Variation in morphology and morphometrics of eggs of Culex quinquefasciatus mosquitoes from different ecological regions of India

Physiological characterization of chitin synthase A responsible for the biosynthesis of cuticle chitin in Culex pipiens pallens (Diptera: Culicidae)

Background

The pathogens transmitted by mosquitoes to humans and animals cause several emerging and resurgent infectious diseases. Increasing insecticide resistance requires rational action to control the target vector population. Chitin is indispensable for insect growth and development and absent from vertebrates and higher plants. Chitin synthase A (CHSA) is a crucial enzyme in chitin synthesis; therefore, identifying and characterizing how CHSA determines chitin content may contribute to the development of novel vector control strategies.

Results

The injection of small interfering RNA targeting CHSA (siCHSA) to knockdown CHSA transcripts in larval, pupal and adult stages of Culex pipiens pallens resulted in the appearance of different lethal phenotypes. When larval and pupal stages were injected with siCHSA, CHSA knockdown prevented larval molting, pupation and adult eclosion, and affected the production of chitin and chitin degradation, which resulted in an ecdysis defect phenotype of mosquitoes. When siCHSA was injected into mosquitoes in the adult stage, CHSA knockdown also affected the laminar organization of the mesoderm and the formation of pseudo-orthogonal patterns of the large fibers of the endoderm.

Conclusion

We provide a systematic and comprehensive description of the effects of CHSA on morphogenesis and metamorphosis. The results show that CHSA not only affects chitin synthesis during molting, but also might be involved in chitin degradation. Our results further show that CHSA is important for the structural integrity of the adult mosquito cuticle.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04741-2.

Establishment of a Colony of Phlebotomus argentipes under Laboratory Conditions and Morphometric Variation between Wild-Caught and Laboratory-Reared Populations

The field-based studies on sand flies are not adequate to uncover information required for the control of the leishmaniasis through reduction of vector populations. Therefore, establishment and maintenance of laboratory colonies of sand flies is an essential step in leishmaniasis research. In the current study, a colony of P. argentipes was established from wild-caught sand flies following standard procedures from the published literature. Morphological measurements of laboratory-reared and wild-caught individual sand flies were compared to assess the difference between two groups. The colony was successfully established under confined laboratory conditions. The comparison of morphometric parameters revealed that the laboratory-reared sand flies are significantly larger than those caught from wild, suggesting a possibility of increased fitness of sand flies under favorable environmental conditions which may cause higher prevalence in the disease. The current study reports the first successful attempt in colonizing sand flies under laboratory conditions. However, the colony data suggest that the conditions extracted from the published literature need to be optimized to suit local settings in order to achieve maximum population sizes within the available amount of resources.

Evaluation of enhanced oviposition attractant formulations against Aedes and Culex vector mosquitoes in urban and semi-urban areas

Surveillance is not only an important tool to assess the population dynamics of vector mosquitoes, but it can also be used to control vector-borne diseases. Mosquito vectors that belong to several genera such as Anopheles, Aedes, and Culex play a crucial role in the transmission of malaria, dengue, chikungunya, Zika, and elephantiasis diseases worldwide. We tested the efficacy of two commercial-grade oviposition attractant formulations that were developed for the container-inhabiting Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus mosquitoes present in urban or semi-urban environments. These attractants can lure gravid females. Field trials were conducted in residential yards during a post-rainy season in September and October. Our data showed considerable efficacy for both attractants. Aedes-attractant collected 1.6-fold more larvae (101.2 ± 10.5 larvae/trap) than the control, and Culex-attractant collected 1.27-fold more larvae (151.2 ± 12.5 larvae/trap) than the control, resulting in 0.8 and 0.7 oviposition attraction indices (OAIs), respectively. Regression analysis indicated that the Aedes-attractant was more stable than the Culex-attractant. Location and time did not alter the efficacy of these attractants. Our experiment suggests that these attractants can be used for the development of species-specific gravid traps to detect, estimate, and control the mosquito population in urban and semi-urban areas.

Differentiation of Aedes aegypti and Aedes notoscriptus (Diptera: Culicidae) eggs using scanning electron microscopy

Aedes notoscriptus and Aedes aegypti are both peri-domestic, invasive container-breeding mosquitoes. While the two potential arboviral vectors are bionomically similar, their sympatric distribution in Australia is limited. In this study, analyses of Ae. aegypti and Ae. notoscriptus eggs were enabled using scanning electron microscopy. Significant variations in egg length to width ratio and outer chorionic cell field morphology between Ae. aegypti and Ae. notoscriptus enabled distinction of the two species. Intraspecific variations in cell field morphology also enabled differentiation of the separate populations of both species, highlighting regional and global variation. Our study provides a comprehensive comparative analysis of inter- and intraspecific egg morphological and morphometric variation between two invasive container-breeding mosquitoes. The results indicate a high degree of intraspecific variation in Ae. notoscriptus egg morphology when compared to the eggs of Ae. aegypti. Comparative morphological analyses of Ae. aegypti and Ae. notoscriptus egg attributes using SEM allows differentiation of the species and may be helpful in understanding egg biology in relation to biotope of origin.

Malaria vector Anopheles culicifacies sibling species differentiation using egg morphometry and morphology

Background

The malaria vector Anopheles culicifacies (sensu lato) is an important malaria vector in Southeast Asia which comprises of five sibling species namely A, B, C, D and E. However, only a few forms have been identified as malaria vectors in various endemic countries. Currently, for the first time egg morphometry and morphology has been used to differentiate the three known vector sibling species of Anopheles culicifacies collected from malaria endemic Madhya Pradesh state of central India.

Methods

The adult An. culicifacies (s.l.) was collected from five districts using standard mosquito collection methods. Adult female mosquitoes were allowed to lay eggs individually. The emerged mosquitoes were identified using allele specific polymerase chain reaction (AS-PCR) to sibling species. Eggs of sibling species A, D and E were studied using scanning electron microscopy (SEM) for morphometric and morphological characteristics.

Results

Currently AS-PCR identified four known sibling species (B, C, D and E) of An. culicifacies in the study area. The surface morphology and morphometric attributes of the sibling species A, D and E eggs considerably differed from each other. An. culicifacies E had a narrow deck as compared to A and D, while An. culicifacies A had a bigger micropyle with 6–7 sectors as compared to D and E that had 6 sectors. An. culicifacies D had the smallest float (the structure present on sides of the egg surface in which air is filled that help in floating) and the number of ribs was also fewer than for An. culicifacies A and E.

Conclusions

The present study provides the first evidence that in addition to PCR assay, sibling species of An. culicifacies can also be differentiated using morphological and morphometric characteristics of the egg stage. The results also advocate that the sibling species of An. culicifacies are morphologically dissimilar and can be resolved using advanced microscopy.

Physical features and chitin content of eggs from the mosquito vectors Aedes aegypti, Anopheles aquasalis and Culex quinquefasciatus: Connection with distinct levels of resistance to desiccation

Mosquito eggs are laid in water but freshly laid eggs are susceptible to dehydration, if their surroundings dry out at the first hours of development. During embryogenesis of different mosquito vectors the serosal cuticle, an extracellular matrix, is produced; it wraps the whole embryo and becomes part of the eggshell. This cuticle is an essential component of the egg resistance to desiccation (ERD). However, ERD is variable among species, sustaining egg viability for different periods of time. While Aedes aegypti eggs can survive for months in a dry environment (high ERD), those of Anopheles aquasalis and Culex quinquefasciatus in the same condition last, respectively, for one day (medium ERD) or a few hours (low ERD). Resistance to desiccation is determined by the rate of water loss, dehydration tolerance and total amount of water of a given organism. The ERD variability observed among mosquitoes probably derives from diverse traits. We quantified several attributes of whole eggs, potentially correlated with the rate of water loss: length, width, area, volume, area/volume ratio and weight. In addition, some eggshell aspects were also evaluated, such as absolute and relative weight, weight/area relationship (herein called surface density) and chitin content. Presence of chitin specifically in the serosal cuticle as well as aspects of endochorion external surface were also investigated. Three features could be related to differences on ERD levels: chitin content, directly related to ERD, the increase in the egg volume during embryogenesis and the eggshell surface density, which were both inversely related to ERD. Although data suggest that the amount of chitin in the eggshell is relevant for egg impermeability, the participation of other yet unidentified eggshell attributes must be considered in order to account for the differences in the ERD levels observed among Ae. aegypti, An. aquasalis and Cx. quinquefasciatus.

Morphometric comparisons of the scanning electron micrographs of the eggs of Anopheles (Nyssorhynchus) darlingi Root (Diptera: Culicidae)

Anopheles darlingi Root is the principal vector of Plasmodium in Brazil, but its biological variability is not well known. Morphometric analyses of scanning electron microscopy images of the eggs of An. darlingi were conducted using individuals collected in nine states of Brazil (Acre, Amapá, Espírito Santo, Pará, Paraná, Rio de Janeiro, Rondônia, São Paulo, and Tocantins). Ten attributes of the eggs (seven continuous variables and three discrete variables) were respectively measured or counted and analyzed to determine if populations from different geographical regions or biomes could be distinguished. Univariate analysis showed that the eggs from Espírito Santo were the narrowest whereas representatives from Tocantins populations had the smallest floats. Results of multivariate analyses of continuous variables showed that the first principal component (PC1), mainly represented by all four float attributes, helped to differentiate populations. The second principal component (PC2) comprised roughly the length and width of the egg. PC1 of discrete variables corresponded to the number of ribs on the float whereas PC2 was approximately equivalent to the number of discs on the micropyle. Based on those variables (continuous and discrete separately), multivariate discriminant analysis indicated that eggs from individuals collected in Tocantins were distinct from the other populations. Among sampled localities, the one from the state of Tocantins was situated within the Cerrado biome whereas the locality from São Paulo state was at the border of Cerrado, within a transition zone of the Atlantic Forest biome. Generally, the climate in the Cerrado biome was more arid than in areas of the Amazon and Atlantic Forest biomes, and the temperature had the highest range. Coincidentally, based on morphometric data, cluster analysis distinguished the population from Cerrado, Tocantins from all other populations. Results of multiple regression analysis of the variables showed no correlation between egg variables and latitude or climatic variables. We concluded that eggs were polymorphic and that some morphological patterns were regional. Although no environmental influence on the egg attributes was unequivocally detected, a potential association cannot be entirely discarded. Consequently, we hypothesize that morphological traits of the immature stages, especially from the eggs, convey evolutionary information regarding to this species.

Integrated proteomic and transcriptomic analysis of the Aedes aegypti eggshell

Background

Mosquito eggshells show remarkable diversity in physical properties and structure consistent with adaptations to the wide variety of environments exploited by these insects. We applied proteomic, transcriptomic, and hybridization in situ techniques to identify gene products and pathways that participate in the assembly of the Aedes aegypti eggshell. Aedes aegypti population density is low during cold and dry seasons and increases immediately after rainfall. The survival of embryos through unfavorable periods is a key factor in the persistence of their populations. The work described here supports integrated vector control approaches that target eggshell formation and result in Ae. aegypti drought-intolerant phenotypes for public health initiatives directed to reduce mosquito-borne diseases.

Results

A total of 130 proteins were identified from the combined mass spectrometric analyses of eggshell preparations.

Conclusions

Classification of proteins according to their known and putative functions revealed the complexity of the eggshell structure. Three novel Ae. aegypti vitelline membrane proteins were discovered. Odorant-binding and cysteine-rich proteins that may be structural components of the eggshell were identified. Enzymes with peroxidase, laccase and phenoloxidase activities also were identified, and their likely involvements in cross-linking reactions that stabilize the eggshell structure are discussed.

Variations in life tables of geographically isolated strains of the mosquito Culex quinquefasciatus

Variations in the life tables and other biological attributes of four strains of Culex quinquefasciatus Say (Diptera: Culicidae) from geographically isolated regions of India that had been reared to the fifth generation in the laboratory were assessed under a standardized rearing regime under constant laboratory conditions. Two strains from arid habitats [Jodhpur (JD) and Bikaner (BKN)], one from a semi-arid inland habitat [Bathinda (BTH)], one from a semi-arid coastal habitat [Jamnagar (JMN)] and a standard laboratory strain (LAB) were compared. Horizontal life-table parameters were measured for each strain. Egg mortality ranged from 4.4% (JD and BTH) to 19.5% (BKN). The lowest rate of adult emergence and highest female : male ratio were found in BKN, and the highest rate of adult emergence and lowest female : male ratio were recorded in BTH. The egg-hatching period was longest in BTH and shortest in LAB. The duration from oviposition to adult emergence was longest in JD and shortest in LAB. Females lived longer than males in all strains. The net reproductive rates (R(0) ) of all field-derived strains (122.9-162.2) differed significantly between strains and were significantly greater than that of LAB (107.6). Similarly, both the intrinsic rate of increase (r(m) ) and finite rate of increase (λ) were found to be lower in LAB than in the field strains, but the mean generation time (T) and doubling time (DT) were longest in LAB. For several life-table attributes, JD and BTH clustered together and were more similar to JMN than to BKN and LAB. The results indicate that BTH, BKN and JD can be characterized as r-strategists, more so than JMN. Overall fecundity increased with age. Differences in annual temperature ranges and mean annual rainfall between locations were positively correlated (r = 0.46-0.97) with egg production, female life expectancy, R(0) , r(m) , λ and T. The results suggest that strains of Cx. quinquefasciatus from different geographical areas with contrasting habitats vary in their survival and reproductive strategies accordingly.

Differentiation of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) with egg surface morphology and morphometrics using scanning electron microscopy

Aedes aegypti and Aedes albopictus are potential arboviral vectors leading to high human fatality worldwide. Efforts in the present study were made to differentiate the eggs of A. aegypti and A. albopictus morphologically and morphometrically using scanning electron microscopy (SEM). Morphometrically, these species' eggs were 48.48% significantly different of the 33 attributes including egg dimensions, micropylar apparatus, dimensions and density of outer chorionic cells (OCCs), tubercles and width of exochorionic network. In comparison to A. aegypti eggs, A. albopictus eggs were significantly smaller and more tapered at the posterior end; however, the micropylar disc of A. aegypti was wider and had incomplete circular sectors whereas it was a narrower polygon without sectors in A. albopictus. These species were also significantly different with regards to OCC which enclose both large central and small peripheral tubercles. Specifically, the exochorionic networks in A. aegypti were interwoven, reticulated and extensively wide whereas they were narrow, prominent and solid-wall-like in A. albopictus. This feature may strengthen A. albopictus eggs against desiccation, when they are laid in containers. The morphometrical and morphological analysis of the egg's attributes of A. aegypti and A. albopictus may be helpful in understanding egg biology as well as in species confirmation.