Biochemical Taxonomy of Marine Phytoplankton by Electrophoresis of Enzymes. Ii. Loss of Heterozygosity in Clonal Cultures of the Centric Diatoms Skeletonema Costatum and Thalassiosira Pseudonana1, 2

An electrophoretic survey of 12 new isolates of Thalassiosira pseudonana Hasle & Heimdal and 25 new isolates of Skeletonema costatum (Grev.) Cleve revealed several heterozygote genotypes at malate dehydrogenase (MDH) and phosphohexose isomerase (PHI) loci. The new clones were maintained in culture for 6 mo and then reasayed at these two loci. All MDH heterozygotes and halj of the PHI heterozygotes had become homozygous. This resulted in a collection of clones that are largely homozygous but that are samples of polymorphic species. The physlogical implications of this loss of heterozygosity in clonal cultures has not been analyzed. Hawever, any change in a clone that is the result of culturing conditions reduces the usefulness of that clone as a laboratory test organism for ecological correlations.