HERPETOLOGICAL JOURNAL 18: 213–222, 2008 Meiotic differentiation in two allopatric population groups of the tetraploid frog Odontophrynus americanus from Argentina C. Lanzone1, D. Baldo2 & S.D. Rosset3 1Grupo de Investigaciones de la Biodiversidad, IADIZA-CRICYT-CONICET, Mendoza, Argentina 2Lab. Genética Evolutiva y Molecular, Dto. de Genética, UNaM, Posadas, Argentina 3Sección Herpetología, Museo de La Plata, Buenos Aires, Argentina The Odontophrynus americanus species complex is widely distributed in South America and is formed of diploid and tetraploid cryptic species. We studied the meiotic prophase stages of two allopatric tetraploid population groups from southeastern and northwestern Argentina. These two allopatric population groups showed a bouquet polarization in zigotene and pachytene, and in the latter stage a complete synapsis of quadrivalents and bivalents. In diakinesis, the frequencies of the different elements per cell were recorded and compared with two theoretical distributions for an autotetraploid organism. Both model tests showed the same overall results. The frequency of quadrivalents did not depart significantly from the models in southeastern populations, and must be considered autotetraploid of recent origin. However, northwestern populations have a significantly reduced number of multivalents, which can be explained by diploidization or allotetraploidy. The reduction of quadrivalents is relatively low when compared with one of the models. Additionally, different chiasmatic frequencies were observed between bivalents and quadrivalents in both population groups. The presence of extra chromosomes detected in southeastern populations and the mitotic and meiotic irregularities reported from other populations, not observed in the present study, account for a high cytological variability between populations of the tetraploid frog Odontophrynus americanus, which can be ascribed to polyploidy. Key words: Anura, bivalents, chiasmata, Cycloramphidae, polyploidy, quadrivalents INTRODUCTION et al., 1985; Matzke et al., 1999; Soltis & Soltis, 1999; Wolfe, 2001). Autopolyploids of ancestral origin may also olyploidy is a mechanism of genome evolution in reduce quadrivalent formation and the ploidy level by Pwhich the diploid chromosome complement multi- diploidization, but this process may be slower than in plies. Traditionally, this process has been viewed as more allopolyploids. Several models of multivalent formation important in plant than in animal evolution (Muller, 1925; have been described and used to test polyploidy in Otto & Whitton, 2000). However, this view has recently plants, but this approach has rarely been applied in ani- been criticized on several of its assumptions (Mable, mals (Driscoll et al., 1979; Jackson & Casey, 1982; Jackson 2003, 2004) and new discoveries of this process have & Hauber, 1994; Gatt et al., 1998). shown that polyploidy is frequent in a wide range of ani- In the evolution of amphibian anurans, polyploidy has mals (Otto & Whitton, 2000; Leggatt & Iwama, 2003). In played an important role. The presence of polyploid spe- spite of this, studies on animals are still scarce compared cies in several non-related lineages points to the fact that to those on plants (Mable, 2003, 2004). this process has emerged independently and on different Polyploidy can occur in two different ways. Autopoly- occasions in this group (Bogart, 1980; Mahony & ploid species are characterized by possessing more than Robinson, 1980; King, 1990; Ptacek et al., 1994; Mable & two copies of the same chromosome complement. Thus, Roberts, 1997; Stöck et al., 2002; Holloway et al., 2006). during meiosis this leads to more than two fully homolo- Odontophrynus americanus is the first sexually repro- gous chromosomes that can pair either as bivalents or as ducing vertebrate for which a polyploid origin was multivalents. On the other hand, the allopolyploids have demonstrated (Beçak et al., 1966, 1967a,b). The high fre- copies of different genomes by hybridization. This can quency of meiotic quadrivalents led to the interpretation lead to preferentially formed bivalents, with strictly homo- that this species is of recent autotetraploid origin (Beçak logue chromosomes, at the expense of multivalents et al., 1966; Schmid et al., 1985). Previous meiotic studies (Jackson & Casey, 1982; Jackson & Hauber, 1994; of O. americanus from Argentina and Uruguay have Cuñado et al., 2005). The reduction in multivalents could shown that the observed frequencies of multivalents are be accelerated after the origin of allopolyploids by diver- not different from those predicted for an autotetraploid gence from the original hybridizing genomic complements organism of recent origin (Rahn & Martínez, 1983; Schmid through genome reorganization and gene silence (Schmid et al., 1985). Other studies, based on low number of cells Correspondence: Cecilia Lanzone, Grupo de Investigaciones de la Biodiversidad, IADIZA-CRICYT-CONICET, CC 507, Mendoza (5500), Argentina. E-mail: [email protected] 213 C. Lanzone et al. Table 1. Specimens, geographic origin and number of meiotic elements (total elements, quadrivalents and bivalents) and chiasmatic frequencies per cell of the Odontophrynus americanus specimens analysed in this study. Values are means ± 1 SD; n = sampling number; * indicates specimens with B chromosome. Number of Number of Number of Chiasmata Locality, specimen total elements quadrivalents bivalents frequencies n NORTHWESTERN POPULATIONS San Pablo de Reyes, Jujuy, MLP DB 2198 15.84±1.77 6.13±1.82 9.68±3.54 39.71±2.36 31 San Pablo de Reyes, Jujuy, MLP DB 2199 15.47±1.59 6.50±1.61 8.93±3.18 41.47±2.30 30 San Pablo de Reyes, Jujuy, MLP DB 2200 15.50±1.53 6.50±1.53 8.93±2.99 39.57±3.43 30 La Merced, Salta, MLP DB 1643 15.83±1.72 6.17±1.72 9.67±3.45 41.22±1.70 18 Encón Grande, Salta, MLP DB 2545 14.72±1.30 7.28±1.30 7.34±2.71 42.78±1.16 32 Encón Grande, Salta, MLP DB 2547 15.33±1.52 6.67±1.52 8.67±3.03 40.63±1.65 30 Encón Grande, Salta, MLP DB 2549 15.07±1.80 6.97±1.69 8.03±3.29 42.40±1.33 30 Encón Grande, Salta, MLP DB 2552 15.47±1.55 6.47±1.57 8.93±3.10 41.53±2.15 30 Encón Grande, Salta, MLP DB 2561 14.97±1.43 7.03±1.43 7.93±2.85 42.53±1.20 30 Encón Grande, Salta, MLP DB 2579 15.20±1.49 6.73±1.55 8.40±2.99 38.90±1.83 30 Encón Grande, Salta, MLP DB 2580 15.11±1.64 6.86±1.65 8.23±3.28 41.43±1.72 35 Encón Grande, Salta, MLP DB 2581 15.08±1.55 6.92±1.55 8.15±3.09 41.08±1.52 26 Encón Grande, Salta, MLP DB 2582 14.73±1.62 7.27±1.62 7.47±3.23 42.10±1.35 30 Salta, Salta, MLP 3716 15.17±1.88 6.83±1.88 8.27±3.70 41.93±1.57 30 Salta, Salta, MLP 3890 15.59±1.78 6.41±1.78 9.07±3.70 42.62±1.18 29 El Mojón, Catamarca, MLP DB 2660 15.10±1.59 6.90±1.63 8.20±3.25 41.00±1.31 30 El Mojón, Catamarca, MLP DB 2745 15.27±1.20 6.73±1.20 8.40±2.54 42.00±1.26 30 El Mojón, Catamarca, MLP DB 2746 15.40±1.61 6.60±1.61 8.60±3.20 41.60±1.30 30 Rosario, Catamarca, MLP DB 2670 15.00±1.47 7.00±1.47 8.00±2.95 42.63±1.21 24 SOUTHEASTERN POPULATIONS Loreto, Corrientes, MLP DB 2311* 14.13±1.53 7.87±1.53 6.27±3.05 42.40±1.54 30 Vera, Santa Fe, MLP DB 2718 14.89±1.59 7.43±1.95 7.32±3.13 40.32±1.87 28 Mojones, Entre Ríos, MLP 3715 15.07±1.74 6.93±1.74 8.13±3.48 42.47±1.41 30 Tres Bocas, Entre Ríos, MLP 3712 14.43±1.36 7.57±1.36 6.87±2.71 41.93±1.82 30 Paraná, Entre Ríos, MLP DB 2109 14.45±1.41 7.55±1.41 6.90±2.82 41.58±1.69 31 Pilar, Buenos Aires, MLP 3888 14.10±1.45 7.93±1.44 6.17±2.88 43.10±0.89 30 Pilar, Buenos Aires, MLP 3931 14.80±1.27 7.20±1.27 7.47±2.46 41.50±1.74 30 Magdalena, Buenos Aires, MLP 3922 14.17±1.42 7.83±1.42 6.33±2.83 42.90±0.99 30 Magdalena, Buenos Aires, MLP 3923 14.57±1.65 7.30±1.68 7.27±3.30 42.33±1.15 30 Magdalena, Buenos Aires, MLP 3924 14.56±1.48 7.40±1.48 7.13±2.86 42.53±0.97 30 Magdalena, Buenos Aires, MLP 3925* 15.53±1.78 6.43±1.83 8.93±3.42 42.10±1.65 30 Magdalena, Buenos Aires, MLP 3980 14.40±1.61 7.60±1.61 6.80±3.22 43.23±0.97 30 Magdalena, Buenos Aires, MLP 4555 14.87±1.46 7.13±1.46 7.73±2.91 42.47±1.17 30 Punta Indio, Buenos Aires, MLP 3713 14.06±1.57 7.94±1.57 6.13±3.14 42.65±1.31 31 Mar Chiquita, Buenos Aires, MLP DB 3489 14.80±2.06 7.17±2.04 7.60±4.12 42.03±1.30 30 from specimens from Brazil, revealed a higher number of MATERIALS AND METHODS multivalents (Beçak et al., 1967a). We studied and compared the synaptic behaviour and We analysed the meiosis of 34 tetraploid males of multivalent formation in two allopatric population groups Odontophrynus americanus from two Argentinean of the tetraploid frog Odontophrynus americanus from allopatric population groups.