Scanning electron microscopy of Heterocapsa pygmaea sp. nov., and evidence for polyploidy as a speciation mechanism in dinoflagellates

The thecal morphology of three isolates of the marine dinoflagellate Heterocapsa pygmaea sp. nov. are here examined by manning electron and light microscopy. The thecal tabulation of these isolates is p.p., c.p., 5′, 3a, 7″, 6c, a.s., r.s., l.a.s., l.p.s., [?a.a.s. and p.a.s.], 5′ ″ and 2″ ″ and is identical to that of H. niei and H. illdefina. The assignment of thecal plates to various series is based on interpretation of plate homologies among peridinioid genera. The above formula represents the basic pattern for Heterocapsa. The cell dimensions of four Heterocapsa species are determined; Heterocapsa pygmaea is the smallest species. Heterocapsa pygmaea differs from the next largest species, H. niei, in having approximately half the number of chromosomes and as such can be interpreted as a case of polyploidy. If so, this is the first evidence of polyploidy as a speciation mechanism in the Pyrrhophyta.