Abstract
In birds, males are the homogametic sex (ZZ) and females are the heterogametic sex (ZW). Here, we investigate the role of chromosomal sex and germ cell competition on avian germ cell differentiation. We recently developed genetically sterile layer cockerels and hens for use as surrogate hosts for primordial germ cell (PGC) transplantation. Using in vitro propagated and cryopreserved PGCs from a pedigree Silkie broiler breed, we now demonstrate that sterile surrogate layer hosts injected with same sex PGCs have normal fertility and produced pure breed Silkie broiler offspring when directly mated to each other in Sire Dam Surrogate mating. We found that female sterile hosts carrying chromosomally male (ZZ) PGCs formed functional oocytes and eggs, which gave rise to 100% male offspring after fertilization. Unexpectedly, we also observed that chromosomally female (ZW) PGCs carried by male sterile hosts formed functional spermatozoa and produced viable offspring. These findings demonstrate that avian PGCs are not sexually restricted for functional gamete formation and provide new insights for the cryopreservation of poultry and other bird species using diploid stage germ cells.
Highlights
Germ cells from different reproductive stages can be directly transplanted into avian embryos in “windowed” eggs to form germline chimeras in a procedure with minimal welfare implications
Embryonic blood was sampled from individual day 2.5 Silkie broiler embryos and primordial germ cell (PGC) were propagated from the embryonic blood in serum-free culture conditions (Whyte et al, 2015). 27 eggs were sampled and 13 PGC cultures were successfully obtained, yielding 6 male and 7 female PGC lines; these were propagated in vitro for 3–4 weeks before being cryopreserved in aliquots of 50,000 cells for future transplantation experiments (Supplementary Table 1)
Host embryos were not sex genotyped before injections so that on average 50% of the surrogate hosts were the reverse sex of the donor PGCs
Summary
Germ cells from different reproductive stages can be directly transplanted into avian embryos in “windowed” eggs to form germline chimeras in a procedure with minimal welfare implications (reviewed in Panda and McGrew, 2021). Chromosomally female (XX) germ cells colonized the male gonads and adult testes, but could not complete meiosis which can be attributed to either the lack of Y chromosome gene products or the presence of two copies of the X chromosome (Palmer and Burgoyne, 1991). These results suggest that sex-restricted gamete differentiation was acquired in mammals during sex chromosome differentiation and vertebrate speciation
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