_____________Mun. Ent. Zool. Vol. 5, No. 2, June 2010__________ 772 MOLECULAR IDENTIFICATION OF IRANIAN ISOLATES OF THE GENUS PHOTORHABDUS AND XENORHABDUS (ENTEROBACTERIACEAE) BASED ON 16S rRNA Mohammad Agazadeh*, Davoud Mohammadi** and Naser Eivazian Kary** * Department of Microbiology, Faculty of Medicine, Tabriz University of Medical Science, Tabriz, IRAN. ** Department of Plant Protection, Faculty of Science, Azarbaijan University of Tarbiat Moallem, Tabriz, IRAN. [Agazadeh, M., Mohammadi, D. & Eivazian Kary, N. 2010. Molecular Identification of Iranian isolates of the genus Photorhabdus and Xenorhabdus (Enterobacteriaceae) based on 16S rRNA. Munis Entomology & Zoology, 5 (2): 772-779] ABSTRACT: Five bacterial strains of Photorhabdus and six strains of Xenorhabdus originating from several geographical isolates of entomopathogenic nematodes have been studied moleculary to distinguish their position among the well defined species. Constructed phylogenetic trees based on the sequences of 16S-rRNA showed all of the studied Photorhabdus belong to the P. luminescens subsp. lumondii and Xenorhabdus were identified as X. bovienii, X. nematophilus and X . budapestensis. Among the Steinernema feltiae symbiont’s one isolate named Xenorhabdus sp. IRA22 made a nearly distinct branch in the group and therefore it is possible to represent new species. KEY WORDS: Entomopathogenic nematodes, Photorhabdus, P. luminescens subsp. lumondii, Xenorhabdus, X. bovienii, X . budapestensis, X. nematophilus. Bacterial symbionts of entomopathogenic nematodes in the family Heterorhabditidae and Steinernematidaeare are members of the family Enterobacteriaceae and belong to the genera Photorhabdus and Xenorhabdus respectively (Thomas et al., 1979). The nematodes invade the larvae of susceptible insects and penetrate to the hemocoel, where they release their symbiotic bacteria (Kaya & Gaugler, 1993). In the hemocoel bacteria replicate within and kill the insect host (ffrench-Constant et al., 2003). Strains of Photorhabdus and Xenorhabdus species occur in two forms. The first is the bacterium isolated from IJ and was named the phase Ι variant. The second variant form, named phase ΙΙ, appears spontaneously during period of an in vitro culture or during nematode rearing on an artificial diet (Boemare et al., 1988.). Different species and strains differ in virulence (Aguillera et al., 1993; Han et al., 1991) and therefore correct identification or grouping of the strains is essential in relation to experimental work. 16s rRNA gene has been revealed as a good marker to determine diversity among bacteria and currently all species description should now include the 16s rRNA sequence species. However, 16S rRNA evolves so slowly and may not always be high enough to distinguish closely related strains (Adams et al., 2006). Three and nineteen species have been described for Photorhabdus and Xenorhabdus respectively: P. luminescens ssp. luminescens (type species), P. luminescens ssp. akhurstii, P. luminescens ssp. laumondii, P. luminescens ssp. kayaii, P. luminescens ssp. thracensis, P. temperata , P. temperata ssp. temperata , and P. asymbiotica ssp. asymbiotica and P. asymbiotica ssp. americana (Akhurst et al., 2004), X. nematophilus ; X. bovienii; X. poinarii; X. beddingii (Akhurst & Boemare, 1988); X. japonica (Nishimura, 1994); X. budapestensis ; X. innexii; X. szentirmaii; X. ehlersii (Lengyel et al., 2005); X.