Review of the Geographic Distribution of Diploid and Tetraploid Populations of the Odontophrynus americanus Species Complex (Anura: Leptodactylidae) SERGIO D. ROSSET, 1,2 DIEGO BALDO, 3 CECILIA LANZONE, 4 AND NE ´ STOR G. BASSO 5 1 Instituto de Limnologı ´a ‘‘Dr. Rau ´l A. Ringuelet,’’ Casilla de Correo 712, 1900 La Plata, Buenos Aires, Argentina 3 Departamento de Gene ´tica, Facultad de Ciencias Exactas, Quı ´micas y Naturales, Universidad Nacional de Misiones, Fe ´lix de Azara 1552, 3300 Posadas, Misiones, Argentina 4 CRICYT–IADIZA–GIB, Parque Gra, San Martı ´n, Av. Ruiz Leal s/n, Casilla de Correo 507, 5500 Mendoza, Mendoza, Argentina 5 Centro Nacional Patago ´nico, Blvd. Brown 3500, 9120 Puerto Madryn, Chubut, Argentina ABSTRACT.—We review the geographic distribution of diploid and tetraploid populations of the Odontophrynus americanus species complex. Ploidy was determined using chromosome count as well as erythrocyte size. We found a complex geographic pattern of populations with different ploidy, including areas of syntopy and sympatry. The tetraploid O. americanus presents three disjunct population groups, which were isolated from one another by diploid populations. One of these tetraploid groups is distributed in central and eastern Argentina, southern Brazil, southern Paraguay, and Uruguay, with the other two population groups of the tetraploid O. americanus inhabiting southeastern Brazil, and northwestern Argentina respectively. We extend the distribution of both diploid Odontophrynus lavillai and Odonto- phrynus cordobae, and we present new records of three more allopatric diploid population groups, referred to here as Odontophrynus sp. Some of these population groups are associated with biogeographic regions. The karyotypes of the three currently known taxa of the O. americanus species complex are given. We report the occurrence of a naturally occurring triploid specimen and the first occurrence of B-chromosomes in the genus. Our review indicates that there is a complex distribution pattern of populations of different ploidy, including areas of syntopy and sympatry, and cytogenetic variability. This could indicate the presence of more species, occurrence of auto and allopolyploidy, and multiple origins of tetraploidy. Polyploidy plays an interesting and impor- tant role in population genetics, speciation, and evolution in anurans (Bogart, 1980; King, 1990). Thirty-six naturally occurring bisexual poly- ploid species have been reported, belonging to 16 genera of different families: Arthroleptidae, Bufonidae, Hylidae, Leptodactylidae, Limnody- nastidae, Microhylidae, Pipidae, and Ranidae (Bogart, 1980; King, 1990; Otto and Whitton, 2000; Sto ¨ ck et al., 2002). This taxonomic distri- bution indicates that the process is not restricted to any particular anuran phylogenetic lineage (Bogart, 1980; Mahoney and Robinson, 1980). The first record of a naturally occurring bisexual polyploid species found in vertebrates was the leptodactylid frog Odontophrynus amer- icanus. Sa ´ez and Brum (1959) and Sa ´ez and Brum-Zorrilla (1966) were the first to find a high and variable chromosome number in this species from populations of Uruguay and Argentina (42, 44, and 50 chromosomes). Bec ¸ak et al. (1966) later demonstrated conclusively that Brazilian specimens were tetraploid (2N 5 4X 5 44). In addition, Bec ¸ak et al. (1970) also found a diploid population (2N 5 2X 5 22) from Brazil, morphologically indistinguishable in external characteristics and size. Since those studies, diploid and tetraploid individuals, showing a geographic mosaic of cryptic populations with different ploidy, have been found in Argentina, Brazil, and Uruguay (Barrio and Pistol de Rubel, 1972; Bec ¸ak and Bec ¸ak, 1974; Cei, 1987). All these diploid and tetraploid cryptic popula- tions currently constitute the O. americanus species complex (Cei, 1987; King, 1990) which are morphologically characterized by absence of tibial and antebraquial glands, and no well- developed postorbital, temporal, and parotoid glands (Savage and Cei, 1965; Cei, 1987). The other morphologically very different species of the genus that have been examined cytogenet- ically (all except Odontophrynus salvatori) have a diploid complement of 2n 5 2X 5 22 chromo- somes (Bec ¸ak and Bec ¸ak, 1974; Ruiz, 1980; Ruiz et al., 1982; Salas et al. 2000). From a taxonomic point of view, the O. americanus species complex is currently repre- sented by three species. The widely distributed O. americanus occurs in central and northern Journal of Herpetology hpet-40-04-07.3d 25/9/06 11:25:01 466 Cust # 06-098R 2 Corresponding Author. E-mail: rosset@ilpla.edu.ar Journal of Herpetology, Vol. 40, No. 4, pp. 466–478, 2006 Copyright 2006 Society for the Study of Amphibians and Reptiles