Sexual reproduction and two different encystment strategies of Lingulodinium polyedrum (Dinophyceae) in culture

Abstract

Unreported aspects in the sexual cycle of the marine dinoflagellate Lingulodinium polyedrum (Stein) Dodge were described. Our observations included the description of two types of hypnozygote formation, because culture planozygotes were observed to encyst in two different ways: an ecdysal sexual stage or a spiny resting cyst. Phosphate deficiency was the main nutritional condition required for fusing gamete pairs to form resting cysts, whereas replete conditions prevented their appearance and favored the formation of ecdysal sexual forms. Mating experiments revealed the existence of two sexual types (+/−), which were enough to explain resting cyst appearance (simple heterothallism). Morphological aspects and timing of gamete mating, fusion, and the efficiency of encystment under different external levels of nitrate and phosphate were analyzed after isolating and monitoring individual pairs of fusing gametes. The staining of sexual stages showed that nuclear fusion was completed at the same time as the cytoplasmic fusion. After 1 to 2 h, the planozygotes presented one quadrolobulated nucleus. Germination of ecdysal sexual stages occurred after <24–72 h, whereas excystment of resting cysts was dependent on the studied parental cross and took place after 2–4 months. Newly germinated cells from both types of cysts had a similar, big, U-shaped nucleus. Twenty-four to 48 h after excystment, the germlings divided by desmoschisis, a process before which enlargement of the nucleus was observed.