Plasticity And Geographical Divergence Of Life History Traits In Western Mosquitofish(Gambusia Affinis)

The Western mosquitofish(Gambusia affinis)was introduced to China in 1927.It nowadays ranks amongst the top 100 invasive species on a global scale,occurring on all continents except Antarctica,and represents one of the most invasive species in China,especially south of the Yangtze River.The widespread distribution of G.affinis points towards a high degree of adaptability and/or phenotypic plasticity.This includes life history traits,i.e.,trait suites related to the allocation of time and resources into somatic growth,maturation and reproduction,etc.In order to adapt to a novel environment,invasive species need to adapt their life histories to a novel set of environmental factors.Of particular interest is how geographically widespread species adapt to climatic variation along their novel distribution range,and whether and how adaptive phenotypic divergence in life history traits represents evolutionary divergence or is made up by plasticity.Study in this direction have important theoretical implications for our understanding of how invasive taxa cope with novel environmental conditions,how this might affect their further spread,and possibly,how indigenous biological diversity can be protected from further biological invasions.In my thesis,I collected a total of 300 females and 346 males G.affinis on a monthly scale in an invasive population in Ankang from May 2016 to April 2017 to investigate life history variation.Studying temporal variation within the same population allowed an estimation of plastic effects(over the course of the year).Furthermore,a total of 96 females and 99 males were collected in five additional cities,namely Shanghai,Wuhan,Kunming,Beihai and Ankang in late April 2017.This sample allowed for an investigation of life history differences across geographically distant populations at a snap-shot in time(i.e.,during the early breeding season).Standards life history traits(reproductive allocation(RA),fecundity,embryo fat content(EFC),mean embryo dry weight(MEDW),somatic fat content(SFC),standard length(SL),gonadosomatic index(GSI))were analyzed by means of multivariate analysis of variance(MANOVA).I also collected environmental factors which,namely water quality parameters(PH,ammonia,nitrite,dissolved oxygen,salinity,conductivity),climatic and geographical factors,and subjected them to principal component analysis(PCA).The resulting PCs served as explanatory variables in the statistical analyses.The main results are as follows:1.There are monthly significant differences in life history parameters(RA,fecundity,EFC,MEDW,SFC,SL)for female.RA maintained a high level(greater than 15%)from April to September and a low level(about 1%)from November to February of the next year.Similarly,fecundity kept a high level(greater than 14)from April to September and a low level(less than 1)from November to February of the next year.So it is obvious that the period from April to September is the reproductive period of female G.affinis in Ankang.EFC and MEDW increased in spring,maintained stability in summer,declined in autumn and not decreased significantly in winter,revealing that the optimum reproductive season is summer.SFC fluctuated in the whole year and it was large(about 5%)in late spring and early summer,small(about 2%)in autumn and winter,showing that female tend to invest more energy in reproduction.The SL of summer was significantly greater than other three seasons,indicating that female G.affinis grow rapidly in summer.2.For male,the monthly differences are also significant.The fluctuation of GSI is smaller than female RA in the whole year,so male reproductive period is not obvious.SFC raised quickly in late spring and early summer,and began to decrease in late autumn,remained a low level(about 1%)in winter,and that may be related to food abundance.The annual variation of SL was smaller than that of female,indicating SL dimorphism is significantly obvious in G.affinis.3.There are spatial geographical divergences about G.affinis life history parameters((somatic dry weight,SDW),fecundity,SFC,RA,MEDW,EFC,SL,GSI)which are influenced by water quality,latitude and altitude.More specifically,the water quality mainly affects SFC,RA and MEDW of female,latitude impacts on female fecundity,SFC,EFC,RA,and MEDW,altitude influences upon female RA,MEDW,SDW,SL and male SFC,GSI,SDW,SL.In summary,there are monthly or geographical divergences of G.affinis life history traits.The great monthly divergences indicate significant plasticity of life history traits and the obvious divergences in geographical aspect show that G.affinis can change their life history strategy according to environmental conditions.