Morphology, phylogeny and life cycle of Fragilidium mexicanum Balech (Gonyaulacales, Dinophyceae)

Abstract

We describe the life cycle, morphology and molecular phylogenetic position of Fragilidium mexicanum, based on vegetative cells established by incubation of resting cysts isolated from sediment trap samples collected at Jinhae-Masan Bay, Korea. Resting cysts of F. mexicanum were spherical and surrounded by transparent mucilage, similar to other Fragilidium species. Germinated vegetative cells were identified as F. mexicanum based on thecal morphology and tabulation; the plate formula was Po, Pc, 4', 8 '',10c, 6s, 7''', 1p, 2 ''''. Molecular phylogenetic analyses of sequence data for small and large subunits of ribosomal DNA genes revealed that Fragilidium species were monophyletic with high supports and that F. mexicanum formed a separate Glade with Fragilidium sp. (JQ317599). Time-lapse photography was used to clarify the life history. Isolated resting cysts germinated into an unarmoured planomeiocyte and directly formed the pellicle cyst, with an armoured planomeiocyte emerging from the pellicle cyst. The armoured planomeiocyte underwent ecdysis and formed a coccoid meiocyte, with a two-celled chain emerging from the coccoid meiocyte. Like the armoured planomeicyte, vegetative cells also underwent ecdysis and then divided into two daughter cells that eventually emerged as motile vegetative cells. The life cycle of F. mexicanum has nonmotile life history stages that regularly alternate with motile stages. The nonmotile stage consists of five forms: resting cyst, pellicle cyst, asexual coccoid stage, coccoid meiocyte, and zygote. The motile stage also has five forms: unarmoured and armoured planomeiocyte, planozygote, gamete and vegetative cell.