Different transmission rates of herpesvirus thymidine kinase reporter transgenes from founder male parents and male parents of subsequent generations

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Abstract
Previously we demonstrated that lines of transgenic mice carrying the herpes simplex type 1 virus thymidine kinase (HSV1‐tk) reporter gene are malesterile. Ectopic transcription of the HSV1‐tk reporter in the testis was initiated downstream of the normal translation initiation codon and truncated proteins consistent with translational initiation at the second and third ATG codons were synthesized. Here we describe the effects on fertility (1) of converting the second and third ATG codons of the HSV1‐tk reporter to CTG codons and (2) of utilizing the HSV type 2 thymidine kinase (HSV2‐tk) reporter gene, in which the second ATG codon is located downstream of the ATP‐binding pocket of the enzyme. Both reporters were coupled to the bovine thyroglobulin promoter (bTG‐tk1α and bTG‐tk2 transgenes). The level of ectopic expression of these transgenes in the testis, relative to expression in the thyroid, was one to two orders of magnitude less than that of bTG‐tk1. Sixty percent of male founders carrying the bTG‐tk1α and bTG‐tk2 transgenes were fertile but did not transmit the transgene. In contrast, most males from subsequent generations were fertile and transmitted the transgenes at the expected frequency. This difference between founder males and male descendants is also observed with certain constructs in which the HSV1‐tk reporter is coupled to other promoters. We attribute the effect to mosaicism among male founders, leading to competition between transgenic and nontransgenic spermatozoa and/or spermatogenic precursor cells and resulting in a lack of fertilization by transgenic sperm that would successfully fertilize eggs in the absence of competition.