National Avian Research Facility
National Avian Research Facility

iCaspase9 surrogate host

Conditionally sterile male and female surrogate host chickens

Summary & Utility

In the iCaspase9 surrogate host line the germ cell lineage of both males and females can be ablated after the introduction of a chemical compound1. These conditionally sterile transgenic chickens provide a major advantage for creating genetically altered (GA) chicken lines. GA primordial germ cells (PGCs) or PGCs from other chicken breeds can be introduced into sterile male and female iCaspase9 embryos. Once the iCaspase9 birds reach sexual maturity, mating between male and female iCaspase9 birds or Sire Dam Surrogate mating results in 100% transmission of the introduced PGCs in the first generation of offspring. The iCaspase9 surrogate hosts cut the time required to create GA chicken lines, and reduces the number of animals required to produce GA chicken lines2. The iCaspase9 surrogate host line has been used to create GA chickens with altered feather traits1 and birds with targeted mutations in the DMRT1 gene to investigate avian sex determination3.  Recently, the iCaspase9 surrogate host line has been used to demonstrate that chicks that have been derived entirely from cryopreserved donor germ cells can be successfully hatched4. This advances our ability to cryopreserve the genetic diversity of both commercial and research chicken lines4.


iCaspase9 schematic
Schematic outlining the production of genetically edited offspring using iCaspase9 Sire Dam Surrogate mating (Ballantyne et al. 2021).

Line origin

The creation of the iCaspase9 line was led by Dr Mike McGrew and funded by Cobb-Europe (Innovate UK Agri-Tech funding - BB/M011895/1) and the BBSRC (grant numbers: BB/P013732/1 and BB/P013759/1). Cobb-Europe holds the commercial rights for this line. The line was generated at the Roslin Institute and originally described by Ballantyne et al. (2021)1; “The inducible caspase-9 (iCaspase9) protein consists of a truncated human caspase-9 protein fused to the FK506-binding protein (FKBP) drug-dependent dimerisation domain5. The chemical compound, AP20187 (B/B), induces the dimerisation of FKBP and subsequent activation of the adjoining caspase-9 protein leading to induced apoptotic cell death. To express the iCaspase9 gene in the germ cell lineage, CRISPR/Cas9-mediated homology-directed repair was used to target the iCaspase9 construct to the 3′-end of the last coding exon of the DAZL gene in PGCs. The chicken DAZL gene is a putative avian germ cell determinant and is highly expressed in migratory PGCs and germ cells in the embryonic gonad6,7”.

DAZL insert
“CRISPR/Cas9-mediated recombination of an iCaspase9 and GFP reporter gene into the final coding exon of the DAZL locus. Red arrow indicates the guide target” from Ballantyne et al. (2021).


For publications please reference; Ballantyne, M., Woodcock, M., Doddamani, D., Hu, T., Taylor, L., Hawken, R. J., & McGrew, M. J. (2021). Direct allele introgression into pure chicken breeds using Sire Dam Surrogate (SDS) mating. Nature communications, 12(1), 1-10.



  1. Ballantyne, M. et al. Direct allele introgression into pure chicken breeds using Sire Dam Surrogate (SDS) mating. Nat. Commun. 12, 659 (2021).
  2. Panda, S. K. & McGrew, M. J. Genome editing of avian species: implications for animal use and welfare. Lab. Anim. 002367722199840 (2021). doi:10.1177/0023677221998400
  3.  Ioannidis, J. et al. Primary sex determination in birds depends on DMRT1 dosage, but gonadal sex does not determine adult secondary sex characteristics. Proc. Natl. Acad. Sci. 118, e2020909118 (2021).
  4.  Hu, T. et al. Direct cryopreservation of poultry/avian embryonic reproductive cells: A low-tech, cost-effective and efficient method for safeguarding genetic diversity. Elife 11, e74036, (2022). 
  5. Straathof, K. C. et al. An inducible caspase 9 safety switch for T-cell therapy. Blood 105, 4247–4254 (2005).
  6. Lee, H. C. et al. DAZL expression explains origin and central formation of primordial germ cells in chickens. Stem Cells Dev. 25, 68–79 (2016).
  7. Kito, G. et al. Temporal and spatial differential expression of chicken germlinespecific proteins cDAZL, CDH and CVH during gametogenesis. J. Reprod. Dev. 56, 341–346 (2010).