Reproductive compatibility among four global populations of the toxic dinoflagellate Gymnodinium catenatum (Dinophyceae)

Reproductive compatibility was examined among 21 strains of Gymnodinium catenatum derived from four different populations from across the globe: Tasmania, Australia (15 strains), Japan (2 strains), Spain (2 strains) and Portugal (2 strains). Pairwise crossing of strains demonstrated extensive intrapopulation compatibility (to resting cyst formation) among all four populations. The observations were most consistent with a heterothallic. multiple-group mating system, requiring more than two groups to explain the pairwise crossing data. Despite the ability of strains from different populations to produce resting cysts, the viability of progeny was highly variable among interpopulation crosses. Cysts from all crosses showed a high germination percentage (93-100%) and released a swimming planomeiocyte. Crosses between different Tasmanian strains, and those between Spanish and Japanese strains, showed high post-meiotic viability (65% and 80%, respectively). However, progeny from Tasmanian-Spanish and Tasmanian-Japanese crosses showed very low post-meiotic viability (5-10%), indicating a higher level of somatic incompatibility between these populations. Significant differences in sexual life-history (e.g. rate of gamete formation and cyst dormancy) were also noted between interpopulation crosses, suggesting genetically determined strain- and population-level differences. The crossing data indicate a high level of mating diversity within the Australian population and show that the Japanese and Spanish populations are more closely related to each other than to Australian populations; this is supported by molecular studies. Implications for the proposed global dispersal of G. catenatum and the use of interbreeding to examine population relationships are discussed. New measures are proposed for examining strain (RC,) and population (RC,) levels of reproductive compatibility, respectively, which are calculated as the product of proportion of successful matings (termed the compatibility index) and the number of cysts produced (average vigour) in successful crosses.