Adaptive Evolution In Life History Of The Global Invasive Plant Of Spartina Alterniflora Across Latitudinal Gradient

Invasive species can often occupy a wide geographic distribution in novel habitats,even beyond the geographic distribution range in native range.In the process of biological invasion,invasive species show a high level of phenotypic variation to adapt to heterogeneous habitats in different geographic regions.This phenomenon can be synthesized as a rapid adaptation of life history.Spartina alterniflora is native to the Atlantic and Gulf Coasts of the United States,has become a global invasive species in coastal wetland,and it has the largest distribution area in invasive China.The various life history traits(e.g.,vegetative growth flowering phenology,and seed set)of S.alterniflora show significant phenotypic variation along latitudes.This provides us an opportunity to explore the patterns,processes,and mechanisms of rapid adaptation of life history in invasive plants across latitudinal gradients.Since most of the previous work on S.alterniflora has only focused on individual traits or traits trade-offs at local scales,we conducted a comprehensive study of multiple life history traits in S.alterniflora at geographic scale.We firstly collected seven populations of S.alterniflora along latitude(2-3 latitudes interval,covering the current distribution range)in invasive China,which were cultivated in two common gardens at the high(38°N)and low(21°N)latitudinal distribution boundaries.We recorded the emergence and duration of flag leaf,flowering and seed maturation,thus we could explore the variation in life history response pattern of the reproductive phase between common gardens.Furthermore,we determined the key phenological stage that affected this variation.Secondly,we combined field survey and common gardens experiment to explore the latitudinal pattern of flowering phenology of S.alterniflora and the abiotic factors in shaping this pattern.Furthermore,the relative importance of different ecological mechanisms in the adaptation process of flowering phenology was detected by comparing the latitudinal patterns between invasive and native populations.Finally,we incorporated nine geographic populations from native range,and compared the trade-offs of different life history traits between invasive and native populations of S.alterniflora.The main findings are as follows:(1)The reproductive phase(including flag leaf,flowering,and seed maturation)was significantly earlier in the low latitude common garden than in the high latitude one,with flag leaf being about 72 days earlier,flowering about 69 days earlier and seed maturation about 57 days earlier.The significant advancement of the life history in S.alterniflora under high temperature may be related to the enhanced physiological metabolism of individuals.In addition,the duration of flowering was significantly longer in the low latitude garden(34.1±1.1 days)than in the high latitude one(23.3±1.0 days),but the duration of flag leaf was not significantly different(low latitude garden:14.7 ± 0.6 days;high latitude garden:13.3± 0.5 days).Correspondingly,the duration of reproductive phase was significantly longer in the low latitude garden(1037.0 ± 49.2 days)than in the high latitude one(575.1 ± 38.9 days).These results preliminarily suggested that the flowering phenology of S.alterniflora may have a greater influence on the reproductive phase in response to environmental changes between gardens.For the variation in the reproductive phase,all the geographical populations of S.alterniflora showed similar degrees of variation.Overall,there was no significant difference in the variation of different life history stages and reproductive phase among the plantations.This phenomenon suggested that the ability of different populations to adjust their life history in response to environmental changes,has not yet evolved in local adaptation under selective pressures.There were no significant differences in the temperature sensitivity of emergence of flag leaf,flowering and seed maturation,while the temperature sensitivity of duration of flowering was significantly higher than flag leaf.That may be due to the leaf tissue receiving more input from protective resources than that of the flower.Thus,the flowering phenology had the greatest influence on the variation in life history pattern of S.alterniflora,and such influence was mainly generated through variation in duration.(2)Flowering time of S.alternflora in the field showed a hump-shaped latitudinal pattern,with plants at mid-latitude sites flowered about 10 days later than at high-latitude sites and about 100 days later than at low-latitude sites.Among the abiotic factors,tidal range and mean annual temperature mainly affecting this latitudinal pattern.However,flowering time of S.alterniflora was significantly delayed in the high latitude garden(257.2 ± 1.9 days)compared to the low latitude one(187.8 ± 2.2 days).Within common gardens,flowering time of S.alternifora was negatively correlated with latitude of origin.These results suggested that there is a certain degree of phenotypic plasticity in flowering phenology in invasive China,meanwhile different populations shown genetic differentiation.The locations of common garden and latitudes of origin had significant effects on the flowering time,respectively,but their interaction was not significant.This result indicated that the phenotypic plasticity of flowering phenology was not genetically controlled.In addition,there was no significant difference in flowering time between invasive and native populations within neither the high latitude garden(invasive populations:260.6 ±2.4 days;native populations:257.3±2.4 days)nor the low latitude one(invasive populations:183.7 ±3.6 days;native populations:187.8 ± 3.6 days).The similar latitudinal patterns of flowering time between invasive and native populations only occurred in the high latitude garden.This result suggested that the genetic differentiation in the flowering phenology of S.alterniflora is likely to be the result of rapid adaptive evolution.(3)In the low latitude garden,there was no trade-off between flowering time and plant height and seed set in either invasive or native S.alterniflora,probably because the high-temperature stress in the low latitude garden limited the expression of genetic variation.In the high latitude garden,flowering time did not correlate with seed set in native populations,but negatively correlated with seed set in invasive populations;native populations showed a significant negative correlation between flowering time and plant height,however,such a trade-off disappeared in invasive populations.Genetic admixture among different introduced origins from the United states generated novel allelic recombination and increase the genetic variation of correlated traits,providing raw materials for natural selection.Under the selective pressure,invasive S.alterniflora has likely broken the genetic constraints of trade-offs between life history traits.Such phenomenon may have contributed to the rapid adaptive evolution of populations in China.In summary,our study firstly revealed the variation in the life history pattern of S.alterniflora in response to environmental change,and the important contribution of flowering phenology to this variation.It provides a systematic and comprehensive view to understand the response of S.alterniflora to changing environments in invasive range.As ambient temperatures increased,more resources were devoted to sexual reproduction,which may contribute to maintaining population fitness.Secondly,the latitudinal patterns,processes and mechanisms of flowering phenology were systematically explored,which elucidated the relative importance of different ecological mechanisms(e.g.,phenotypic plasticity,interaction between genetic and environment,and adaptive evolution).On this basis,combining with the earlier flowering time and limited vegetative growth at high and low latitudes in the field,we could predict that the rapid dispersion of S.alterniflora on the East Coast of Asia may be coming to an end.Finally,the trade-offs between flowering time with plant height and seed set varied significantly from the native to the invasive populations.That may be due to the combination of genetic admixture and selective pressures.The altered trade-offs may promote the rapid adaptive evolution of S.alterniflora populations in China.The seed set of northern populations was significantly higher than that of other populations,thus the control stage should focus on limiting their sexual reproduction and the formation of viable seeds.Our study systematically elucidates the adaptive evolution in life history of a global plant invader,which enriches the theory of invasion biology and evolutionary biology.It further provides a theoretical basis for understanding the adaptive potential and predicting invasion dynamics of invasive species in coastal wetland ecosystems in the context of global change,and the advice for the control of S.alterniflora in China.