Although Ae. albopictus has long been considered a vector of secondary importance, however, Ae. aegypti was distributed limited, and Ae. albopictus widely in China. Recent Dengue and Chikungunya outbreaks were caused by Ae. Albopictus [4]. Knowledge about the genetic diversity and the genetic structure of the mosquito could help researchers evaluate the risk of disease and insecticide resistance spread, as well as identify the origins and frequencies of introductions.
In this study, we collected samples from the temperate region to the tropical region in China and conducted a population genetic analysis of Ae. albopictus based on the 8 polymorphic microsatellite loci. The variation analysis showed that most variation occurred within individuals, whereas only about 9% of the total variation was detected among populations. There was no relation between genetic distance and geographical distance, and the co-ancestry of the populations in the South of China was heterogeneous. We also observed the trend that the greater the genetic difference (FST), the lower the wild-type frequency of F1534 of VSGC gene in the same population.
Sampling strategy and geographic coverage greatly influence the analysis and interpretation of the data generated from the samples. In this study, Ae. albopictus mosquitoes were collected from 17 sites in 11 provinces, covering the main distribution range of China from 16°N to 40°N [2], as well as tropical, subtropical, and temperate climate zones. In addition, it is crucial to clearly establish whether the collected samples are eggs, larvae, or flying adults. This is important because in Ae. albopictus, larvae, or eggs collected from the same breeding site are likely to belong to the same progeny. In the study, this potential drawback was avoided by collecting a few individuals per breeding site and setting multiple traps throughout the sampling site.
Useful genetic markers are expected to have several key features: selective neutrality, ease of scoring in all specimens of the species, and sufficient variability to allow for measures of genetic differentiation, and genetic clustering of individuals. More than ten microsatellites DNA loci from works of literatures were used in these Ae. albopictus populations and eight of them were highly polymorphic and amplification efficiency, and thus were used for exploring population genetic structure. Although, their location with respect to chromosomes was unknown, no consistent pattern of linkage disequilibrium between any particular pair of loci was observed, suggesting that they are at least statistically independent and might distribute across the genome-wide.
The high allelic diversity and expected heterozygosity were observed in most of the populations (HO= 0.32–0.57), which was similar to the level of diversity in the Ae. albopictus of the coastal areas in southern China (HO= 0.384–0.641) [23], and 17 populations from 3 climatic regions of China (HO= 0.467–0.627) [27], 34 localities across China (HO= 0.551–0.633) [28], and eight samples from Thailand, Réunion, and Northern Italy (HO= 0.22–0.28). In China, Ae. albopictus occurs in different climate zones, populations undergo marked seasonal variations in abundance, reaching high densities only during the summer months. The high level of genetic diversity suggests the Ae. albopictus are able to maintain a relatively large effective population size in spite of the seasonality of low temperatures in cold winter or dry season.
Notably, the FST value of GDST population was minimum (0.087), meanwhile carrying no mutation in F1534 [34]. As the collection site is close to the seaside wharf, therefore it was speculated that GDST population mosquito was imported from ships and colonized locally. Local environment was suitable for the survival of Ae. albopictus, thus the population size was abundance. Because the colonization time maybe not be long, the genetic difference within the population was small, and 92.8% of the individuals were allocated to cluster III (blue), these could be attributed to the founder effect. The oral interviews revealed that almost no insecticides were used in the local area, so they were all wild-type alleles of F1534. A similar population is HNSS, which was collected from a tropical island, far from the Chinese mainland, and has no fresh water on it. It was likely to be brought to the island by ships from Hainan Island, so both HNSS and HNSY populations were belonging to cluster II (green). In addition, some microsatellite loci of many individuals in the GDGZ population could not be amplified, resulting in more invalid data, which may be caused by the genetic diversity of the species [55], or it may be related to the population's susceptibility to dengue virus [56].
The STRUCTURE analysis showed that these Ae. albopictus populations were divided into three clusters. Each cluster could be considered as a gene pool, the overall trend was that the northern Chinese Ae. albopictus populations belong to gene pool I (red), the eastern to III (blue), while the southern populations belong to three different gene pools. The genetic difference among the three gene pools was small, only 0.027 between gene pool II (green) and III (blue). As it was known, Ae. albopictus was also named the Asian tiger mosquito, which has been described as one of the 100 worst invasive species in the world. Originating from South and East Asia, this species has spread throughout the world mostly since the second half of the twentieth century, and it is now found on every continent except Antarctica [57]. The new areas colonized by Ae. albopictus include such disparate environments as tropical South America, Africa, and the most temperate areas of Northern America and Europe. In China, the south of China should be the first place to invade, and the local area is in the tropical and subtropical regions. Over there, Ae. albopictus can colonize and expand its population, and it has more generations every year, even does not overwinter. In addition, there would be many times to invade from different places, therefore, the genetic pools of the population in the southwest and south were relatively complex, supported by this STRUCTURE analysis. To analyze the migration route of Ae. albopictus in China, it should be that the southern population of Ae. albopictus was introduced into the eastern and northern (the temperate regions) of China, with the movement of people and goods by a chaotic propagule distribution mediated by human activity, and forming a relatively simple gene pool.
Thus, Ae. albopictus from eastern and northern China should be temperate-diapausing populations, and there was sufficient gene flow with tropical non-diapause populations. Particularly, the photoperiodical diapause, which has a demonstrated genetic basis, seems to be an important component of climatic adaptation, favoring the invasive success of Ae. albopictus. On the other hand, anthropogenic activities also create new breeding and trophic niches of adaptation in close proximity to human living sites, impacting their relationships with humans.
However, there are still some imperfections in this study. For a geographical area as large as China, the sampling points could be expanded to get more detailed data. In addition, the correlation between kdr mutation and genetic structure did not show statistical differences, possibly because the sample size was not large enough. Therefore, we will continue to collect a large number of samples of Ae. albopictus in the future to obtain more data to further explore the correlation between resistance mutations and genetic background.