Sexual Isolation and Genetic Diversification Among Some Strains of Crypthecodinium cohnii-like Dinoflagellates Evidence of Speciation

SYNOPSIS. Nine new isolates of Crypthecodinium-like dinoflagellates from diverse geographic locations, together with 3 established strains from Woods Hole and Puerto Rico, were analyzed for sexual compatibility by means of a complementation test using motility mutants. The results indicate that 5 of the 12 are mutually compatible and thereby represent one species. Five others are clearly reproductively isolated from this group and from each other and therefore may belong to separate species. The position of 2 other isolates remains uncertain. Only one, MC-5, is markedly distinct morphologically from all the others. It is also sympatric with one of the others, strain G, and its separation from Crypthecodinium cohnii seems therefore more fully justified than that of the other sexually isolated strains. G and MC-5 would be considered "good" species by an evolutionary biologist. The others are from widely separate geographic origins and. though still poorly characterized, all superficially resemble C. cohnii . Comparison of many characters (DNA profile, radiation response, drug sensitivity, macroalgal association, etc.) of the incompatible strains show significant disparities which are discussed in considering speciation.