High heterozygosity and phenotypic variation of zoids in apomictic Ulva prolifera (Ulvophyceae) from brackish environments

• hsp90 gene analyses revealed 14 genotypes in Ulva prolifera from brackish habitats.
• 99% of the specimens examined were heterozygous.
• The cell features and phototactic behavior of zoids differed between the genotypes.
• The parthenogenetic thallus had the same genotype and zoid features to the parents.
• These data suggest clonal apomictic reproduction of this species.

Apomictic reproduction without alternation of generations has been reported in Ulva prolifera Müller from some brackish environments, and few genetic and phenotypic variations are expected in such apomictic populations. We collected 534 specimens of U. prolifera throughout the year from brackish lakes and a river and genotyped all the samples using a nuclear-encoded hsp90 gene sequence. In total, 14 genotypes were detected, 12 of which were heterozygous (n = 531), and the genotype composition differed between the lakes and estuaries as well as between the high and low brackish sites. We tried to induce discharge of zoids from the collected thalli, and the phenotypes of zoids were examined in total 309 specimens. The flagellum number of the zoids differed among the eight most frequently encountered genotypes, i.e., six genotypes released only biflagellate zoids, whereas the other two produced quadriflagellate zoids. The ratio of specimens that produced positively phototactic zoids, which varied among the genotypes regardless of the flagellum number, was relatively high in the high-salinity brackish regions. The parthenogenetic thallus had the same genotype, flagellum number, and direction of phototaxis relative to the parental thallus in all the genotypes examined, which suggested clonal apomictic reproduction. Apomixis induction through outcrossing between genetically different entities has been reported in other algae and this may have occurred many times in U. prolifera.