The cilium like region of theDrosophilaspermatocyte: an oncoming flagellum?

Abstract

SummaryPrimary cilia and flagella are distinct structures with different functions in eukaryotic cells. Despite the fact that they share similar basic organization and architecture, a direct developmental continuity among them has not been reported until now. The primary cilium is a dynamic structure that typically assembles and disassembles during mitotic cell cycles, whereas the sperm axoneme is nucleated by the centriole inherited by the differentiating spermatid at the end of meiosis. Fruit flies display a remarkable exception to this general rule. Drosophila spermatocytes have an unusual axoneme-based structure reminiscent of primary cilia (the cilium-like region, or CLR). This structure persists through the meiotic divisions when it is internalized with the centriole to organize the centrosome and is finally inherited by young spermatids. Examination of elongating spermatids by transmission electron microscopy (EM) and cold regrowth experiments suggests that the motile axoneme derives directly from the elongation and remodelling of the immotile CLR. Both the CLR and elongating spermatid flagella have incomplete C-tubules that form longitudinal sheets associated with the B-tubule wall, unlike axonemes of other organisms in which C-tubules stop growing at the transition between the basal body and the axonemal doublets. Moreover, both the CLR and spermatid flagella lack a structured transition zone, a characteristic feature of ciliated cells. Uncoordinated (unc) mutants that lack C-remnants have short centrioles, suggesting that the C-sheets play a role in the elongation of the centriole after it docks to the cell membrane. The structural similarities between CLR and sperm axoneme suggest that the CLR can be considered as the basal region of the future axoneme and could represent the start point for its elongation.

The Drosophila centriole: conversion of doublets to triplets within the stem cell niche

“…It has been suggested that the evolutionary pressure applied to the ciliary and flagellar axoneme might affect the architecture of the basal bodies (Carvalho-Santos et al, 2011). Thus, the structural differences we observed might be explained by the special role of the germline centrioles to nucleate the cilium-like projections of the primary spermatocytes, which at the onset of spermatid differentiation elongate in the sperm axoneme (Gottardo et al, 2013;Basiri et al, 2014). However, the female germ cells also have centrioles with triplets (Mahowald and Strassheim, 1970) but lack ciliary projections.…”

“…Moreover, the presence of the C-tubule seems to be dispensable in Drosophila for the proper assembly of an axoneme since the sensory cilia of auditory and olfactory sensilla are assembled from basal bodies composed of only doublets. However, when the C-tubule dynamics is defective in unc Drosophila mutants the growth of centrioles and axonemes is severely affected (Gottardo et al, 2013). Thus, at least during Drosophila male gametogenesis, the C-tubule seems to be essential for the proper dynamics of centrioles and to the correct assembly of the ciliary projections.…”

“…Testes dissected from pupae were fixed overnight in 2.5% glutaraldehyde in PBS at 4°C and processed as described previously (Gottardo et al, 2013). Ultrathin sections were stained with uranyl acetate and lead citrate, and then observed with a Tecnai G2 Spirit EM (FEI) equipped with a Morada CCD camera (Olympus).…”

Procentriole assembly without centriole disengagement: a paradox of male gametogenesis

“…1a,b) (Henneguy, 1898;Meves, 1902;Friedländer and Wahrman, 1970;Wolf andTraut, 1987, Yamashiki andKawamura, 1998). Unlike most animal cells, in which the cilia are reabsorbed to release the centriole that then organizes the spindle poles, the cilium-like projections persist during insect spermatogenesis in order to later organize the sperm axoneme (Riparbelli et al, 2012;Gottardo et al, 2013). Thus, the centriole performs the double role of spindle pole organizer and axoneme constructor.…”

“…Testes isolated from pupae were fixed in 2.5% glutaraldehyde in PBS overnight at 4˚C and further processed as described previously (Gottardo et al, 2013). Thin sections were stained routinely with uranyl acetate and lead citrate, and then observed with a Tecnai Spirit EM (FEI) equipped with a Morada CCD camera (Olympus).…”