Abstract
BackgroundAphids display “cyclic parthenogenesis,” in which parthenogenetically and sexually reproducing morphs seasonally alternate in the aphid annual life cycles. There are various characteristics that differ between asexual viviparous and sexual oviparous females. In oviparous females, swollen cuticular structures (~ 10 μm in diameter), called “scent plaques,” are scattered on the surface of hind tibias, and secrete monoterpenoid sex pheromones. However, the developmental processes of the pheromone glands and the biosynthetic pathways of monoterpenoid pheromones have yet to be elucidated.ResultsComparisons of the developmental processes that form hind tibias between sexual and parthenogenetic females revealed that, in sexual females, the epithelial tissues in proximal parts of hind tibias become columnar in fourth instar nymphs, and circular pheromone glands with Class 1 gland cells appear in adults, although they do not appear in parthenogenetic females. Furthermore, by comparing the expression levels of genes involved in the mevalonate pathway, which is required for monoterpenoid synthesis, we show that genes that encode the downstream enzymes in the pathway are highly expressed in hind tibias of sexual females.ConclusionGlandular tissues of scent plaque are differentiated from the fourth instar in sexual females, while parthenogenetic females lack the glandular cells. Only the downstream steps of the mevalonate pathway appear to occur in scent plaques on hind tibias of sexual females, although the upstream steps may occur somewhere in other body parts.
Highlights
Aphids display “cyclic parthenogenesis,” in which parthenogenetically and sexually reproducing morphs seasonally alternate in the aphid annual life cycles
Scent plaques in the pea aphid are suggested to be composed of glandular cuticle, and the epithelial cells under the scent plaques are classified as Class 1 gland cells, suggesting that sex pheromones are synthesized in the gland cells under the scent plaques
The Quantitative RT-PCR (qRT-PCR) results (Fig. 5) indicated that the downstream enzymes (MevPK, mevalonate 5diphosphate decarboxylase (MevPPD), and isopentenyl pyrophosphate isomerase (IPPI)) and farnesyl diphosphate synthetase (FPPS) were responsible for monoterpene biosynthesis in oviparous females (Figs. 2–4), which is consistent with the fact that only oviparous females possess Class 1 gland cells on hind tibias. These results indicate that oviparous females synthesize monoterpenoid sex pheromones by up-regulating gene expression of downstream enzyme genes in the Class 1 epidermal gland cells of hind tibias
Summary
Aphids display “cyclic parthenogenesis,” in which parthenogenetically and sexually reproducing morphs seasonally alternate in the aphid annual life cycles. There are various characteristics that differ between asexual viviparous and sexual oviparous females. Swollen cuticular structures (~ 10 μm in diameter), called “scent plaques,” are scattered on the surface of hind tibias, and secrete monoterpenoid sex pheromones. Most aphid species show cyclic parthenogenesis, in which both sexual and asexual reproduction are performed depending on season [1,2,3,4] (Fig. 1). The following spring, parthenogenetic females known as “fundatrices”. Sexual aphid females are known to secrete sex pheromones to attract males [9,10,11,12]. In many aphid species, including the pea aphid Acyrthosiphon pisum, two Murano et al Zoological Letters (2018) 4:9
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