The Stellate-made crystals formation in spermatocytes is the phenotypic manifestation of a disrupted crystal-Stellate interaction in testes of Drosophila melanogaster. Stellate silencing is achieved by the piRNA pathway, but many features still remain unknown. Here we outline the important role of the crystal-Stellate modifiers. These have shed light on the piRNA pathways that defend genome integrity against transposons and other repetitive elements in the gonads. In particular, we illustrate the finding that HSP90 participates in the molecular pathways of piRNA production. This observation has relevance for the mechanisms underlying the evolutionary canalization process. 1. The Stellate-Made Crystals in Spermatocytes Are the Phenotypic Manifestation of a Disrupted crystal-Stellate Interaction in Testes of Drosophila melanogaster The history of the crystal-Stellate system started in 1961 when Meyer and collaborators discovered the presence of crystalline aggregates in primary spermatocytes of D. melanogaster X/O male testes. They also described the morphological differences between needle-shaped and star-shaped crystals [1]. In 1983, Gatti and Pimpinelli provided a detailed cytological description of the chromosome. They showed that the hll region contains the genetic determinants for normal chromosome behavior during male meiosis and for the suppression of Stellate-made crystals formation in spermatocytes [2]. This region was called the Suppressor of Stellate [Su(Ste)] locus, also referred to as crystal (cry) [3]; in this paper we use “crystal.” Afterwards, different groups established that both the morphology of the crystalline aggregates and the severity of the meiotic defects in X/O and males depend on the Stellate (Ste) locus on the chromosome [4–6]. Two regions containing clustered Stellate elements have been identified on the chromosome: 12E1 in euchromatin and h27 in heterochromatin. Stellate and crystal are both repetitive sequences and they share sequence homology [6–8]. At the molecular level, the loss of the crystal region results in the production of a testes-specific Stellate mRNA of 750 nucleotides in length. The product of this mRNA is the Stellate protein [8, 9]. In 1995 there was a fundamental discovery: the Stellate protein is the main component of the crystals in the primary spermatocytes [10] and Figure 1. Figure 1: Testes of males immunostained with anti-Stellate antibody, (a) magnification 20x; (b) magnification 40x. 2. The Regulation of the crystal-Stellate Interaction The first indication about the mechanism that regulates the
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