Xenogenesis is an innovative technology for hybrid catfish (9 channel catfish, Ictalurus punctatus, x male blue catfish, I. furcatus) embryo production. The xenogeneic process can be accomplished by transplanting undif-ferentiated diploid germline stem cells derived from donor fish into sterile recipients. This methodology enables recipients to produce donor-derived gametes. Until recently, the timing of transplantation of donor cells into hosts was done with limited knowledge of the best age to inject cells. The age of the host could critically affect the success of germ cell transplantation. The present study aimed to identify the best age of the triploid channel catfish to transplant blue catfish stem cells for production of xenogeneic catfish. Triploid channel catfish fry were injected with blue catfish stem cells labeled with PKH26 dye from 0 to 18 days post-hatch (DPH). Then at 50 DPH (1st time interval) and 90 DPH (2nd time interval), total length (TL), weight (BW), and survival of recipients were evaluated. Colonization of donor cells was evaluated in recipients using PKH26 dye fluorescence to calculate percent cell (<150 mu m2) and cluster areas (>150 mu m2). PCR determined the percentage of xenogens from gonads. Day of stem cell injection had no impact on TL and BW of recipient fish when evaluated at both sampling intervals. Survival of recipients injected with blue catfish stem cells increased from 0 to 5.4 DPH. After 5.4 DPH, survival remained high (>= 82%) for fry injected until 18 DPH. At the 1st time interval, cell and cluster area increased as recipients fish injected from 0 to 5.4 DPH and 0 to 5.6 DPH, respectively. Thereafter, fluo-rescent cell and cluster area in the host declined with no further decrease after 11.3 and 10.4 DPH, respectively. At the 2nd time interval, cell and cluster area increased as recipients were injected from 0 to 5.8 DPH and 0 to 5.7 DPH and significantly declined with no further decrease after 10.2 and 11.3 DPH, respectively. At the 1st time interval, the highest percentage of xenogens were detected when recipients were injected from 3 to 5 DPH (83.3%), while at the 2nd time interval, the highest percentage of xenogens was detected from 4 to 6 DPH (83.3%). Our results show that 4 to 6 DPH is a suitable timespan to inject donor-derived stem cells into re-cipients. These findings will enhance the efficiency of germ cell transplantation for commercial-scale hybrid catfish production.
