Effect of progesterone on sexual reproduction of Brachionus manjavacas (Rotifera)

Using hormones to regulate reproduction is a common theme among animals, but little is known about the hormones of most invertebrates. The use of oxidized sterols as hormones is widespread, as is binding to specific nuclear receptors. As part of a transcriptome sequencing project, we discovered a receptor similar to the vertebrate progesterone receptor in the rotifer Brachionus manjavacas. This motivated us to investigate the effects of progesterone on various aspects of rotifer reproduction, including the switch from asexual to sexual reproduction, male production and behavior, resting egg production and hatching, and comparative species sensitivity. Progesterone had no significant effect on asexual population growth rate (r) of B. manjavacas up to 10 mg/L, however, at 14 mg/L r turned strongly negative. At 5 mg progesterone/L, resting egg production was 3.9X greater than in the control and at 10 mg/L resting egg production was 4.4X higher. Resting egg production was completely inhibited at 14 mg progesterone/L. The higher resting egg production at 10 mg/L was accomplished by a 2.2X higher rate of sexual reproduction than the control. The number of males and resting eggs produced per mictic female were not significantly different at 10 mg progesterone/L. The progesterone antagonist mifepristone at 10 mg/L had no significant effect on resting egg production, sexual reproduction rate, and males or resting eggs produced per mictic female. Exposure of males to concentrations of progesterone up to 10 mg /L for one hour had no significant effect on their circling behavior, however, exposure to 4 mg/L increased male copulation by 2.1X over the control. When females were exposed to 4 mg/L progesterone during resting egg formation, a 1.8X greater percentage of their resting eggs hatched. The enhancement of resting egg production by progesterone exposure was greater in B. manjavacas (7.8X higher than control) than B. ibericus (3.9X higher). Progesterone exposure did not enhance resting egg production in B. plicatilis or B. calyciflorus.