| The research of invasion mechanism is always the hotspot and frontier of invasive biology.It is found that the frequency of polyploidy is very high in the invasion hot spots and even in the global investigation of invasive plant species.Therefore,more and more scientists have viewed polyploidy as an important mechanism for the success of alien plant species invasion.Nevertheless,the mechanism of higher fitness and broader ecological tolerance of polyploids than that of diploids needs more empirical research.Another scientific issue related to the mechanism of plant invasion is that does polyploidy facilitate plant invasion through pre adaptation,post adaptation evolution or both?Solidago canadensis L.(Asteraceae),a rhizomatous perennial herb,originated in North America,now widely invades into Europe and East Asia.In 1935,it was introduced into China as a garden flower,and then it became an invasive weed,causing serious ecological and economic losses.There are various ploidy levels in S.canadensis,and the common ones are diploid(2n=18),tetraploid(2n=36),and hexaploid(2n=56).Previous studies showed that the main invaders in China were hexaploids and a small number of tetraploids.Polyploidization contributes to facilitate the invasion of S.canadensis into China.Nevertheless,at present,the mechanism of polyploidy leading to the global distribution and adaptive differentiation of S.canadensis is not particularly clear.Based on the previous research of our laboratory,the number of test population was further expanded and the global geographical distribution and climate niche differentiation of different geo-gytotypes S.canadensis was analyzed.The effects of polyploidy on the characteristics related to adaptability,such as heat tolerance,growth,reproduction,and competitiveness,were analyzed by using the common garden experiments in native and introduced range.We aimed to elucidate the key mechanism of successful invasion of S.canadensis and reveal the pre adaptation and post adaptation in the invasion of S.Canadensis in China.The main research contents and results are as follows:Based on the previous research,the ploidy level of newly collected plants was detected by flow cytometry,and the number of population was further expanded on 150.And these combined with 321 published sample points,as a sample data set.Based on the geocytological analysis in the different S.canadensis geo-cytotypes,this phenomenon that the shifts on geographical distribution and climatic factors among different S.canadensis geo-cytotypes were highly overlapped,but polyploids slightly shifted to the low latitude in geographical distribution,and to the higher temperature and precipitation on the climatic factors were found.The differentiation between diploid and polyploidy was significant in geographical distribution and climatic factors in introduced range.The diploids mainly invaded areas with similar climate as the native range,while polyploid successfully invaded areas with lower latitude and higher temperature.MaxEnt climate niche model was used to predict the potential distribution of S.canadensis under the current climate and RCP4.5 scenario.The results showed that that the distributions of three cytotype of S.canadensis will expand with the climate warming,and large parts of Europe,which are currently invaded by diploids only,may see increased infestation by polyploids.These results indicate that temperature is the key climatic factor affecting the geographical distribution of different cytotype of S.canadensis.The global invasion of S.canadensis may be the result of the interaction between adaptive differentiation caused by polyploidy and selection pressure in new environment.To test the above findings,and study the heat tolerance of S.canadensis seedlings with different geo-cytotypes,and its physiological and biochemical mechanism,the heat tolerance index of seedlings of 86 populations(grow in above common garden)was measured after exposure at 45? for 10 hours and the antioxidant enzyme activity,antioxidant and osmoregulation substance content in leaves of the same populations was measured after exposure at 45? for 0,2 and 4 hours.The results show that the heat tolerance index of polyploids was significantly higher than that of diploids.The heat tolerance index was negatively correlated with latitude and positively correlated with the mean temperature of the hottest month(July temperature).Polyploid can maintain higher antioxidant enzyme activity,and their hydrogen peroxide(H2O2)content was significantly lower than diploids.The antioxidant enzyme activity of diploids was lower,but their ascorbic acid(AsA)and soluble protein content was higher than that of polyploids.These results showed that polyploidy enhanced the heat tolerance of S.canadensis and facilitated the pre adaptation to warm climate?This may be related to the adaptation to the ambient temperature of the sample sites.Furthermore,the effects of heat stress on the growth,asexual and sexual reproduction of S.canadensis with different geo-cytotypes also was studied.Two conditions were set up in the artificial climate chamber,i.e.low temperature(LT)group(20/15?)and high temperature(HT)group(3 0/25?),and the plants from 16 S.canadensis populations of different geo-cytotypes were grown under the two conditions for 2.5 months.The results show that the biomass,stem diameter,rhizome biomass and length of polyploids were significantly higher than that of diploids both in LT and HT group.Compared with HT group,the plants tend to allocate more biomass to the underground part in LT group.Compared with LT group,the rhizome growth of diploids from HT group was inhibited,while the number,length and biomass of rhizomes in introduced polyploids increased significantly.In terms of sexual reproduction,The results showed that the native populations and the introduced diploid populations,which could not produce viable seeds under the natural condition(June to August)of Nanjing common garden,produced a certain number of viable seeds at 24/18?,while the number of viable seeds in introduced polyploid population,which could produce a large number of viable seeds under the natural condition(October to November)of Nanjing common garden,decreased significantly at 36/30? than under natural condition.The results indicated that temperature and ploidy were the main factors affecting the asexual and sexual reproduction of S.canadensis.Polyploidy increased the asexual and sexual reproduction capacity of S.canadensis under heat stress through enhancing heat tolerance.In order to fully reveal the effect of polyploidy on the adaptive phenotypes of S.canadensis,seedling morphology characteristic,flowering period and fruit capacity of adult plants of 95 populations were measured in Nanjing common garden.The results show that that polyploids have larger leaves,more and longer roots,stronger and higher stems,lower root shoot ratio,higher biomass and growth rate,later flowering and higher fruit capacity than diploids.These indexes were significantly correlated with latitude,showing cline along temperature and precipitation gradient,and were mainly caused by polyploidy.After introduction,the above advantages of tetraploid and hexaploid in introduced range were further enhanced compared with the corresponding native cytotypes,but there was no significant difference between the native diploids and introduced ones.The results show that the polyploidy drives the adaptive differentiation of S.canadensis and contributes to the formation of cline along latitudinal gradient.The polyploid had a pre adaptive evolution to the high temperature and rainy climate in the lower latitude.After introduction,the rapid post adaptive evolution under the selection pressure in new habitat facilitated the successful invasion of S.canadensis in China.Nevertheless,the results got in single common garden need to be verified in multiple common gardens.It is difficult to exclude the influence of climate on the test results in single common garden.To confirm that polyploidy is the key factor to facilitate the adaptive character differentiation of S.canadensis,we set up four common gardens in the high and low latitudes in native(North America)and introduced(China)range were set up,and the morphological characteristics,phenology,fruit production and survival of the different geo-cytotype populations in the seedling and the mature stage were compared.The results show that the polyploids had higher plant height,number of leaves,total biomass and lower root shoot ratio of seedlings than the diploids both in native and introduced range.The introduced polyploids had better performance than the corresponding native polyploids.These results were basically consistent among the four common gardens.Polyploids had higher plant height and later flowering in the mature stage.The diploids had higher fruit production than polyploids in high latitude common gardens,but lower in low latitude common gardens.The introduced polyploids had the best performance than other geo-cytotypes in low latitude common gardens,but could not bear fruit because of their late flowering or even survive the winter in high latitude common gardens.The results indicated that polyploidization drove the adaptive differentiation of S.canadensis in native range,and the polyploid evolved the characteristics adapting to warm and humid climate.Polyploids had undergone further adaptive evolution in introduced range(China).There was a trade-off between phenotypic evolution and cold tolerance,which may be the result of natural selection.Autopolyploidy has been shown to significantly contribute to successful invasion of China by Solidago canadensis that is native to North America.But how polyploidization improves competitive ability and determines the dominance of invasive species when competing with a plant community in the introduced range remains unclear.We manipulated the initial plant composition of plowed land and subsequently allowed natural colonization by S.canadensis in a five-year common garden experiment.Diploid,tetraploid and hexaploid populations collected in North America(native range)and East Asia(introduced range)were separately planted and allowed to compete with associated weeds in individual plots.The diversity and compositional variation of the plant communities and the growth characteristics of S.canadensis were investigated in summer and autumn each year.Based on how the community assembled,three outcomes were found:1)S.canadensis outcompeted local vegetation:tetraploids and hexaploids from the introduced range outcompeted associated weeds and were dominant at equilibrium;2)S.canadensis coexisted with local vegetation:hexaploids from the native range were competitive but ultimately could not outcompete the local vegetation;and 3)S.canadensis became extinct:diploids from both the native and introduced ranges and tetraploids from the native range went extinct.Concomitantly,diversity was low in the first group and high in the second and third.Therefore,polyploidization contributes to the pre differentiation of competitive ability among native S.canadensis populations,facilitating the invasion of China by this species.The competitive ability of polyploids was enhanced through possible rapid post introduction evolution after their introduction to China,which could be the crucial factor of successful invasion by S.canadensis.In summary,polyploidization drives climate niche shifts of S.canadensis and facilitates the range expansion of polyploids to climate zone with lower latitude and higher temperature,and may further expand with global warming.Polyploidization drives pre adaptation to warm environment,which laid the foundation for S.canadensis to invade China.The advantages of growth and reproduction characteristics related to the adaptability of introduced polyploids were further enhanced.Multiple common garden experiments confirmed that polyploidy drove the pre and rapid post adaptation of S.canadensis,and facilitated its successful invasion in China.The rapid post evolution of competition ability may be the key to the successful invasion of China by S.canadensis. |
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