Identification of Genes Involved in Transmission Ratio Distortion and Male Fertility in Mice

Abstract

Spermatozoa are considered phenotypically equivalent because, during spermatogenesis, their precursor cells are organized in a syncytium where they share mRNA through intercellular bridges. This assumed phenotypic equivalence is challenged by a selfish genetic element encoded by the t-haplotype. The mouse t-haplotype is a naturally occurring variant of mouse chromosome 17 that shows high transmission from heterozygous (t/+) males due to a poison/antidote mechanism. t-Distorter genes exert detrimental effects on all sperm of these males. The t-responder, which can counterbalance these effects, remains restricted to the t-sperm, leading to their selective rescue and transmission ratio distortion (TRD) in favor of the t-chromosome. I first investigated whether energy metabolism contributes to the altered flagellar function, causative for t-haplotype TRD via the glycolysis enzyme phosphoglycerate kinase 2 (Pgk2). Pgk2 is overexpressed in mice with t-chromosomes. I generated male mutants carrying a t-specific Pgk2 knockout allele (tw5Pgk2∆/+) and showed that sperm of mutant tw5Pgk2∆/+ mice have increased progressivity, which suggests an improvement of wild-type sperm performance (which in t/+ are compromised by t-distorters). An important goal of this work was to evaluate whether TRD is a more widespread phenomenon and whether other genes, independent of the t-haplotype, can also cause TRD. Four candidate genes, Spata45, Tex29, Tex46, and Txndc2, were selected based on two criteria: late expression during spermatogenesis and minimal sharing among haploid spermatids. I generated reporter mouse lines of each gene and analyzed the distribution of their transcripts in the seminiferous tubules. Tex29 was shown to be restricted to the haploid spermatid encoding it. I then generated a Tex29 heterozygous loss-of-function mutant and showed that the transmission of the mutant allele was not affected. In addition, to study the function of Spata45, Tex29 and Tex46. I generated homozygous knockout mutants of these genes and examined whether they exhibit sterility or subfertility phenotypes. Spata45-/- and Tex29-/- males showed no fertility defects. However, Tex46-/- males are sterile demonstrating that Tex46 is essential for male fertility. Tex46-/- mice have normal testes, typical seminiferous tubule structures and can produce spermatozoa. However, the morphology of the sperm head reveals malformations affecting the arrangement of the plasma membrane, the internal and external acrosomal membranes, and the apical hook. Although Tex46-/- sperm cannot fertilize oocytes with zona pellucida, they successfully fertilize zona pellucida-free oocytes. It is proposed here that Tex46-/- sperm have an impaired acrosome reaction and are therefore unable to cross the zona pellucida. In this work a novel gene has been identified, that is critical for male fertility in mice, and may also play an important role in human male fertility.