Genetic structure of the whitefly Bemisia tabaci populations in Colombia following a recent invasion

Disparities in Genetic Diversity Drive the Population Displacement of Two Invasive Cryptic Species of the Bemisia tabaci Complex in China

Within the whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) complex, two cryptic species, namely Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED), are important invasive pests affecting global agriculture and horticulture. They were introduced into China sequentially in the mid-1990s and around 2003, respectively. Subsequently, the latter invader MED has outcompeted the earlier invader MEAM1, becoming the dominant population in the field. Although extensive studies have explored the underlying mechanisms driving this shift, the contribution of population genetics remains notably underexplored. In this study, we analyzed the genetic diversity and structure of 22 MED and 8 MEAM1 populations from various regions of China using mitochondrial DNA sequencing and microsatellite genotyping. Our results indicate low and moderate levels of genetic differentiation among geographically separate populations of MED and MEAM1, respectively. Median-joining network analysis of mtCOI gene haplotypes revealed no clear geographic structuring for either, with common haplotypes observed across provinces, although MED had more haplotypes. Comparative analyses revealed that MED presented greater genetic diversity than MEAM1 on the basis of two markers. Furthermore, analysis of molecular variance supported these findings, suggesting that while some genetic variation exists between populations, a significant amount is also present within populations. These findings reveal the population genetics of the two invasive cryptic species of the B. tabaci complex in China and suggest that the disparities in genetic diversity drive the displacement of their populations in the field. This work also provides valuable information on the genetic factors influencing the population dynamics and dominance of these invasive whitefly species.

Population structure of Taenioides sp. (Gobiiformes, Gobiidae) reveals their invasion history to inland waters of China based on mitochondrial DNA control region

Taenioides sp. is a small temperate fish originally known to inhabit muddy bottoms of brackish waters in coastal areas of China. However, it began to invade multiple inland freshwaters and caused severe damage to Chinese aquatic ecosystems in recent years. To investigate the sources and invasive history of this species, we examined the population structure of 141 individuals collected from seven locations based on partial mitochondrial D-loop regions. The results revealed that the genetic diversity gradually decreased from south to north, with the Yangtze River Estuary and Taihu Lake populations possessing the highest haplotype diversity (Hd), average number of differences (k), and nucleotide diversity (π) values, suggesting that they may be the sources of Taenioides sp. invasions. Isolation-by-distance analysis revealed a non-significant correlation (p = 0.166) between genetic and geographic distances among seven populations, indicating that dispersal mediated through the regional hydraulic projects may have played an essential role in Taenioides sp. invasions. The population genetic structure analysis revealed two diverged clades among seven populations, with clade 2 only detected in source populations, suggesting a possible difference in the invasion ability of the two clades. Our results provide insights into how native estuary fish become invasive through hydraulic projects and may provide critical information for the future control of this invasive species.

Natural habitat connectivity and organic management modulate pest dispersal, gene flow, and natural enemy communities

The simplification and fragmentation of agricultural landscapes generate effects on insects at multiple spatial scales. As each functional group perceives and uses the habitat differently, the response of pest insects and their associated natural enemies to environmental changes varies. Therefore, landscape structure may have consequences on gene flow among pest populations in space. This study aimed to evaluate the effects of local and landscape factors, at multiple scales, on the local infestation, gene flow and broad dispersion dynamics of the pest insect Bemisia tabaci (Genn.) Middle East-Asia Minor 1 (MEAM-1, former biotype B) (Hemiptera: Aleyrodidae) and its associated natural enemies in a tropical agroecosystem. We evaluated the abundance of B. tabaci populations and their natural enemy community in 20 tomato farms in Brazil and the gene flow between farms from 2019 to 2021. Landscapes dominated by agriculture resulted in larger B. tabaci populations and higher gene flow, especially in conventional farms. A higher density of native vegetation patches disfavored pest populations, regardless of the management system. The results revealed that whitefly responds to intermediate spatial scales and that landscape factors interact with management systems to modulate whitefly populations on focal farms. Conversely, whitefly natural enemies benefited from higher amounts of natural vegetation at small spatial scales, while the connectivity between natural habitat patches was beneficial for natural enemies regardless of the distance from the focal farm. The resulting dispersion model predicts that the movement of whiteflies between farms increases as the amount of natural vegetation decreases. Our findings demonstrate that landscape features, notably landscape configuration, can mediate infestation episodes, as they affect pest insects and natural enemies in opposite ways. We also showed that landscape features interact with farm traits, which highlights the need for management strategies at multiple spatial scales. In conclusion, we demonstrated the importance of the conservation of natural areas as a key strategy for area-wide ecological pest management and the relevance of organic farming to benefit natural enemy communities in tropical agroecosystems.

Native and Non-Native Bemisia tabaci NAFME Haplotypes Can Be Implicated in Dispersal of Endemic and Introduced Begomoviruses in Oman

Irrigated agriculture and global trade expansion have facilitated diversification and spread of begomoviruses (Geminiviridae), transmitted by the Bemisia tabaci (Gennadius) cryptic species. Oman is situated on major crossroads between Africa and South Asia, where endemic/native and introduced/exotic begomoviruses occur in agroecosystems. The B. tabaci 'B mitotype' belongs to the North Africa-Middle East (NAFME) cryptic species, comprising at least eight endemic haplotypes, of which haplotypes 6 and/or 8 are recognized invasives. Prevalence and associations among native and exotic begomoviruses and NAFME haplotypes in Oman were investigated. Nine begomoviral species were identified from B. tabaci infesting crop or wild plant species, with 67% and 33% representing native and exotic species, respectively. Haplotypes 2, 3, and 5 represented 31%, 3%, and 66% of the B. tabaci population, respectively. Logistic regression and correspondence analyses predicted 'strong'- and 'close' virus-vector associations involving haplotypes 5 and 2 and the exotic chili leaf curl virus (ChiLCV) and endemic tomato yellow leaf curl virus-OM, respectively. Patterns favor a hypothesis of relaxed virus-vector specificity between an endemic haplotype and the introduced ChiLCV, whereas the endemic co-evolved TYLCV-OM and haplotype 2 virus-vector relationship was reinforced. Thus, in Oman, at least one native haplotype can facilitate the spread of endemic and introduced begomoviruses.

Low Genetic Variability in Bemisia tabaci MEAM1 Populations within Farmscapes of Georgia, USA

Simple Summary

Sweetpotato whitefly, Bemisia tabaci Gennadius, is a serious pest of many agricultural crops worldwide. Numerous studies have examined the genetic structure of whitefly populations separated by geographical barriers; however, very few have assessed the population structure of B. tabaci at a farmscape level. A farmscape in this study is defined as heterogenous habitat with crop and non-crop areas spanning approximately 8 square kilometers. To assess the roles of farmscapes as drivers of B. tabaci genetic variation, thirty-five populations of the sweetpotato whitefly were collected from crop and non-crop plant species from fifteen farmscapes. Using mitochondrial COI gene sequences (mtCOI) and six nuclear microsatellite markers, the genetic diversity and genetic differentiation among collected B. tabaci MEAM1 populations were examined. Haplotype analysis using mtCOI sequences revealed the presence of a single B. tabaci MEAM1 haplotype across farmscapes of Georgia. Results from microsatellite markers further showed no significant genetic structuring among populations that corresponded to plant species or farmscapes from which they were collected. Annual whitefly population explosions and subsequent dispersal might have facilitated the persistence of a single panmictic B. tabaci population over all sampled farmscapes in this region.

Abstract

Bemisia tabaci is a whitefly species complex comprising important phloem feeding insect pests and plant virus vectors of many agricultural crops. Middle East–Asia Minor 1 (MEAM1) and Mediterranean (MED) are the two most invasive members of the B. tabaci species complex worldwide. The diversity of agroecosystems invaded by B. tabaci could potentially influence their population structure, but this has not been assessed at a farmscape level. A farmscape in this study is defined as heterogenous habitat with crop and non-crop areas spanning ~8 square kilometers. In this study, mitochondrial COI gene (mtCOI) sequences and six microsatellite markers were used to examine the population structure of B. tabaci MEAM1 colonizing different plant species at a farmscape level in Georgia, United States. Thirty-five populations of adult whiteflies on row and vegetable crops and weeds across major agricultural regions of Georgia were collected from fifteen farmscapes. Based on morphological features and mtCOI sequences, five species/cryptic species of whiteflies (B. tabaci MEAM1, B. tabaci MED, Dialeurodes citri, Trialeurodes abutiloneus, T. vaporariorum) were found. Analysis of 102 mtCOI sequences revealed the presence of a single B. tabaci MEAM1 haplotype across farmscapes in Georgia. Population genetics analyses (AMOVA, PCA and STRUCTURE) of B. tabaci MEAM1 (microsatellite data) revealed only minimal genetic differences among collected populations within and among farmscapes. Overall, our results suggest that there is a high level of gene flow among B. tabaci MEAM1 populations among farmscapes in Georgia. Frequent whitefly population explosions driven by a single or a few major whitefly-suitable hosts planted on a wide spatial scale may be the key factor behind the persistence of a single panmictic population over Georgia’s farmscapes. These population structuring effects are useful for delineating the spatial scale at which whiteflies must be managed and predicting the speed at which alleles associated with insecticide resistance might spread.

What Is the Spatial Extent of a Bemisia tabaci Population?

Effective pest management depends on basic knowledge about insect dispersal patterns and gene flow in agroecosystems. The globally invasive sweet potato whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is considered a weak flier whose life history nonetheless predisposes it to frequent dispersal, but the scale over which populations exchange migrants, and should therefore be managed, is uncertain. In this review, we synthesize the emergent literature on B. tabaci population genetics to address the question: What spatial scales define B. tabaci populations? We find that within-species genetic differentiation among sites is often low, and evidence of population structuring by host plant or geography is rare. Heterozygote deficits prevail among populations, indicating that migrants from divergent populations are frequently sampled together. Overall, these results suggest that there is high ongoing gene flow over large spatial extents. However, genetic homogeneity typical of recently invading populations could obscure power to detect real isolation among populations. Genome-wide data collected systematically across space and time could distinguish signatures of invasion history from those of ongoing gene flow. Characterizing the spatial extent of B. tabaci populations could reveal whether insecticide rotations can be tailored to specific commodities or if coordination across linked commodities and regions is justified.

Population genetic structure of Bemisia tabaci MED (Hemiptera: Aleyrodidae) in Korea

The sweet potato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a major agricultural pest that causes economic damages worldwide. In particular, B. tabaci MED (Mediterranean) has resulted in serious economic losses in tomato production of Korea. In this study, 1,145 B. tabaci MED females from 35 tomato greenhouses in different geographic regions were collected from 2016 to 2018 (17 populations in 2016, 13 in 2017, and five in 2018) and analyzed to investigate their population genetic structures using eight microsatellite markers. The average number of alleles per population (NA) ranged from 2.000 to 5.875, the expected heterozygosity (HE) ranged from 0.218 to 0.600, the observed heterozygosity (HO) ranged from 0.061 to 0.580, and the fixation index inbreeding coefficient (FIS) ranged from -0.391 to 0.872 over the three years of the study. Some significant correlation (p < 0.05) was present between genetic differentiations (FST) and geographical distance, and a comparatively high proportion of variation was found among the B. tabaci MED populations. The B. tabaci MED populations were divided into two well-differentiated genetic clusters within different geographic regions. Interestingly, its genetic structures converged into one genetic cluster during just one year. The reasons for this genetic change were speculated to arise from different fitness, insecticide resistance, and insect movement by human activities.

Differential wing polyphenism adaptation across life stages under extreme high temperatures in corn leaf aphid

Polyphenism, a common phenomenon in nature, is an important form of adaptation in a diverse environment. Corn leaf aphid (CLA), Rhopalosiphum maidis, (Hemiptera: Aphididae), exhibit wing polyphenism in response to poor habitat quality. In this study, we focused on the effects of crowding and thermal cues on morph determination of CLA. Five developmental stages of aphids (1^st to 4^th nymphs and maternal adults) with increased population densities, were tested under two kinds of temperature patterns, i.e., A) a constant temperature of 22 °C with 2 h exposure to high temperature in the range of 35 to 39 °C during mid-photophase and B) different constant temperatures in the range of 22-30 °C with 2 h exposure to high temperature of 39 °C during mid-photophase. Crowding was found to directly impact winged induction. The 1^st and 2^nd nymphs were more sensitive for alate morphs induction under high density. In addition, temperature played a significant role in wing production, with the temperature setting of 26/39 °C in pattern B inducing higher alate morphs and survival than other temperature settings. Therefore, we hypothesize that warmer climate with brief high temperature is more favourable for survival and alate morphs production, but cool weather and transient extreme high temperature (>39 °C) is detrimental for CLA. Our results provide a new perspective on understanding the interactions between changes in extreme high temperatures and insect densities that differentially affect wing polymorphism for further demographic and distribution rates of species across temporal and spatial scales.

Rapid adaptation of invertebrate pests to climatic stress?

There is surprisingly little information on adaptive responses of pests and disease vectors to climatic stresses even though the short generation times and large population sizes associated with pests make rapid adaptation likely. Most evidence of adaptive differentiation has been obtained from geographic comparisons and these can directly or indirectly indicate rates of adaptation where historical data on invasions are available. There is very little information on adaptive shifts in pests detected through molecular comparisons even though the genomes of many pests are now available and can help to identify markers underlying adaptation. While the limited evidence available points to frequent rapid adaptation that can affect pest and disease vector control, constraints to adaptation are also evident and a predictive framework around the likelihood and limits of rapid adaptation is required.