may have a ‘‘general-purpose genotype’’ facilitating its invasion. In 1965 Herbert Baker coined the term ‘‘general-purpose genotype’’ to describe colonizing species that thrive in a wide range of environmental conditions through phenotypic or developmental plasticity (Baker, 1965). A general-purpose genotype allows for the success of populations founded by small numbers of individuals through reproductive systems such as autogamy that do not promote genetic exchange but do provide reproductive assurance. In the United States, the two distinct biotypes (broadstemmed and narrow-stemmed) are genetically different and represent separate introductions of previously differentiated populations into the United States from origins in South America (Kay and Haller, 1982; Wain et al., 1984). In China, contrarily, the two biotypes appear to be same in genetic composition and the morphological variations may be assigned to environmental induced changes. It has been known that alligator weed exhibits considerable morphological variation, much of which is directly attributable to environmental conditions (Wain et al., 1985). Our study suggests that the genetic diversity between land- and water-grown populations of alligator weed is not the reason for the differing effect of A. hygrophila as a biological control agent in the two ecological conditions in China. Another difference from the United States is that alligator weed was introduced to China in the 1930s from
Japan (Zhang et al., 2001), and the populations in China may contain a mere subset of the genetic diversity represented in A. philoxeroides Japanese populations. The biological control mechanism of A. hygrophila on alligator weed can be explained as follows. The larvae and adult flea beetles chew the upper leaves and tender stems of the weed; on the other hand, the 3-year larvae pupate in stems, prohibit the growth of internodes and secrete toxins that may inhibit growth and eventually kill the plants (Coulson, 1977; Stewart et al., 2000). Pupation is the key phase for the beetle to complete its life cycle and control alligator weed effectively. Vogt et al. (1979) noted that small stem diameters restricted the population of the flea beetle even though prepupae could enlarge the stem cavity somewhat in the more slender stems and pupate (Kay and Haller, 1982). And from our observation, we also agree with the previous conclusion that the stem diameter factor, as larger pupation chambers may favour insect survival. Another possible reason may be the phytochemical basis of agent– weed interaction which needs further research. Acknowledgements We are grateful to Dr. Zhang-Ming Wang and Dr. Xue-Jun Ge for giving advice. We thank G. Bowes and two anonymous reviewers for helpful comments on the manuscript. This work was supported by the State Key Basic Research and Development Plan (G2000046803).
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References Baker, H.G., 1965. Characteristic and modes of origin of weeds. In: Baker, H.G., Stebbins, G.L. (Eds.), The Genetics of Colonizing Species. Academic Press, New York, pp. 147–168. Buckingham, G.R., Boucias, D., Theriot, R.F., 1983. Reintroduction of the alligatorweed flea beetle (Agasicles hygrophila Selman and Vogt) into the United States from Argentina. J. Aquat. Plant Manage. 21, 101–102. Carter, M.C., Sytsma, M.D., 2001. Comparison of the genetic structure of North and South American populations of a clonal aquatic plant. Biol. Invasions 3, 113–118. Chapman, H.M., Robson, B., Pearson, M.L., 2004. Population genetic structure of a colonising, triploid weed, Hieracium lepidulum. Heredity 92, 182–188. Coulson, J.R., 1977. Biological control of alligatorweed, 1959–1972: a review and evaluation. U.S. Department of Agriculture Technical Bulletin No. 1547, p. 98. Doyle, J., 1991. DNA protocols for plants—CTAB total DNA isolation. In: Hewitt, G.M., Johnston, A. (Eds.), Molecular Techniques in Taxonomy. Springer, Berlin, Germany, pp. 283–293. Ge, X.J., Sun, M., 1999. Reproductive biology and genetic diversity of a cryptoviviparous mangrove Aegiceras corniculatum (Myrsinaceae) using allozyme and intersimple sequence repeat (ISSR) analysis. Mol. Ecol. 8, 443–451. Hasan, S., Jenkins, P.T., 1972. The effect of some climatic factors on infectivity of the skeleton weed rust, Puccinia chondrillina. Plant Dis. Rep. 56, 858–860. Hollingsworth, M.L., Bayley, J.P., 2000. Evidence for massive clonal growth in the invasive weed Fallopia japonica (Japanase knotweed). Bot. J. Linn. Soc. 133, 463–472. Julien, M.H., 1995. Alternanthera philoxeroides (Mart.) Griseb. In: Groves, R.H., Shepherd, R.C.H., Richardson, R.C. (Eds.), The Biology of Australian Weeds. R.G. and F.J. Richardson, Frankston, Victoria, pp. 1–12. Julien, M.H., Broadbent, J.E., 1980. The biology of Australian weeds 3, Alternanthera philoxeroides (Mart.) Griseb. J. Aust. Inst. Agric. Sci. 46, 150–155. Julien, M.H., Chan, R.R., Low, V., 1992. Growth of the weed Alternanthera Philoxeroides (Martius) Grisebach, (alligator weed) in aquatic and terrestrial habitats in Australia. Plant Prot. Q. 7, 1–7. Julien, M.H., Skarratt, B., Maywald, G.F., 1995. Potential geographical distribution of alligator weed and its biological control by Agasicles hygrophila. J. Aquat. Plant Manage. 33, 55–60. Kay, S.H., Haller, W.T., 1982. Evidence for the existence of distinct alligatorweed biotypes. J. Aquat. Plant Manage. 20, 37–41. Lym, R.G., Nissen, S.J., Rowe, M.L., Lee, D.J., Masters, R.A., 1996. Leafy spurge (Euphorbia esula) genotype affects gall midge (Spurgia esulae) establishment. Weed Sci. 44, 629–633.
McNeely, J., 1997. IUCN (International Union for Conservation of Nature and Natural Resources) alien invasive species: a framework for action. In: Rubec, C.D.A., Lee, G.O. (Eds.), Conserving Vitality and Diversity: Proceedings of the World Conservation Congress Workshop on Alien Invasive Species, IUCN and Environment Canada, Ottawa, pp. 3–9. Nissen, S.J., Masters, R.A., Lee, D.J., Rowe, M.L., 1995. DNA based marker systems to determine genetic diversity of weedy species and their application to biocontrol. Weed Sci. 42, 504–513. Sainty, G.R., MuCorkelle, G., Julien, M., 1998. Control and spread of alligator weed Alternanthera philoxeroides (Mart.) Griseb., in Australia: lessons for other regions. Wetlands Ecol. Manage. 5, 195–201. Spencer, N.R., Coulson, J.R., 1976. The biological control of alligator weed, Alternanthera philoxeroides, in the United States of America. Aquat. Bot. 2, 177–190. Stebbins, G., 1942. The genetic approach to problems of rare and endemic species. Madrono 6, 242–272. Stewart, C.A., Chapman, R.B., Emberson, R.M., Syrett, P., Frampton, C.M.A., 1999. The effect of temperature on the development and survival of Agasicles hygrophila, a biological agent for alligatorweed (A. philoxeroides) NZ J. Zool. 26, 11–20. Stewart, C.A., Chapman, R.B., Frampton, C.M.A., 2000. Growth of alligator weed (Alternanthera philoxeroides) and population development of Agasicles hygrophila in northern New Zealand. Plant Prot. Q. 15, 95– 101. Vogt, G.B., McGurie Jr., J.U., Cushman, A.D., 1979. Probable evolution and morphological variation in South American Disonychine flea beetles (Coleoptera: Chrysomelidae) and their Amaranthaceous hosts. USDA Technical Bulletin. 1593, p. 148. Wain, R.P., Haller, W.T., Martin, D.F., 1984. Genetic relationship among two forms of alligatorweed. J. Aquat. Plant Manage. 22, 104–105. Wain, R.P., Haller, W.T., Martin, D.F., 1985. Isozymes in studies of aquatic plants. J. Aquat. Plant Manage. 23, 42–45. Wang, B.R., Li, W.G., Wang, J.B., 2005. Genetic diversity of Alternanthera philoxeroides in China. Aquat. Bot. 81 (3), 277–283. Wang, R., 1990. Biological control of weeds in China: a status report. In: Proceedings of the Eighth International Symposium on Biological Control of Weeds. Rome, Italy. Istituto Sperimentale per la Patologia Vegetale, Ministero dell’Agricoltura e delle Foreste. Xu, C.Y., Zhang, W.J., Fu, C.Z., Lu, B.R., 2003. Genetic diversity of alligator weed in China by RAPD analysis. Biodivers. Conserv. 12, 637–645. Ye, W.H., Li, J., Cao, H.L., Ge, X.J., 2003. Genetic uniformity of Alternanthera philoxeroides in South China. Weed Res. 43, 297–302. Zhang, B., Jin, Y.G., Huai, H.G., Shi, H.Y., 2001. The comparative leaf anatomy of alligator weed under two ecological conditions. Weed Sci. 4, 6–8.