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 dinoflagellateHeterocapsa pygmaea sp. nov. are here examined by manning electronand light microscopy. The thecal tabulation of these isolatesis 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 thatof H. niei and H. illdefina. The assignment of thecal platesto various series is based on interpretation of plate homologiesamong peridinioid genera. The above formula represents the basicpattern for Heterocapsa. The cell dimensions of four Heterocapsaspecies are determined; Heterocapsa pygmaea is the smallestspecies. Heterocapsa pygmaea differs from the next largest species,H. niei, in having approximately half the number of chromosomesand as such can be interpreted as a case of polyploidy. If so,this is the first evidence of polyploidy as a speciation mechanismin the Pyrrhophyta. ¹Present address: Department of Botany, Duke University, DurhamNorth Carolina 27706 ²Present address: The Biological Laboratories, Harvard University,Cambridge, Massachusetts 02138