Adaptive Evolution Of The Invasive Plant Spartina Alterniflora Across Latitudinal Gradients In Its Invasive And Native Ranges

Biological invasion has been recognized as the environmental and economic problems,which is one of the greatest challenges for mankind in the 21st century.Especially,it is very important to study the invasion pattern,process and mechanism of alien invasive species across latitude under global change.Also,most of invasive species have re-established across vast latitude in the invasive range which is similar with or lower than those in the native range.As the weaker interspecific competition in the isalnds and coastal wetland compared with terrestrial ecosystem,alien invasive species often invade into lower latitudes than in the native range.Therefore,parsing how multiple factors and possible interactions drive the success of invasive species becomes an urgent to deal with the serious threat of biological invasion in the coastal wetland.Biological invasions are mainly facilitated by phenotypic plasticity and pre-adaptation.Also,it is possible for alien species to undergo rapid evolution by responding to the selection pressure from liberation of exotic plants from their natural enemies and geographic gradients in abioptic conditions,intraspecific or interspecific hybridization and genetic drift.The rapid invasion of the global salt-marsh grass Spartina alterniflora along the coast of China(19°-41°N)and the Southern Hemisphere(Brazil,Argentina,South Africa,and New Zealand:24°-40°N),where it extends to latitudes lower than it occupies in its native range(United States,27°-43°N),provides a unique opportunity to study how the phenotypic plasticity,pre-adaptation,rapid evolution or the possible interactions affect geographic clines in the native and introduced ranges.We studied patterns in vegetative growth and sexual reproduction of global invasive grass S.alterniflora across latitudes in the introduced range at the end of the 2012-2014 growing seasons and native range at the end of the 2014 growing seasons by field survey,we also identified correlations between plant traits and abiotic conditions(cliamte,hydrology and soil etc.)which were collected from field locations To assess the role of environment vs.genetic differentiation in these patterns,we grew plants from the 10 field sites in three common gardens at low(20.9°N),mid(28.3°N),and high(38.0°N)latitudes to maturity from the year 2014 to 2015.To examine the role of phenotypic plasticity,pre-adaptation and rapid evolution in the trait differences and evolution across geographic clines and among continents of the intertidal grass S.alterniflora within its invasive and native ranges,we grew 10 Chinese populations and 16 native USA populations in a common garden in a greenhouse at the Xiang'an campus of Xiamen University,Xiamen,Fujian(24.62°N,118.31°E)from the year 2015 to 2017.According to field survey across latitude in the invasive and native ranges,multiple common gardens,greenhouse common garden,and literatures,we found the main results as follows:1)Field plant height showed a hump-shaped relationship across latitude(Low-latitude:129±3 cm;Mid-latitude:201±7 cm;High-latitude:158±5 cm),but this pattern of different latitudinal provenances disappeared in multiple common gardens.However,field seed set increased linearly with latitude(10%-80%)and persisted in mid-and high-latitude common gardens,with the greatest slope in the high-latitude garden,lower slope in the mid-latitude garden and no relationship in the low-latitude garden,indicating a provenance by environment interaction.The number of surviving plants also suggested a provenance by environment interaction;no relationship with latitude among provenances in the two southern gardens and increasing survival with latitude in the northern high-latitude garden,which is consistent with field seedling survey-seedling density was greater at high(206±22 culms/m2)than low-latitude sites(1.8± 0.7 culms/m2).2)Chinese plants were taller(Field:20%;Greenhouse common garden 20%-40%),denser(Field:17%;Greenhouse ommon garden:15%-30%),and set up to four times more seed(Field:200%;Greenhouse common garden:400%)than US plants in both the field and common garden.Plants at the three US sites that sourced the invasion did not differ from others in the US in vegetative or reproductive traits.Thus,there is no evidence that superior performance of Chinese plants was due to founder effects or peculiar trait values of the sources.The common garden experiments showed a striking genetic cline of greater seed set at higher latitudes in the introduced but not the native range.In contrast,there was slight genetic cline in some vegetative traits in the native but not the introduced range.3)Aboveground biomass of S.alterniflora in the field in its native range declined with increasing latitude,a pattern that persisted in a greenhouse common garden,indicating a genetic basis of this aspect.In China,aboveground biomass displayed a hump-shaped relationship with latitude(Low-latitude:850 ± 167 g/m2;High-latitude:532±52 g/m2;Mid-latitude:3709±310 g/m2),a pattern that disappeared in common garden experiments,indicating phenotypic plasticity.In the greenhouse,Chinese plants were larger than their North American ancestors.Aboveground biomass in the Southern Hemisphere also showed a hump-shaped relationship with latitude(Low-latitude:116 g/m2;High-latitude:283 g/m2;Mid-latitude:1129 g/m2).Relationships between aboveground biomass and latitude in all geographic regions were well explained by annual temperature.In conclusions,firstly,we found a provenance by environment interaction of sexual reproduction,which evolved a latitudinal cline in China.The profuse seed germination and recruitment in the north could have created high selection intensity resulting in evolution of reproductive traits at high latitudes with the result that the mechanisms of the invasion differ with latitude.Secondly,S.alterniflora has evolved different trait clines in the native and introduced ranges that likely reflect rapid spread in the introduced range versus intraspecific vegetative competition in the native range Thirdly.S.alterniflora has evolved greater aboveground biomass during its invasion into China,but it has not yet evolved the geographic clines in biomass that are seen in its native range.This finding suggests that although plant-sediment feedbacks may enhance salt marsh resistance to sea level rise at high latitudes,global warming will lead to less productive and therefore less resistant marshes at lower latitudes.Our results will enrich the theory of invasion ecology and shed insight into understanding biological invasion of the coastal wetland.Furthermore,our study will provide scientific support for controling S.alterniflora along latitude,tackling global change,and ecological and environmental conservation.