Nuclear DNA content variation in the freshwater red algal orders Batrachospermales and Thoreales (Florideophyceae, Nemaliophycidae)

Abstract

D.F. Kapraun, K.S. Braly and D.W. Freshwater. 2007. Nuclear DNA content variation in the freshwater red algal orders Batrachospermales and Thoreales (Florideophyceae, Nemaliophycidae). Phycologia 46: 54–62. DOI: 10.2216/06-18.1A microspectrophotometric investigation of red algae in the exclusively freshwater orders Batrachospermales and Thoreales was initiated to determine the extent of nuclear DNA content variation and to corroborate the alternation of haploid and diploid nuclear DNA contents in gametophyte and sporophyte life history stages. Partial RuBisCO subunit (rbcL) sequences were generated and included in a larger data set for phylogenetic analyses to explore possible correlations between genome size and evolution. Results of rbcL analyses reflected those of previous studies, including the monophyly of the genus Sirodotia and family Lemaneaceae, and paraphyly of Batrachospermum. Static microspectrophotometry and the DNA-localizing fluorochrome DAPI (4′,6′-diamidodino-2-phenylindole) and red blood cell (chicken erythrocytes) standard were used to estimate nuclear DNA contents for 11 species in 6 genera in the Batrachospermales and the Thoreales. Estimated nuclear DNA content values ranged from 0.23 to 3.20 pg. Both estimated genome sizes and published chromosome numbers suggest a discontinuous distribution that can be explained in terms of ancestral polyploidy events. Results of this study suggest a possible correlation between polyploidy and the expression of the Batrachospermum or Lemanea morphological phenotypes. For isolates of three species, If (fluorescence) levels in 2C nuclei in presumptive gametophytes closely approximated 50% of the 4C values in presumptive sporophytes, consistent with an alternation of ploidy levels in a sexual life history.