The growth of the ciliary axonemes mainly depends on the evolutionary conserved intraflagellar transport (IFT) machinery. However, insect spermatocytes are characterized by cilium-like regions (CLRs) that elongate in the absence of IFT. It is generally believed that the dynamics of these structures relies on the free diffusion of soluble tubulin from the cytoplasm. However, this passive process could allow the elongation of short ciliary axonemes, but it is unclear whether simple diffusion of tubulin molecules can ensure the correct assembly of elongated ciliary structures. To decipher this point we analyzed the assembly of the CLRs held by the primary spermatocytes of Drosophila bifurca. These ciliary structures consist of a very elongated axoneme that grows without IFT and, therefore, could represent a good model in which to evaluate the role played by the free diffusion of soluble tubulin. The observation of wavy microtubules in the axonemal lumen of fully elongated CLRs of D. bifurca may be consistent with the diffusion of tubulin within the axonemal lumen. Progressive consumption of soluble tubulin used for axoneme growth at the apical tip of the CLRs could result in a gradient sufficient to move tubulin from the cytoplasm to the apical end of the forming ciliary structure. When the axoneme reaches its full length, tubulin molecules are not drawn to the tip of the CLRs and accumulate at the base of the axoneme, where its concentration may exceed the threshold need for microtubule polymerization. The presence of γ-TuRCs at the proximal ends of the supernumerary microtubules could enhance their nucleation.

中文翻译：

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果蝇二叉精母细胞的纤毛样区域：无鞭毛内运输的巨大轴丝的伸长

纤毛轴丝的生长主要依赖于进化保守的鞭毛内运输（IFT）机制。然而，昆虫精母细胞的特征是纤毛样区域（CLR）在没有 IFT 的情况下会伸长。人们普遍认为这些结构的动力学依赖于可溶性微管蛋白从细胞质的自由扩散。然而，这种被动过程可能允许短纤毛轴丝的伸长，但尚不清楚微管蛋白分子的简单扩散是否可以确保伸长的纤毛结构的正确组装。为了解释这一点，我们分析了果蝇二叉果蝇初级精母细胞所持有的 CLR 的组装。这些纤毛结构由非常细长的轴丝组成，无需 IFT 即可生长，因此可以代表一个很好的模型，用于评估可溶性微管蛋白自由扩散所起的作用。在D. bifurca完全伸长的 CLR 的轴丝管腔中观察到的波状微管可能与微管蛋白在轴丝管腔内的扩散一致。用于 CLR 顶端轴丝生长的可溶性微管蛋白的逐渐消耗可能会产生足以将微管蛋白从细胞质移动到形成纤毛结构顶端的梯度。当轴丝达到其全长时，微管蛋白分子不会被吸引到 CLR 的尖端，而是积聚在轴丝的基部，其浓度可能超过微管聚合所需的阈值。多余微管近端的 γ-TuRC 的存在可以增强其成核。