crease in the expression of male-determining genes SOX9 and AMH and increase in the female marker FOXL2 in oestrogen-treated male gonads. Only MAP3K1 expression increased in male gonads in response to oestrogen while other MAPK genes remained unaffected. This study suggests that MAP3K1 can be influenced by exogenous oestrogens during gonadal differentiation in this marsupial. © 2020 S. Karger AG, Basel Disorders of sex development are common birth abnormalities in humans that affect 1.7% of live births [Fausto-Sterling, 2000]. These disorders frequently lead to infertility or gonadal cancers and have profound effects on the individuals, their families, and society. Despite the progress made in identifying genes controlling sexual differentiation of the gonad, the aetiology of disorders of sex development in most cases remains unknown, indicating there are other genetic or environmental factors at play in gonadal differentiation. Testis determination in man, mouse, and marsupial is under primary genetic control of Abstract MAPKs affect gonadal differentiation in mice and humans, but whether this applies to all mammals is as yet unknown. Thus, we investigated MAPK expression during gonadal differentiation and after treatment with oestrogen in a distantly related mammal, the marsupial tammar wallaby, using our model of oestrogen-induced gonadal sex reversal. Highthroughput RNA-sequencing was carried out on gonads collected from developing tammar 2 days before birth to 8 days after birth to characterise MAPK and key sexual differentiation markers. Day 25 foetal testes were cultured for 120 h in control medium or medium supplemented with exogenous oestrogen and processed for RNA-seq to identify changes in gene expression in response to oestrogen. MAPK pathway genes in the tammar were highly conserved at the sequence and amino acid level with those of mice and humans. Marsupial MAP3K1 and MAP3K4 clustered together in a separate branch from eutherian mammals. There was a marked de-