A Comparison of Genome Sizes and Thermal-Denaturation-Derived Base Composition of DNAs from Several Members of Entamoeba (Histolytica Group)

Abstract

The DNA base composition and genome size of several strains of Entamoeba that form mature 4-nucleated cysts (histolytica group) was compared. These 2 measurements of genetic potential were used to differentiate species. Significant differences in genome size were observed for E. histolytica, E. histolytica-like amebae, and E. moshkovskii. Significant differences in DNA base composition were found in strains of amebae thought to be of the same species. These data clearly indicate the existence of more than one genospecies for classical E. histolytica, E. histolytica-like amebae, and E. invadens. Members of the genus Entamoeba can be readily divided into four major morphological groups on the basis of the existence or absence of a cyst stage and by the number of nuclei in the mature cyst. One group is characterized by having uninucleated cysts, another has 4-nucleated cysts, a third has 8-nucleated cysts, and one is without any known cyst stage. Levine (1961) conveniently designates these groups as bovis, histolytica, coli, and gingivalis, respectively. Within each of these groups separation into species is especially difficult. Currently, the classification is based primarily on morphology, host specificity, and pathogenicity. This system is inadequate. There are few consistent morphological differences among members of a group other than size. Host specificity is not a particularly reliable character, and pathogenicity is a highly variable property. Additional criteria for classification must be sought. In recent years, advances in our knowledge of the biology of the Entamoeba, gained primarily through study of monoxenically and axenically cultivated organisms, gives promise that the antigenic, biochemical, metabolic, and physiological properties of the amebae can be used to develop more meaningful systems of classification as has been done for bacteria. We have been characterizing entamoebal DNA and using specific differences as a means of distinguishing members of the histolytica group, or those Entamoeba forming 4-nucleated Received for publication 1 December 1970. cysts. In our previous study (Gelderman et al., 1971) we demonstrated that three members of the group, classical Entamloeba histolytica, E. histolytica-like ameba, and E. moshkovskii were sufficiently different with respect to genome size, DNA reassociation kinetics, mole per cent guanine plus cytosine (%GC), and DNA to DNA homologies to be considered distinct species. In that work only one strain of each type was studied. In this study, the results of two parameters of genetic potential (genome size and %GC) are reported for several strains of the histolytica group. GENOSPECIES AND TAXONOMY The ideal taxonomic classification must consider the widest possible range of relatedness such as morphology, host specificity, metabolism, growth requirements, immunological characteristics, specific isozymes, and DNA composition. Many of the parameters change with differing environmental conditions, thus requiring a different part of the DNA to be expressed. However, genome size and %GC of a cell is a biological constant and any significant change in the nucleotide composition of the DNA of an organism with 107 to 108 nucleotide pairs would be lethal to the cell. The DNA base ratio has been extensively studied in bacteria and used as a taxonomic tool (Mandel, 1969). The %GC of the DNA from all bacteria studied varied from 27 to 72%(Shapiro, 1968). The range of the DNA base ratios from the whole family of Enterobacteriaceae varied by only 20% (Shapiro, 1968). The