DNA-BASE COMPOSITION HETEROGENEITY IN SOME AGAROPHYTES (GRACILARIALES, RHODOPHYTA) FROM MEXICO AND THE PHILIPPINES

Estimates of nuclear DNA base composition by determination of thermal denaturation temperatures (T-m) indicate guanine + cytosine (G + C) le vels of 35.4-46.8% for ten species of the Gracilariaceae, representing the genera Gracilaria and Hydropuntia. T-m values were found to be re producible with variation among most samples and replicates of less th an 1 degrees C and 2 mol%. Interspecific variation in G + C values was less than 11.4% among Gracilaria species. Calculation of intragenomic base pair composition distribution based on mid-resolution thermal de naturation (Delta 1 degrees C/min with 4s interval Delta H and dT logg ing) indicated an inverse relationship between maximum similarity valu es and taxonomic rank. Intraspecific (population level) maximum simila rity (homology) values were estimated to range from 79-90% in Gracilar ia tikvahiae (4 isolates). Interspecific values of 46-69% were found i n 13 species of Gracilaria. Nucleotide distribution similarity values for the Gracilariaceae are compared with previous information for geno me organization and complexity, genome size and karyotype patterns.