Developmental Regulation of Corazonin, Eclosion Hormone, and Bursicon Messages and RNAi Suppression of Corazonin in Adult, Female American Dog Ticks, Dermacentor variabilis

TL;DR

This study explores how molting-related hormones in insects might also regulate reproduction in adult female American dog ticks.

Contribution

The study identifies and functionally tests the role of molting-related neuropeptides in tick reproduction for the first time.

Findings

Corazonin, eclosion hormone, and bursicon messages are present in adult, non-molting female ticks.

RNAi suppression of corazonin reduced egg maturation and oviposition in ticks.

Bursicon β transcript levels were 32-fold higher post-drop-off, suggesting a role in egg-laying.

Abstract

The insect molting process including their shedding of the old cuticle and hardening of the new cuticle is regulated by a cascade of peptide hormones, including corazonin, eclosion hormone and α and β bursicon. The messenger RNA for these hormones were found in adult, female American dog ticks. Since adult ticks do not molt, this finding suggests the hormones that regulate insect molting might also control female reproduction. Changes in the developmental expression levels of the messages of these hormones during tick host seeking, blood feeding, mating and oviposition argues they are involved in reproduction. The artificial suppression of corazonin that initiates the molting cascade reduced the maturation level of eggs and reduced egg deposition in the American dog tick.

The insect molting process is critical to growth and development and is regulated in part by the neuropeptides corazonin, eclosion hormone, and α and β bursicon. We found messages in a synganglion transcriptome from adult, female American dog ticks, Dermacentor variabilis (that do not molt), with a high similarity to the larval insect neuropeptides that control molting. The phylogenetic analysis of the tick putative neuropeptides compared to other arthropods is discussed in detail. The relative gene expression of these peptides was determined by quantitative PCR during the following adult developmental stages: (i) virgin, unfed 0–24 h after entering the adult stage (non-host-seeking), (ii) host-seeking, unfed, and not mated (3 d after emergence), (iii) part-fed (unmated, attached to host; 1st and 3rd day after emergence), (iv) mated (females are part-fed; allowed to mate for ≤1 day, 7th day after emergence), (v) mated repletes (completion of blood feeding but still attached to host), and (vi) post-drop-off (from host) with egg laying starting within 1 d of detachment. Eclosion hormone transcript levels peaked at mating and at drop-off. Bursicon α levels were highest just after molting into adults, with a second smaller peak in replete females. Bursicon β levels were highest (32-fold) post-drop-off. Corazonin message levels peaked in part-feds and were much higher (40-fold) in repletes compared to 0–24 h after emergence. RNAi suppression of the corazonin message by injection in newly molted ticks reduced oviposition and the number of vitellogenic eggs in the ovaries at drop-off but had no apparent effect on host-seeking, partial feeding, mating, feeding to repletion, and drop-off. The possible roles of these transcripts in adult, female tick development are discussed.

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