Digoxigenin (DIG)-labeled RNA probes used for in situ hybridization were synthesized by in vitro transcription with a DIG RNA labeling kit (Roche Diagnostics) using a 244-bp fragment (position 528–771 bp) of the crab Dmrt cDNA subcloned into pGEM-T Easy vectors (Promega) as a template. The transcriptions were performed from 1 mg of linearized plasmid using either T7 (antisense) or SP6 (sense) RNA polymerases. In situ hybridization was performed essentially as described previously (Qiu et al. 2008). Paraformaldehyde-fixed ovary samples were routinely embedded in paraffin, sectioned (5 μm), de-paraffinized, and then rehydrated. Gonad tissue sections were hybridized with DIGlabeled sense or antisense RNA probes, and hybridization signals were detected with alkaline phosphatase-conjugated anti-DIG antibody using a DIG Nucleic Acid Detection Kit
(Roche Diagnostics).