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

Abstract

The dinoflagellate genus Fragilidium has rarely been reported in taxonomic checklists. We describe the life cycle, morphology and molecular phylogenetic position of F. mexicanum, based on vegetative cells established by incubation of resting cysts isolated from sediment trap samples collected at Jinhae-Masan Bay, South Korea. Resting cysts of F. mexicanum are 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", 1 Oc, 6s, 7"', 1 p, 2"". Molecular phylogenetic analyses of sequences data for small and large subunits of ribosomal DNA genes (SSU and LSU rDNA) revealed that Fragilidium species were monophyletic with high supports and that F. mexicanum formed a separate clade with Fragilidium sp.(JQ317599). Time-lapse photography was used to clarify the life history. Isolated resting cysts germinated into an unarmored planomeiocyte and directly formed the pellicle cyst, with armoured planomeiocyte emerging from the pellicle cyst. The armored planomeiocyte underwent ecdysis and formed a coccoid meiocyte, with atwo-celled chain emerging from the coccoid meiocyte. Like the armoured planomeicyte, vegetative cells also underwent ecdysis (asexual coccoid stage) and then divided into two daughter cells that eventually emerged as motile vegetative cell