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 polymerase chain reaction (RT-PCR). In the tammar, the gonadal primordium is first apparent at day 21 of the 26.5 day gestation. By day 25 the cytoplasmic:nuclear ratio in male pre-Sertoli cells is greater than in female cells, but it is not until two days post-partum that seminiferous cords are obvious with putative Leydig cells appearing in the interstitium(9). We initially tested for the presence of SRY transcripts from the day after the genital ridge is established (day 22 of gestation) until two days after the development of seminiferous tubules (day 4 post partum). Surprisingly, SRY expression was detected throughout this entire developmental period in male fetal tissue and in gonads from male pouch young (Fig. 1a). We therefore extended our sampling period from 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, expression of the tammar SRY gene does not result in immediate Sertoli cell differentiation.