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Table of Contents
Year : 2018  |  Volume : 55  |  Issue : 3  |  Page : 235-238

Adaptation of Aedes aegypti to salinity: Characterized by larger anal papillae in larvae

1 Department of Zoology, Faculty of Science, University of Jaffna, Jaffna, Sri Lanka
2 Department of Mathematic and Statistics, Faculty of Science, University of Jaffna, Jaffna, Sri Lanka
3 Department of Chemistry, Faculty of Science, University of Jaffna, Jaffna, Sri Lanka
4 ID-FISH Technology Inc., Palo Alto, USA

Date of Submission18-Oct-2017
Date of Acceptance25-Jul-2018
Date of Web Publication4-Jan-2019

Correspondence Address:
S N Surendran
Department of Zoology, University of Jaffna, Jaffna–40000
Sri Lanka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-9062.249482

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Keywords: Aedes aegypti; anal papillae; brackish water; fresh water; salinity-tolerant mosquitoes; Sri Lanka; vector biology

How to cite this article:
Surendran S N, Sivabalakrishnan K, Jayadas T, Santhirasegaram S, Laheetharan A, Senthilnanthanan M, Ramasamy R. Adaptation of Aedes aegypti to salinity: Characterized by larger anal papillae in larvae. J Vector Borne Dis 2018;55:235-8

How to cite this URL:
Surendran S N, Sivabalakrishnan K, Jayadas T, Santhirasegaram S, Laheetharan A, Senthilnanthanan M, Ramasamy R. Adaptation of Aedes aegypti to salinity: Characterized by larger anal papillae in larvae. J Vector Borne Dis [serial online] 2018 [cited 2023 Feb 6];55:235-8. Available from: http://www.jvbd.org//text.asp?2018/55/3/235/249482

Aedes aegypti is considered the principal vector of several human arboviral diseases, including dengue, chikungunya, West-Nile encephalitis and Zika. Many of the approximately 5% of mosquito species that are salinity tolerant[1], are vectors of important human and animal diseases; and such vectors can increase in number and invade to new territories in the context of global warming and other anthropogenic environmental changes[2],[3],[4]. Aedes aegypti has previously been regarded to oviposit and undergo preimaginal development only in fresh water (FW) collections, and hence, larval control efforts are solely directed towards FW habitats in the vicinity of human dwellings[5],[6]. We recently showed that Ae. aegypti and the closely related arboviral vector Ae. albopictus can undergo preimaginal development in brackish water (within discarded food and beverage containers) of up to 15 parts per thousand (ppt) salt and 14 ppt salt, respectively in northern Sri Lankan beaches[7],[8]. Saline, brackish water (BW) and FW are defined as containing >30, 0.5–30 and <0.5 ppt salt, respectively in this context[2]. Aedes aegypti larvae have also been observed in brackish domestic wells of up to 9 ppt salt in the coastal Jaffna peninsula of northern Sri Lanka[9]. Similarly, BW development of Ae. aegypti or Ae. albopictus has been observed in coastal locations of Brunei Darussalam[10], USA[11] and Brazil[12]. Physiological changes accompanying BW adaptation in Ae. aegypti include the greater ability to oviposit in 10 ppt salinity; and significantly greater, and partly inheritable, larval salinity tolerance[13].

The anal papillae of Ae. aegypti are reported to transport Na+ and Cl from a FW environment into the hemolymph[14],[15]. There is also a salinity-related differential expression of specific aquaporins in Ae. aegypti anal papillae[16]. These findings suggest a possible role of anal papillae in short-term osmoregulation in Ae. aegypti larvae. In an attempt to explore the mechanisms, underlying the adaptation to BW, this study investigated the changes in anal papillae size in Ae. aegypti mosquitoes adapting to BW.

Self-mating laboratory colonies of Ae. aegypti were established from larvae collected from FW ovitraps in Thirunelvely (9° 41' N: 80° 1' E) and BW domestic wells in coastal Kurunagar (9° 39' N: 80° 1' E) in the Jaffna peninsula of Sri Lanka. The FW colony was maintained in potable tap water supplied from a dug-well located in the University of Jaffna campus in Thirunelvely, and the BW colony in sea water collected from Jaffna lagoon, diluted with tap water to yield ~10 ppt salt (measured by a refractor Salinometer, Atago, Japan). The adult and larvae were maintained at 28–30°C with relative humidity ~75% and 12 h dark and light conditions. Eggs from the BW and FW colonies were collected in 10 ppt saline BW and 0 ppt saline FW egg-laying surfaces, respectively and allowed to hatch into larvae in the corresponding BW and FW. Larvae were reared in 24 × 16 cm plastic trays with 1.5 litre water with maximum number of 150 larvae per tray and fed with fish meal powder thrice a day. Adult females were blood-fed on mice according to a protocol approved by the Institutional Animal Ethics Review Committee of the University of Jaffna (June, 2014; AERC/2014/02).

The FW colony maintained up to 20 generations in the laboratory and BW colony maintained up to 24 generations were used for experimental purposes. For experiments to determine the effects of reversing larval rearing salinity (salinity reversal experiments), FW colony eggs were collected on FW egg-laying surfaces and then reared in 10 ppt salinity BW, and also separately in FW in parallel. For the corresponding reversal of salinity, BW colony eggs were collected on BW egg-laying surfaces and reared in FW, and also parallelly in 10 ppt salinity BW.

On an average, >90% of the larvae in the rearing trays in each experiment were of the same age and size. Late III instar larvae from the four test samples were identified as such by counting the number of comb spines[17] and pecten teeth[18]. For measuring the size of anal papillae, late III instar larvae were dorso-ventrally placed individually on a clean glass microscope slide and the four lobes were individually termed I–IV as shown in [Figure 1]a. The maximal length and width of individual anal papillae were measured as indicated in [Figure 1]b using an CX21 (Olympus, Tokyo) light microscope equipped with an ocular micrometer. The maximum lengths of the larvae were also measured at the same time. All the measurements were done and recorded independently by two persons and the average value was used for statistical analysis.
Figure 1: (a) Photograph of posterior portion of dorso-ventrally placed larvae showing the numbering system of anal papilla lobes; and (b) Photograph of anal papillae of (i) FW larvae, and (ii) BW larvae.

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The Student's two-tailed t-test was performed when mean dimensions of specific lobes were initially compared. Tukey's method for multiple comparisons in the analysis of variance (ANOVA) was used to compare group means at 95% CI, where dimensions of all four anal papillae lobes were compared in salinity reversal experiments.

Delayed pre-imaginal development previously observed during the early stages of BW adaptation (up to five generations) in Ae. aegypti[13] was not observed in the present experiments, most likely due to the longer time of adaptation to salinity. Initially, the length of a single anal papilla was measured. The mean anal papilla IV length of BW larvae (15th generation) was 0.55 ± 0.5 mm, and this was significantly greater than the mean anal papilla IV length of FW (10th generation) larvae of 0.50 ± 0. 04 mm (p < 0.05). In a subsequent experiment, the mean anal papilla IV length of BW (18th generation) larvae was 0.57± 0.07 mm and this was significantly greater than the mean anal papilla IV length of FW (14th generation) larvae of 0. 45 ± 0.06 mm.

As the initial measurements suggested that there were significant differences in the sizes of anal papillae between FW and BW larvae, the experiment was further modified to measure the length and width of the four anal papillae (I–IV) that involved reversing larval-rearing salinity. The maximum length (L-I to L-IV) and corresponding width (W-I to W-IV) of each anal papilla and the total length of the larvae [Figure 1]a were measured in 10 late III instar larvae of FW (generation 20) and BW (generation 24). The dimensions of the late III larvae anal papillary lobes and the outcome of the statistical analysis are presented in [Table 1]. The BW larvae reared in BW possessed significantly larger anal papillae lobes in both length and width than FW larvae reared in FW [Figure 1]b, [Table 1]. When FW larvae were shifted to 10 ppt salinity BW for rearing, they tended to have longer and wider lobes than FW larvae maintained in FW, but the differences were not statistically significant. However, when the larvae from the BW colony larvae were subjected to a reversal in salinity and reared in FW, there was a significant reduction in their mean lobe length and width. The BW colony larvae reared in FW had anal papillae that were significantly longer (but not significantly wider) than FW colony larvae that were reversed to develop in BW [Table 1]. Although larvae developing in BW tended to have longer mean whole larval lengths than larvae developing in FW, statistically significant differences could not be established except that BW colony larvae in BW were longer than in BW colony eggs allowed to develop as larvae in FW [Table 1].
Table 1: Morphometric analysis of larval anal papillae of Ae. aegypti reared in brackish and fresh water

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The results, therefore indicate that the anal papillae of Ae. aegypti larvae increase in size when adapted to BW and this is partly reversed when they are reared in FW. The corresponding changes in larvae of Ae. aegypti adapted to FW and reared in BW, although not statistically significant, tend to support this conclusion.

The only other study to our knowledge on size variations in anal papillae of Ae. aegypti in response to salinity changes was performed in UK in 1938[19]. This reported that when larvae of long established laboratory colonies of Ae. aegypti maintained in FW are placed in solutions of increasing NaCl concentration up to 0.9% or 9 ppt, the length of the anal papillae decreased up to 17%. The changes were attributed to atrophy of the anal papillae associated with a decreased need to take up NaCl to maintain hemolymph NaCl concentrations in a more saline environment[19]. The different findings of this study might be due to the use of larvae from colonies adapted relatively to FW and BW from recently field collections made in the Jaffna peninsula rather than a long established FW Ae. aegypti colony in UK, and the use of diluted sea water in present experiments instead of NaCl solutions to better reflect the natural coastal BW habitats of Ae. aegypti.

Larvae of some salinity-tolerant mosquito species have evolved various physiological mechanisms associated with specific anatomical changes to cope with salinity in the larval environment. Aedes taeniorhynchus ingest the surrounding fluid and excrete Na+ and Cl from an additional differentiated rectal segment (the posterior rectum) to produce a hyperosmotic urine[1]. Larvae of the euryhaline malaria vector An. albimanus are able to differentially localize sodium-potassium ATPase in specialized anterior dorsal cells of their rectum in fresh or saline water for likely osmoregulation through ion excretion[20]. There is no evidence yet to suggest that Ae. aegypti have specialized rectal structures that are associated with salinity adaptation[21]. The present findings, and related physiological observations on anal papillae[14],[15],[16], lead us to propose that changes in ion and water transport in Ae. aegypti anal papillae are reflected in size change, and that the underlying osmoregulatory mechanisms may be critical to permit their preimaginal development in BW habitats.

In conclusion, the potential importance of BW adaptation in Ae. aegypti to their anthropogenically-induced spread and increased transmission of arboviral diseases[2],[21], warrant further studies to elucidate the relevant biological adaptive mechanisms, e.g. through ongoing differential gene expression studies on anal papillae of Ae. aegypti. This is important since the BW developing Ae. aegypti and Ae. albopictus originating in the Jaffna peninsula, like their FW counterparts, can be infected with dengue virus and transmit the virus transovarially to their progeny[21].

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