Meeting Abstract
Diadema setosum and D. savignyi are sympatric throughout much of the Tropical West Pacific and Indian Oceans and exhibit no spatial isolation on coral reefs. Diadema savignyi typically spawns on the full moon and D. setosum around the time of the new moon, the out of phase spawning rhythms functioning as a temporal reproductive barrier to hybridization. In the wild hybrids are rare. However, artificial cross-fertilization experiments have produced viable hybrids at equal rates and sperm concentrations relative to intraspecific crosses. This suggests that temporal reproductive isolation is important for maintaining specific integrity. In this investigation we assessed temporal expression profiles of mRNAs from cryptochromes (Cry) (which include members that are blue-light photoreceptors and can detect moonlight), and timeless genes (tim), which are involved in regulating the circadian clock. Expression levels of mRNAs were analyzed in the spine-base skin and muscle, tubefeet, and gonads from D. savignyi and D. setosum collected from their natural habitats, every 6 hrs on the moons first quarter, full moon, third quarter and new moon, over one lunar month. In D. savignyi, dCry1,vCry1-lt, Cry-D and tim2 were significantly up-regulated in all three tissues on the full moon, with greatest significant up-regulation of vCry1-lt at 18:00 hrs and dCry1, Cry-D and tim2 at midnight. Such up-regulation is likely to result in increased sensitivity to moonlight, with increased lunar radiance acting as a stimulus for spawning. In D. setosum, greatest significant up-regulation of dCry1, vCry1-lt and tim2 was recorded at 06:00 hrs on the new moon in all three tissues. This is when the most abrupt change in illumination occurs during the night/dawn transition and is likely to influence spawning time in D. setosum. Both dCry1 and vCry1-lt were found to have regions under positive selection, and are strong candidate genes for regulatory control of lunar spawning in sympatric Diadema.
