Genetic Structure of Water Chestnut Beetle: Providing Evidence for Origin of Water Chestnut

A Phylogeographical Analysis of the Beetle Pest Species Callosobruchus chinensis (Linnaeus, 1758) in China

Simple Summary

Callosobruchus chinensis, a stored product pest, is difficult to control. In the study, the goal was to explore the causes of the demographic history, dispersal path and genetic variations underlying the spatial and temporal distribution of C. chinensis in China. The phylogeography of C. chinensis was analyzed by distribution modelling (SDM) under six periods and the least-cost path (LCP) based on combined mitochondrial DNA. Our data showed that the geographical isolation of the genetic lineages and the distribution range of C. chinensis were restricted by climate in different times. The landscape structure had influence on the genetic differentiation of C. chinensis. Although the migration ability of C. chinensis is limited, the development of transportation and trade is helping the insect spread, along with the beans of its host.

Abstract

Callosobruchus chinensis (Coleoptera Bruchidae), is a pest of different varieties of legumes. In this paper, a phylogeographical analysis of C. chinensis was conducted to provide knowledge for the prevention and control of C. chinensis. A total of 224 concatenated mitochondrial sequences were obtained from 273 individuals. Suitable habitat shifts were predicted by the distribution modelling (SDM). Phylogeny, genetic structure and population demographic history were analyzed using multiple software. Finally, the least-cost path (LCP) method was used to identify possible dispersal corridors and genetic connectivity. The SDM results suggested that the distribution of C. chinensis experienced expansion and contraction with changing climate. Spatial distribution of mtDNA haplotypes showed there was partial continuity among different geographical populations of C. chinensis, except for the Hohhot (Inner Mongolia) population. Bayesian skyline plots showed that the population had a recent expansion during 0.0125 Ma and 0.025 Ma. The expansion and divergent events were traced back to Quaternary glaciations. The LCP method confirmed that there were no clear dispersal routes. Our findings indicated that climatic cycles of the Pleistocene glaciations, unsuitable climate and geographic isolation played important roles in the genetic differentiation of C. chinensis. Human activities weaken the genetic differentiation between populations. With the change in climate, the suitable areas of C. chinensis will disperse greatly in the future.

Mitochondrial DNA analysis reveals spatial genetic structure and high genetic diversity of Massicus raddei (Blessig) (Coleoptera: Cerambycidae) in China

The oak longhorned beetle (OLB), Massicus raddei (Blessig, 1872) (Coleoptera: Cerambycidae), is widely distributed in Asia (China, the Korean Peninsula, Japan, Vietnam and the Russian Far-East), but pest outbreaks have occurred only in Liaoning Province and Jilin Province of China. In order to explore possible mechanisms of local population outbreaks and characterize the genetic diversity and genetic structure of M. raddei across its range in China, three mitochondrial genes (COI, Cytb, and COII) were sequenced and analyzed for seven M. raddei populations collected from six provinces in China. From these different populations, we found a high haplotype and nucleotide diversity. Haplotype networks and phylogenetic analyses both demonstrate apparent genetic diversification between SC (southern China) and NC (northern China) population groups. A set of 21 pairwise comparisons for Fst (pairwise fixation indices) and Nm (genetic flow index) showed significant genetic differentiation and limited gene flow except for two pairs, Shandong (SD) and Liaoning (LN), and Anhui (AH) and Henan (HN). This pattern suggested that the periodic outbreak of the LN population could not be attributed to the absence of genetic flow with other spatial populations and that regional environmental factors might be responsible. AMOVA (Analysis of molecular variance) showed that the greater molecular genetic variation was among populations. Based on Tajima's D statistic, Fu's Fs, and the mismatch distribution test, we determined that the seven populations sampled were stable and had not experienced any recent population expansion. The fact that all the sampled populations showed only unique haplotypes and lacked shared or ancestral haplotypes, as well as the nonstar-like distribution of haplotype network for concatenated genes, collectively provided powerful evidence of the stable and isolated nature of most populations. The high genetic differentiation and spatial genetic structuring among populations are both likely related to the beetle's moderate flight capacity, regional variation in host tree species and microclimate, as well as the geographic distance between sampling sites.

Neolithic cultivation of water chestnuts (Trapa L.) at Tianluoshan (7000-6300 cal BP), Zhejiang Province, China

Water chestnuts (Trapa) are frequently recovered at Neolithic sites along the Lower Yangtze River Valley and have been important components of the diets of prehistoric people. However, little systematic research has been conducted to determine their cultural and dietary importance. Excavations at the Tianluoshan site produced large quantities of well-preserved specimens, which provide an excellent collection for studying morphological changes with time. Using modern wild and domesticated water chestnuts (n = 447) as a reference, we find Neolithic samples (n = 481) at Tianluoshan are similar in shape but smaller in size compared to the domesticated species Trapa bispinosa. In particular, the Tianluoshan water chestnuts have bigger seeds than the wild species Trapa incisa. Further, water chestnuts diachronically increased in size at the Tianluoshan site with significant differences (one-way, ANOVA) observed for length (p = 7.85E-08), height (p = 3.19E-06), thickness (p = 1.2E-13), top diameter (p = 5.04E-08) and bottom diameter (p = 1.75E-05) between layers 7 (6700-6500 cal BP) and 6 (6500–6300 cal BP). These results suggest that water chestnuts were actively selected based on size (big), shape (full fruit, two round horns, wide base, etc.) and were an important non-cereal crop to the agricultural practices at the Tianluoshan site.

Genetic Structure of the Bacterial Endosymbiont Buchnera aphidicola from Its Host Aphid Schlechtendalia chinensis and Evolutionary Implications

Buchnera aphidicola is a primary symbiotic bacterium which provides essential amino acids to aphids. In this study, we sequenced nuclear 16s rDNA and atpAGD genes for 156 individuals of B. aphidicola from eight geographically distant populations to investigate the genetic diversity and structure of B. aphidicola associated to the sumac gall aphid Schlechtendalia chinensis in central and southern China. Our analyses of the combined sequences showed that B. aphidicola from S. chinensis had high haplotype and nucleotide diversity (h = 0.893; π = 0.00164). One of the 16 haplotypes detected had a wide geographic distribution across the central and southern China and was probably the ancestral haplotype of B. aphidicola from S. chinensis. A network and phylogenetic analysis revealed a geographic structure in which the 16 haplotypes of B. aphidicola were divided into the northern and southern clades separated by the Yangtze River. The two clades diverged from each other at 22.1 ± 3.7 Mya according to our divergence time estimations. Therefore, the modern genetic structure in B. aphidicola from S. chinensis has been probably impacted by historical geological events. Combined with the data from GenBank, we also reconstructed the phylogenetic relationships of three aphid subfamilies and their symbiont bacteria. The results indicated significant topological correlations between the aphid and bacterial phylogenies at interspecific levels.