Effects of maternal androgens and their metabolite etiocholanolone on prenatal development in birds.

Offspring phenotypes can be affected by maternal testosterone (T) and androstenedione (A4), which are considered a tool of mothers to adjust offspring in a fluctuating environment. Yet T and A4 are found to be very rapidly metabolized by developing avian embryos, suggesting that either the maternal T and A4 have potent organizational effects on the embryos extremely early before being metabolized or it's the metabolites that evoke phenotypic variation in the offspring. One of the metabolites, etiocholanolone, increases substantially during early embryonic development and is a likely candidate for mediating maternal effects as it can promote erythropoiesis. To investigate and compare the effects of T and A4 with the possible effects of etiocholanolone during prenatal embryonic development, we increased their levels in Black-headed gull eggs (Larus ridibundus), and used sham-injected eggs as controls. This species usually has 3-egg clutches in which maternal androgen levels increase with the egg-laying sequence. We analyzed embryonic heart rates, peri-hatching biometric traits, the ratio of white to red blood cells (W/R), and bursa development. We found that T and A4 treatment increased embryonic heart rate irrespective of egg-laying sequence, decreased bill length and W/R, whereas etiocholanolone did not mimic these effects. Instead, etiocholanolone treatment decreased tarsus length and brain mass. Our finding that etiocholanolone does not mimic the effects induced by T and A4 suggests that the embryonic metabolism of maternal T and A4 can potentially diversify the function of these maternal androgens.