WIDESPREAD EXPRESSION OF THE TESTIS-DETERMINING GENE SRY IN A MARSUPIAL

Total RNA was extracted from pooled tammar fetuses and neonatal gonads and analysed for SRY expression using reverse transcription and polym erase chain reaction (RT-PCR). In the tammar, the gonadal primordium i s first apparent at day 21 of the 26.5 day gestation. By day 25 the cy toplasmic:nuclear ratio in male pre-Sertoli cells is greater than in f emale cells, but it is not until two days post-partum that seminiferou s cords are obvious with putative Leydig cells appearing in the inters titium(9). We initially tested for the presence of SRY transcripts fro m the day after the genital ridge is established (day 22 of gestation) until two days after the development of seminiferous tubules (day 4 p ost partum). Surprisingly, SRY expression was detected throughout this entire developmental period in male fetal tissue and in gonads from m ale pouch young (Fig. 1a). We therefore extended our sampling period f rom day 19 of gestation (early head-fold; similar to 20 somites) to 40 days after birth and established that in the tammar SRY is expressed from at least 6 days before the appearance of Sertoli cells until long after the immature testis has bully differentiated (Fig. 1b). Thus, e xpression of the tammar SRY gene does not result in immediate Sertoli cell differentiation.