Effects Of Long-term Sexual Reproduction And Parthenogenesis Of D.pulex On Population Fitness And Genetic Diversity

Experimental evolution is a means to study the evolution process of experimental groups in laboratory environment.It has been extensively used to estimate population evolution parameters and test various evolutionary hypotheses.Daphnia pulex has a short generation cycle and two reproductive modes.Due to the easy operation,it becomes an excellent model organism for studying experimental evolution.In this research,D.pulex was used as the experimental subject,and serial cultures were performed in the manner of sexual reproduction and parthenogenesis respectively.Population fitness was evaluated via life table experiment and the genetic diversity information of each generation population was obtained through the technology of microsatellite neutral marker.In addition,the life history parameters and genetic parameters were used to monitor the population fitness of D.pulex,and the dynamic changes of genetic diversity during the cultivation process.The scientific problem that this research aims to solve is as followed: In the process of the long-term sexual reproduction and parthenogenesis,is it true that D.pulex of single genotype can all change their population fitness and genetic diversity? Detailed results are as follows:1.The dynamic changes of D.pulex in population fitness and genetic diversity during the long-term sexual reproduction.In this article,120 D.pulex clones produced by sexual reproduction was detected by life table assay,all of which were hatched from dormant eggs laid out by the sexual reproduction group F8-F13 and each generation included 20 clones established.Afterwards,the effect on genetic diversity of D.pulex exerted by long-term sexual reproduction was explored with the method of microsatellite markers.The experiment results showed that the R value of the intrinsic increasing rate of D.pulex fed with nutritious chlorella(Chlorella pyrenoidosa)was significantly higher than that of those fed with toxic Microcystis aeruginosa.One-way analysis of variance(ANOVA)showed D.pulex's resistance to toxic Microcystis aeruginosa varied from generation to generation in sexual reproduction significantly.Although no obvious change rule was observed,the resistance of each generation of sexual reproduction had a relatively large change interval.On top of that,other life history parameters also had significant clonal differences and generational differences.Thus,it can be seen that D.pulex of a single genotype can bring about a variety of phenotypic changes to the population after ongoing sexual reproduction,and besides,fitness will be significantly changed.Ten microsatellite loci were selected in our research,four of which belongs to highly polymorphic site(PIC > 0.5)and the rest moderate polymorphic site(0.2 < PIC < 0.5).It indicated that those ten microsatellite loci selected in this research can provide a more reliable genetic information to the population of D.pulex.The mean of the genetic differentiation index Fst of the sexual reproductive population of D.pulex was 0.246,which showed the presence of a more obvious differentiation between the six sexual reproduction populations.In sexual reproduction F8-F13 generation,the number of effective alleles and observed heterozygosity were both high,providing a rich raw material for the evolutionary process of D.pulex's resistance to toxicogenic Microcystis aeruginosa.According to all above experiments,we can conclude that D.pulex's sexual reproduction can lay a genetic basis for phenotypic diversity,and a single sexual reproduction can bring about much phenotypic changes and abundant genetic variation for the population.2.The dynamic changes of fitness and genetic diversity of D.pulex in long-term parthenogernesis process.In this study,a single female D.pulex hatched by dormant eggs was subcultured by way of parthenogernesis.As the parthenogernesis process continued,though D.pulex's resistance did not present significant regular changes,it also did not manifest with a significant downward trend.One-way analysis of variance(ANOVA)showed D.pulex's resistance to toxigenic Microcystis aeruginosa resistance in different generations of parthenogenesis has significant differences.In this study,STR genotyping technology was adopted and the results showed that the genetic parameters of the 12 D.pulex parthenogenesis population has significant differences.However,it showed no downward trend either,which means that D.pulex's long-term parthenogenesis did not dramatically reduce the genetic diversity of the population.Moreover,it also provided raw material for the evolution of the resistance to toxigenic Microcystis aeruginosa.By detecting the dramatic changes in the resistance to toxigenic Microcystis aeruginosa and the multiple genetic parameters during the parthenogenesis process,it was found that the resistance and the heterozygosity both fluctuated obviously as the parthenogenesis went on.When heterozygosity was at a high level,the population fitness of that generation was not high,and after each peak of heterozygosity,the population fitness would show a peak,too.In the course of D.pulex's everlasting parthenogenesis,although genetic drift was possible,the loss of the fitness was not apparent.The possible reason for this situation is that the occasional harmful mutations in small groups are quickly neutralized by beneficial mutations in subsequent generations,and these beneficial mutations specifically compensate for the negative effects of these mutations.In conclusion,D.pulex's parthenogenesis can also provide a genetic basis for phenotypic diversity;and in the absence of genetic recombination,the genetic diversity and fitness of parthenogenesis population did not decrease,that is to say,asexual reproduction is not a ‘dead end' for biological evolution.To sum up,a single genotype of D.pulex is able to provide sufficient raw materials for its resistance to Microcystis aeruginosa in the course of long-term sexual reproduction and parthenogenesis.In this study,multicellular plankton D.pulex was used as the experimental subject,and by combining the molecular data and the ecology of zooplankton,some hypothesis of adaptative evolution confirmed in unicellular organisms was expanded in multicellular organisms.