A Molecular Approach in the Systematics of Higher Organisms

SUMMARYThe relatedness between several nomen species of the PseudomonasXanthomonas group and some other organisms was numerically fixed through D N A homology and D N A base composition. For Pseudomonas the numerically analysed strains proposed by Lysenko as neotypes were used. The mean Yo (G+C) was in the range 60-67.5. 14C-DNA from either Fseudomonasfluorescens or P. putida was hybridized with D N A from 17 different species centres and the D N A homology was in the range 50-100 %.Genetic species differentiation in the genus Pseudomonas seems justified. In three border cases (P. iodinum, P. diminuta and P. atlantica) D N A homology was only 28-50%, so the inclusion of these organisms in the genus Pseudomonas is uncertain. The species centres P. pavonacea and F. rubescens are omitted from the genus Pseudomonas because of their very low D N A homology and aberrant D N A base composition. Twentyeight nomen species of Xanthomonas all form a narrow group in the range 63.5-69y0 (G+C). With two exceptions D N A homology with a median strain Xanthomonas pelargonii was always over 75 yo and frequently nearly complete. 14C-DNA from P. fluorescens hybridized with Xanthomomas-DNA to the same extent as with the pseudomonads proper. The Xanthomonas cluster overlapped perfectly with part of the Pseudomonas group. It is proposed therefore to gather all xanthomonads in a single genetic species P. campestris. This is such a dense cluster that the preservation of separate species names for the border cases seems undesirable. About one half to two-thirds of Pseudomonas-and Xanthomolzas-DNA is identical. The genera Rhixobium, Axotobacter and Axomonas appeared to be rather closely related to Pseudomonas since they shared some 40-50y0 of their DNA. The genus Serratia appeared to be more closely related to Pseudomonas than to Escherichia. The genera Gluconobacter, Acetobacter, Serratia and Escherichia shared some 20-30y0 D N A with Pseudomonas, but Bacillus-DNA was almost entirely different. From a comparison between D N A homology and taximetric similarity, it appeared that most pseudomonads would not contain unused genes. The advantages of a classification based on % (G+C) and D N A homology are obvious.

Section: Discussionmentioning

confidence: 99%

DNA Homology and Taxonomy of Pseudomonas and Xanthomonas

Ley¹,

Park²,

Tijtgat³

et al. 1966

“…Filters with immobilized DNA were preincubated in the medium of Denhardt (I 966) for 2 to 6 h. at the temperature to be used for hybridization, drained and blotted, and then placed in vials which contained I pg, homologous or heterologous labelled DNA fragments in twofold concentrated SSC. For competition experiments (Hoyer, McCarthy & Bolton, 1964), a large excess of unlabelled DNA fragments was added to the reaction mixture (final volume of 0.5 ml.) to compete with labelled DNA fragments for binding sites on the immobilized DNA.…”

Section: Methodsmentioning

confidence: 99%

Division of Mycoplasmas into Subgroups

Neimark

1970

S U M M A R YThe deoxyribonucleic acid (DNA) base compositions of 12 recently isolated or new species of mycoplasmas fell within the range known for the Mycoplasma group. By DNA-DNA hybridization a new sterol-nonrequiring strain, S-743, was unrelated to Mycoplasma laidlawii or M . granularum, the latter two strains showing moderate cross-reactivity ; swine strain B 3 appeared to be related to M . mycoides var. Capri. From these results and information in the literature, the mycoplasmas have been divided into six provisional subgroups on the basis of physiological characters, DNA base composition and nucleic acid hybridization. Some speculations are made on the significance of heterogeneity among mycoplasmas.

“…Percentage of DNA relatedness Direct hybridization is not the best method for the determination of the percentage DNA relatedness (Hoyer, McCarthy & Bolton, 1964), and it was not our explicit purpose to measure it. The calculation of a high or moderate percentage DNA relatedness between individual strains relative to a reference strain is not meaningful in direct hybridization because of the high standard deviation of about 20%.…”

Section: Stability Of Molecular Hybrids Formed At 42 "Cmentioning

confidence: 99%

Thermal Stability of DNA: DNA Hybrids within the Genus Agrobacterium

Ley

Tijtgat

Smedt

et al. 1973

S U M M A R YMolecular hybrids were prepared between unlabelled DNA from representative strains of eleven genetic races of Agrobacterium and [I4C]DNA from typical strains of each of the three main races. The thermal stability of each hybrid was determined. The nature of the hybrids formed varied with the incubation temperature and the kind of DNA used. Hybridization in 2 x SSC-30 % dimethylsulphoxide below 59 "C yielded two kinds of hybrids: a labile one of unknown nature, denaturing below 59 "C, and a more or less stable hybrid denaturing above that temperature. The latter was the only one formed in hybridizations at or above 59 "C. There were three kinds of stable hybrids. Within each of the main Agrobacterium races thermal stability of the molecular hybrid was about the same (within 2 "C) as for the homoduplex. Between two races of 50 % DNA relatedness, the duplexes were about 6 "C less stable. Between races of 10 to I 5 % DNA relatedness, the duplexes were weak, and the stability was at least 13 "C lower. The stability of the hybrids decreased concomitantly with the degree of DNA relatedness. The decreased hybrid denaturation curve is not due to AT-rich sequences. The less two races of agrobacteria appeared to be evolutionarily related, the more mutations occurred within the common part. I N T R O D U C T I O NFrom previous studies it appeared that the genus Agrobacterium is genetically very heterogeneous. DNA of cluster I (typical Agrobacterium tumefaciens and A. radiobacter strains), cluster 2 ( A . rhizogenes and atypical A . tumefaciens strains), the 'rubi' group and two very small groups hybridize at about 10 to 15 %. Within cluster I the DNA of the seven groups hybridize at about 50 % and within each group at least 80 % ( Mutational events modified considerable parts of the genomes, preventing in vitro molecular hybridization. Considering the evolutionary history of a bacterial genus, one can wonder whether mutations also occurred within the common DNA parts. For an experimental answer to that question we prepared a number of DNA-hybrids between the genomes of different Agrobacterium groups and clusters, and determined their thermal stability which is interpreted as a measure of base-pairing imperfections, and mutational differences (Brenner & Cowie, 1967). We established that decreased hybrid denaturation curves were not due to preferential binding of heterologous AT-rich DNA sequences.