Sex and stripping

Abstract

Wolbachia pipientis is known to infect only arthropods and nematodes (mainly filarial worms). A unique feature shared by the two Phyla is the ability to replace the exoskeleton, a process known as ecdysis. This shared characteristic is thought to reflect a common ancestry. Arthropod moulting is induced by the steroid hormone 20-hydroxyecdysone (20E) and a role for ecdysteroids in nematode ecdysis has also been suggested. Interestingly, Wolbachia removal in filarial worms is detrimental to the host that is subjected to an impaired development. From analyses of the genome of Wolbachia harboured by the filarial nematode Brugia malayi and that of its host, the bacterium may provide a source of heme, an essential component of cytochromes P450 that are necessary for biosynthetic pathways of steroid hormones.

In arthropods, Wolbachia is a reproductive manipulator, inducing various phenotypic effects that may be due to differences in host physiology, and in particular to endocrine-related processes governing development and reproduction. At this regard, insect steroids have a well-defined role in the coordination of multiple developmental processes, and in adults they control important aspects of reproduction, including ovarian development, oogenesis, sexual behaviour, and in some taxa vitellogenin biosynthesis.

According to some authors ecdysteroids may also act as sex hormones. In insect sex differentiation is generally thought to be a strictly genetic process, where each cell decides its own sexual fate based on its sex chromosome constitution, but, surprisingly, recent data demonstrate that in Drosophila sex determination is non cell-autonomous, as it happens in mammals. Thus the presence of signals coordinating the development of a gender-specific phenotype cannot be excluded. This could explain why Wolbachia interferes with insect reproduction; and also could explain why Wolbachia interferes with insect development. Thus, is “sex (= reproduction) and stripping (= ecdysis)” the key to the intimate relationship between Wolbachia and its host?

Figures and Tables

Figure 1 Insect moulting is induced by the circulating steroid hormone 20-hydroxyecdysone (20E) whose biosynthesis takes origin from dietary cholesterol. The process is initiated by secretion of the brain neuropeptide prothoracicotropic hormone (PTTH). PTTH acts on the prothoracic glands, triggering the synthesis and secretion of a pre-ecdysteroid which is sequentially converted to active 20E. Cholesterol is converted to 20E thanks to a number of hydroxylation reactions catalysed by cytochrome P450 enzymes of microsomal and/or mitochondrial origin. Then, 20E binds heterodimeric nuclear receptors composed of EcR (Ecdysone Receptor) and Usp (Ultraspiracle), and activates the transcriptional processes underlying the cellular and morphogenetic moulting cascade events. Even if PTTH is the the major tropic hormone driving ecdysone secretion, also insulin-like peptides (ILPs) may induce ecdysteroidogenesis, through stimulation of the prothoracic gland growth. BR (brain); ILPs (insulin-like peptides); PTTH (prothoracicotropic hormone); PG (prothoracic gland); EcR/USP (Ecdysone Receptor/Ultraspiracle).