View Program - 63rd Annual Drosophila Research Conference

During gametogenesis, male and female germ cells develop as syncytia connected by intercellular bridges that facilitate cell-cell communication and the transport of molecules and organelles between sister cells. Functional studies demonstrate the requirement of germline bridges for fertility, yet the mechanism underlying their formation is still unclear. Immunofluorescence studies of fixed Drosophila tissues have suggested that these bridges, otherwise known as ring canals, arise from arrested constriction of the contractile ring. In contrast, studies in the mouse testis have suggested that germline bridges are derived from the midbody matrix. These observations, combined with the compositional differences between Drosophila male and female ring canals and mouse and Drosophila male ring canals, raise the question of whether there is one underlying, conserved mechanism of ring canal formation. To investigate how ring canals are formed, we performed time-lapse imaging of the conserved ring canal component Pavarotti/MKLP1/kinesin-6 during the incomplete mitotic divisions of the male and female Drosophila germlines. In both testes and ovaries, rather than contractile ring arrest, contractile rings constrict completely to form a dense, transient midbody intermediate that reorganizes into an open ring canal in less than one hour. The midbody-to-ring canal transition appears to be a conserved feature of gametogenesis as we also observed kinesin-6-labeled midbodies in fixed preparations of mouse and Hydra vulgaris male germlines.

To further characterize the midbody-to-ring canal transition, we imaged known midbody ring components in the Drosophila testis. We observed that the germline midbody is compositionally similar to, but functionally distinct from, the midbodies formed during complete cytokinesis. Known midbody ring components Septin 2 and Citron kinase/Sticky localize to the germline midbody, but as expected, germline midbodies fail to initiate microtubule severing and abscission. Interestingly, in addition to the midbody ring localization, Sticky localizes to a subset of ring canals, presumably nascent ring canals, but not mature ring canals suggesting a possible role in the midbody-to-ring canal transition. In fact, knockdown of sticky in the male germline (nos>sticky RNAi) results in persistent midbody-like foci. We are now working to understand the precise role of Sticky during the midbody-to-ring canal transition and identify additional components required.

Evolutionarily conserved midbody reorganization precedes ring canal formation during gametogenesis