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Embryonic desiccation resistance in Aedes aegypti: presumptive role of the chitinized serosal cuticle.

Rezende GL, Martins AJ, Gentile C, Farnesi LC, Pelajo-Machado M, Peixoto AA, Valle D - BMC Dev. Biol. (2008)

Bottom Line: It has been argued that mosquito SC contains chitin as one of its components, however conclusive evidence is still missing.We observed an abrupt acquisition of desiccation resistance during Ae. aegypti embryogenesis associated with serosal cuticle secretion, occurring at complete germ band extension, between 11 and 13 hours after egglaying.With this embryological approach we expect to shed new light regarding this important physiological process related to the Ae. aegypti life cycle.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratório de Fisiologia e Controle de Artrópodes Vetores, Instituto Oswaldo Cruz, FIOCRUZ and Laboratório de Entomologia, Instituto de Biologia do Exército, Rio de Janeiro, Brazil. rezendeg@ioc.fiocruz.br

ABSTRACT

Background: One of the major problems concerning dengue transmission is that embryos of its main vector, the mosquito Aedes aegypti, resist desiccation, surviving several months under dry conditions. The serosal cuticle (SC) contributes to mosquito egg desiccation resistance, but the kinetics of SC secretion during embryogenesis is unknown. It has been argued that mosquito SC contains chitin as one of its components, however conclusive evidence is still missing.

Results: We observed an abrupt acquisition of desiccation resistance during Ae. aegypti embryogenesis associated with serosal cuticle secretion, occurring at complete germ band extension, between 11 and 13 hours after egglaying. After SC formation embryos are viable on dry for at least several days. The presence of chitin as one of the SC constituents was confirmed through Calcofluor and WGA labeling and chitin quantitation. The Ae. aegypti Chitin Synthase A gene (AaCHS1) possesses two alternatively spliced variants, AaCHS1a and AaCHS1b, differentially expressed during Ae. aegypti embryonic development. It was verified that at the moment of serosal cuticle formation, AaCHS1a is the sole variant specifically expressed.

Conclusion: In addition to the peritrophic matrix and exoskeleton, these findings confirm chitin is also present in the mosquito serosal cuticle. They also point to the role of the chitinized SC in the desiccation resistance of Ae. aegypti eggs. AaCHS1a expression would be responsible for SC chitin synthesis. With this embryological approach we expect to shed new light regarding this important physiological process related to the Ae. aegypti life cycle.

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Embryo viability in dry conditions is greatly improved after serosal cuticle formation. Pools of synchronized developing eggs were transferred to dry conditions after 10, 15 or 20 HAE (A, B, C, respectively), a period that encompasses SC formation. In each case, embryos were kept on dry for 25 (1), 48 (2) or 120 (3) hours, as indicated. Hatching of samples was then checked (see item 4 of Methods for details). Data are expressed as percent viability, corrected from control samples, kept on moist conditions throughout development. Blue stripe: putative period between beginning of SC formation and its complete maturation (see Discussion). Dashed line: end of embryogenesis. Hatching stimuli was performed at the end of embryogenesis, or when indicated by *.
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Figure 2: Embryo viability in dry conditions is greatly improved after serosal cuticle formation. Pools of synchronized developing eggs were transferred to dry conditions after 10, 15 or 20 HAE (A, B, C, respectively), a period that encompasses SC formation. In each case, embryos were kept on dry for 25 (1), 48 (2) or 120 (3) hours, as indicated. Hatching of samples was then checked (see item 4 of Methods for details). Data are expressed as percent viability, corrected from control samples, kept on moist conditions throughout development. Blue stripe: putative period between beginning of SC formation and its complete maturation (see Discussion). Dashed line: end of embryogenesis. Hatching stimuli was performed at the end of embryogenesis, or when indicated by *.

Mentions: To evaluate the physiological relevance of desiccation resistance acquisition, we quantified the viability of developing eggs transferred to dry conditions at different ages and for varying periods of time, from 25 to 120 hours (Fig. 2, see item 4 of Methods for details). No hatching was observed if eggs were exposed to dry conditions before SC formation (10 HAE, Fig 2A). In contrast, samples dried at 20 HAE were fully viable (Fig 2C). Intermediate values were obtained if eggs were transferred to dry conditions in a period close to SC formation (15 HAE, Fig 2B). In this case viability was inversely proportional to the period of time spent in dry conditions. Since 120 hours on dry, starting from 20 HAE, corresponds to approximately 3 days after the end of embryogenesis, we conclude that the fully formed SC at 20 HAE is competent to impede embryonic and larvae desiccation during and after completion of embryogenesis (see Discussion).


Embryonic desiccation resistance in Aedes aegypti: presumptive role of the chitinized serosal cuticle.

Rezende GL, Martins AJ, Gentile C, Farnesi LC, Pelajo-Machado M, Peixoto AA, Valle D - BMC Dev. Biol. (2008)

Embryo viability in dry conditions is greatly improved after serosal cuticle formation. Pools of synchronized developing eggs were transferred to dry conditions after 10, 15 or 20 HAE (A, B, C, respectively), a period that encompasses SC formation. In each case, embryos were kept on dry for 25 (1), 48 (2) or 120 (3) hours, as indicated. Hatching of samples was then checked (see item 4 of Methods for details). Data are expressed as percent viability, corrected from control samples, kept on moist conditions throughout development. Blue stripe: putative period between beginning of SC formation and its complete maturation (see Discussion). Dashed line: end of embryogenesis. Hatching stimuli was performed at the end of embryogenesis, or when indicated by *.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2561029&req=5

Figure 2: Embryo viability in dry conditions is greatly improved after serosal cuticle formation. Pools of synchronized developing eggs were transferred to dry conditions after 10, 15 or 20 HAE (A, B, C, respectively), a period that encompasses SC formation. In each case, embryos were kept on dry for 25 (1), 48 (2) or 120 (3) hours, as indicated. Hatching of samples was then checked (see item 4 of Methods for details). Data are expressed as percent viability, corrected from control samples, kept on moist conditions throughout development. Blue stripe: putative period between beginning of SC formation and its complete maturation (see Discussion). Dashed line: end of embryogenesis. Hatching stimuli was performed at the end of embryogenesis, or when indicated by *.
Mentions: To evaluate the physiological relevance of desiccation resistance acquisition, we quantified the viability of developing eggs transferred to dry conditions at different ages and for varying periods of time, from 25 to 120 hours (Fig. 2, see item 4 of Methods for details). No hatching was observed if eggs were exposed to dry conditions before SC formation (10 HAE, Fig 2A). In contrast, samples dried at 20 HAE were fully viable (Fig 2C). Intermediate values were obtained if eggs were transferred to dry conditions in a period close to SC formation (15 HAE, Fig 2B). In this case viability was inversely proportional to the period of time spent in dry conditions. Since 120 hours on dry, starting from 20 HAE, corresponds to approximately 3 days after the end of embryogenesis, we conclude that the fully formed SC at 20 HAE is competent to impede embryonic and larvae desiccation during and after completion of embryogenesis (see Discussion).

Bottom Line: It has been argued that mosquito SC contains chitin as one of its components, however conclusive evidence is still missing.We observed an abrupt acquisition of desiccation resistance during Ae. aegypti embryogenesis associated with serosal cuticle secretion, occurring at complete germ band extension, between 11 and 13 hours after egglaying.With this embryological approach we expect to shed new light regarding this important physiological process related to the Ae. aegypti life cycle.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratório de Fisiologia e Controle de Artrópodes Vetores, Instituto Oswaldo Cruz, FIOCRUZ and Laboratório de Entomologia, Instituto de Biologia do Exército, Rio de Janeiro, Brazil. rezendeg@ioc.fiocruz.br

ABSTRACT

Background: One of the major problems concerning dengue transmission is that embryos of its main vector, the mosquito Aedes aegypti, resist desiccation, surviving several months under dry conditions. The serosal cuticle (SC) contributes to mosquito egg desiccation resistance, but the kinetics of SC secretion during embryogenesis is unknown. It has been argued that mosquito SC contains chitin as one of its components, however conclusive evidence is still missing.

Results: We observed an abrupt acquisition of desiccation resistance during Ae. aegypti embryogenesis associated with serosal cuticle secretion, occurring at complete germ band extension, between 11 and 13 hours after egglaying. After SC formation embryos are viable on dry for at least several days. The presence of chitin as one of the SC constituents was confirmed through Calcofluor and WGA labeling and chitin quantitation. The Ae. aegypti Chitin Synthase A gene (AaCHS1) possesses two alternatively spliced variants, AaCHS1a and AaCHS1b, differentially expressed during Ae. aegypti embryonic development. It was verified that at the moment of serosal cuticle formation, AaCHS1a is the sole variant specifically expressed.

Conclusion: In addition to the peritrophic matrix and exoskeleton, these findings confirm chitin is also present in the mosquito serosal cuticle. They also point to the role of the chitinized SC in the desiccation resistance of Ae. aegypti eggs. AaCHS1a expression would be responsible for SC chitin synthesis. With this embryological approach we expect to shed new light regarding this important physiological process related to the Ae. aegypti life cycle.

Show MeSH
Related in: MedlinePlus