Date Published: April 3, 2017
Publisher: Public Library of Science
Author(s): Li-Juan Zhang, Wan-Zhi Cai, Jun-Yu Luo, Shuai Zhang, Chun-Yi Wang, Li-Min Lv, Xiang-Zhen Zhu, Li Wang, Jin-Jie Cui, Tzen-Yuh Chiang.
Lygus pratensis (L.) is an important cotton pest in China, especially in the northwest region. Nymphs and adults cause serious quality and yield losses. However, the genetic structure and geographic distribution of L. pratensis is not well known. We analyzed genetic diversity, geographical structure, gene flow, and population dynamics of L. pratensis in northwest China using mitochondrial and nuclear sequence datasets to study phylogeographical patterns and demographic history. L. pratensis (n = 286) were collected at sites across an area spanning 2,180,000 km2, including the Xinjiang and Gansu-Ningxia regions. Populations in the two regions could be distinguished based on mitochondrial criteria but the overall genetic structure was weak. The nuclear dataset revealed a lack of diagnostic genetic structure across sample areas. Phylogenetic analysis indicated a lack of population level monophyly that may have been caused by incomplete lineage sorting. The Mantel test showed a significant correlation between genetic and geographic distances among the populations based on the mtDNA data. However the nuclear dataset did not show significant correlation. A high level of gene flow among populations was indicated by migration analysis; human activities may have also facilitated insect movement. The availability of irrigation water and ample cotton hosts makes the Xinjiang region well suited for L. pratensis reproduction. Bayesian skyline plot analysis, star-shaped network, and neutrality tests all indicated that L. pratensis has experienced recent population expansion. Climatic changes and extensive areas occupied by host plants have led to population expansion of L. pratensis. In conclusion, the present distribution and phylogeographic pattern of L. pratensis was influenced by climate, human activities, and availability of plant hosts.
Cytoplasmic and nuclear data combined with coalescent theory are commonly used for phylogeographic studies. They are powerful tools for evaluating the possible influence of climate changes, geological events, environmental changes and hosts on extant population structure and tracing the possible evolutionary history of species . Molecular phylogeographic results have been used to reconstruct the evolutionary history of species by revealing colonization history, range expansion, and spatial and temporal genetic variation [2, 3]. The phylogeographic patterns of many organisms, whose evolution was affected by vicariance [4–6], climatic changes , hosts  and human interference events [9–11] have been studied using molecular data.
Studies on the relationship between population genetic structure and host plants of agricultural insect pests are difficult and complicated, especially for species that have diverse host plants and high dispersal capacity. Our previous study of the population dynamics of L. pratensis on different host plants in northwest China, showed that this species mainly feeds on cotton and alfalfa. However, population size is obviously reduced in groups feeding on native host plants (e.g. Sophora alopecuroides Linn, Artemisia frigida Willd, Alhagi sparsifolia Shap). Therefore, we did not examine molecular markers on samples from native host plants of L. pratensis. We only collected samples from cultivated cotton and alfalfa. Native host plants might play a role in the population genetics of L. pratensis, but this possibility was not discussed here because no insect samples from these hosts were available.