A Tousled-like kinase (TLK) homolog required for male germline stem cell homeostasis in the planarian Schmidtea mediterranea.

Tousled-like kinases (TLKs) are serine/threonine kinases that are conserved across metazoan life and involved in DNA stability and cell division. Mammalian TLKs are most highly expressed in the testes, but their role in spermatogenesis has remained elusive because mutations result in embryonic lethality. Here, we identify two TLK homologs in the planarian Schmidtea mediterranea (Smed-TLK1a and Smed-TLK1b) and reveal a testis-specific requirement for Smed-TLK1a. Preferential expression of Smed-TLK1a was detected in testes of S. mediterranea hermaphrodites by whole-mount in situ hybridization, and more specifically in germline stem cells and spermatogonia by fluorescence in situ hybridization. RNA-interference disruption of Smed-TLK1a expression revealed that its function is required for development of normal testes. Detailed analyses using stage-specific genetic markers revealed that Smed-TLK1a is required for development and/or maintenance of male germline stem cells. In contrast, ovary morphology, oogenesis, and development of yolk glands were indistinguishable between control planarian hermaphrodites and Smed-TLK1a knockdowns, indicating that normal expression of this gene is not essential during development of female reproductive structures. Given the mechanisms of TLK action in other systems and phenotypic specificity to sperm development, we hypothesize that Smed-TLK1a drives changes in chromatin structure that are required for differentiation and/or maintenance of male germline stem cells.