Gem formation upon constitutive Gemin3 overexpression in Drosophila

Gems or ‘Gemini of Cajal bodies’ are spherical nuclear aggregates of SMN (survival of motor neurons) complexes that frequently overlap Cajal bodies. Although described and characterized in mammalian tissues, gems have not been reported in invertebrates. Stimulation of gem formation in the fruitfly Drosophila melanogaster was investigated through the constitutive overexpression of a fluorescently tagged transgene of a DEAD-box SMN complex member, Gemin3, in wild-type tissues. Although expression was predominantly cytoplasmic in the larval brain cells, Gemin3 was found enriched in multiple discrete bright foci in the nuclei of several tissues including epidermis, muscle and gut. Similar to their mammalian counterparts, Drosophila gems contained endogenous SMN and at times overlapped with Cajal bodies. These findings support the hypothesis that gems are storage sites for excess nuclear SMN complexes and their frequent association with Cajal bodies might imply recruitment for nuclear ribonucleoprotein assembly reactions.