Identification and characterisation of chicken DMRT1 transcript isoforms

In birds, the mechanism of sex determination is unknown: it is uncertain whether the female-specific W-chromosome encodes an ovary-determining gene analogous to the mammalian testis-determining gene, or whether testis determination is dependent on a double dose of a Z-chromosome gene(s) that escapes inactivation. In addition, birds do not carry a homologue to the Sry gene and the identity of the proposed sex determining gene is unknown. Currently, the gene considered the strongest avian candidate for a 'classical' testis-determining gene is DMRT1 (doublesex and Mab-3-related transcription factor 1). DMRT1 is conserved across vertebrates and is expressed in the developing gonads of mammals, birds, reptiles and fish, with higher expression in male than in female gonads. Chicken DMRT1 is located on the Z-chromosome and is expressed at higher levels in male (ZZ) gonads than in female (ZW) gonads, both at and after the suspected point of sex determination (day 5.5/stage 28). Recently we have identified six different transcript isoforms of DMRT1 in the chicken. Sequence analysis of these cDMRT1 cDNAs revealed that they encode a combination of DM domain (a conserved zinc-finger-like DNA-binding domain) and/or a SP domain (conserved domain with unknown function). The identification of five different 5'UTR sequences indicates five different transcription initiation sites. PolyA Northern analysis shows that one transcript is expressed at higher levels in male embryonic gonad than in female embryonic gonad from day 5 to 9. Currently, a transgenic chicken with cDMRT1 has been generated and further analysis is underway.