Host Related Species Diversification And Population Genetic Diversity Of Acorn Weevils

The inter-trophic interactions play an important role in the formation and development of ecosystem functions, which is supposed much stronger in subtropical forests. Under the background of global biodiversity loss, studying the interaction of inter-trophic biodiversity is not only in favor of further understanding the emergence of biodiversity, but also of the proper conservation of biodiversity.Here, taking the inter-trophic acorn weevils and their Fagaceae host plants as the study system, I investigated the species diversity, host specificity, and species diversification of acorn weevils based on mitochondrial and nuclear genes. Further I tested the correlations between genetic diversity of Curculio bimaculatus, one of the most generalist acorn weevils and species diversity of plants in Gutianshan National Nature Reserve, by the developed polymorphic microsatellite loci.The main results are as following.1. From 2009 to 2012, larva samples of weevils were collected from the seeds of 19 Fagaceae species in 17 sites of Zhejiang and Northeast Fujian provinces. A total number of 515 larvae were sequenced in mitochondrial COI and nuclear ArgK and EF1-α, which induced 107,127 and 89 haplotypes, respectively.Both the mitochondrial and nuclear haplotypes clustered into 10 clades, and K2P genetics distance suggested that these clades had diverged into species level. These clades clustered with the Curculio species exclusively. Two clades formed by two morpho-diverged weevil species reared under laboratory conditions were assigned to Curculio davidii and C. bimaculatus, respectively, and the other 8 may be undescribed species due to their independent phylogenetic locations of known weevil species in phylogenetic tree.2. Among the 10 weevil species,4 are generalist and the others are specialist. The two identified weevil species, C. davidii and C. bimaculatus are generalist, C. bimaculatus is parasitic on the most diverse plants including 14 species from 4 genera of Fagaceae. Host specificity may link to the geographic range of weevils, since weevils with lower specificity distributed in a wider geography range, while weevils with higher specificity were found in only one sampling site.Nested dendrogram of generalist weevils to clades of specialist were found frequently. Cophylogenetic analyses between weevils and hosts suggested that no cospeciation event had happened in their evolutionary histories (Cospeciation= 0). Host shift had led to species divergence once, while species duplication events had happened frequently (Duplication= 8). It is suggested that the weevil species diversification were driven mainly by the oscillation of host-plant range.3. Using the biotin-streptavidin capture method, eleven microsatellite loci were developed for C. bimaculatus. Polymorphism was tested using 48 individuals from 2 populations. Ten loci were polymorphic with 4 to 26 alleles per locus (mean= 13). The observed and expected heterozygosity ranged from 0.167 to 1.000 and from 0.451 to 0.958, respectively.4. Using 8 neutral microsatellite loci, the genetic analyses for 15 C. bimaculatus populations in Gutianshan National Nature Reserve were conducted. Results showed that recently genetic bottleneck had occurred in 7 populations, in which 5 lived in early successional stage communities, while anther 2 in middle successional stage communities. Genetic differentiation between populations exhibited a rather low level (FST ranged from 0.004 to 0.074), however, significant genetic differentiations (P< 0.05) were found between most of the populations (87.6% of the population pairs). For all of the populations, no significant IBD pattern was found. The genetic diversity of weevil populations showed a high variation. Allelic richness varied from 5.37 to 10.62, and the private allelic richness varied from 0.00 to 1.25, while population observed heterozygosity ranged from 0.5437 to 0.7500, and the expected heterozygosity ranged from 0.6676 to 0.7663. By Spearman test, a significant correlation (P< 0.05) between genetic diversity of weevil populations and successional stage of communities were revealed. Immigration analyses showed that the generational immigrant number of weevils between populations ranged from 0.303 to 5.762, with an average of 1.813. Furthermore, an asymmetric immigration between late successional stage communities and the early ones was found, in which weevils emigrated from the former into the latter in a much larger individual number than the reverse.5. Genetic diversity of C. bimaculatus populations showed a positive correlation to plant species diversity of forests communities in Gutianshan. At alpha diversity level, the allelic richness, but not the private allelic richness of weevil populations positively correlated (P< 0.05) to species richness of all wood plants in forest communities, the observed and expected heterozygosities were also positively correlated (P< 0.05) to the Simpson diversity, Shannon-Wiener diversity and Pielou evenness of all wood plants in forest communities. On the other hand, both the allelic richness and the private allelic richness of weevil population were positively correlated (P< 0.05) to species richness of Fagaceae plants in forest communities, but the observed and expected heterozygosities of weevil populations were not significantly correlated to the Simpson diversity, Shannon-Wiener diversity and Pielou evenness of Fagaceae plants. At beta diversity level, Bray-Curtis distances of alleles in weevil populations were significantly correlated to those of plant species in both all wood plants (r=0.306, P=0.002) and Fagaceae plants (r=0.208, P=0.032) of forest communities. It was pointed out that the loss of correlations between weevil genetic heterozygosity and the individual number-based diversity of Fagaceae plants was caused by the uneven distribution of the large number of pre-mature Fagaceae species individuals among communities. Further analyses revealed that the positive correlations between genetic diversity of weevil populations and species diversity of plant communities were caused by both the parallel dispersal of weevils and plants, and host related genetic differentiation of weevils.It concludes that the species diversity of acorn weevils in studied region may be far beyond the recognized point, and a good number of undescribed species are to be discovered. Host specificity of weevils may deeply affect the species diversification in this herbivore taxon, where expansion of host range could subsequently facilitate the expansion of geographic range of weevils, and ultimately lead to local adaptation and speciation. It is also provided that species and genetic diversity correlations can be linked between inter-trophic organisms, however, the underlying mechanisms can not be fully explained by the theoretic frame developed from the interaction of organisms in the same trophic.