Karyotype description of two species of the genus A kodon ( Rodentia , Sigmodontinae ) in the northern Rio Grande do Sul state , Brazil

Among the sigmodontine rodents, the genus Akodon is the second in numerical terms, with about 45 recognized species. In the Rio Grande do Sul state, there are 5 registered species. This article shows data on the distribution of the species of the genus Akodon in the northern Rio Grande do Sul state, as well as a description


Introduction
Among the South American rodent families, Cricetidae stands out, encompassing about 55% of the species described (MUSSER; CARLETON, 2005).This family is divided into 2 subfamilies: Sigmodontinae and Neotominae, and among the irst one, there is the tribe Akodontini, which has 12 genera and 63 species, accounting for 35% of the total diversity of the group.The genus Akodon Meyen, 1833 is the second most specious in South America, and due to its complex taxonomy, the number of species has ranged from 90 (TATE, 1932) to 33 (HONACKI et al., 1982), also being divided into groups since its description (MYERS et al., 1990).Nowadays, about 45 species are accepted for this genus (SMITH; PATTON, 2007;BRAUN et al., 2008;D'ELÍA et al., 2011).
The genus Akodon includes species with high morphological similarity, thus showing a complex and poorly understood taxonomy (CHRISTOFF et al., 2000;HASS et al., 2008).For this reason, karyotypic analyzes are necessary in order to identify many Brazilian species (CHRISTOFF et al., 2000;GONÇALVES et al., 2007).However, this trait contrasts with its large multiplicity of the species' karyotypes (SBALQUEIRO; NASCIMENTO, 1996;SILVA;YONENAGA-YASSUDA, 1998;CHRISTOFF et al., 2000).In this study, data on the distribution of the genus Akodon in the northern portion of Rio Grande do Sul state, as well as a description of the karyotypes of the species from this region were shown.
The region has the Araucaria Forest as the typical forest formation of southern Brazil, which occurs in the higher parts of the South Brazilian Plateau, and it is restricted to high altitudes (HUECK, 1972).In several areas of the region, the Araucaria Forest shares many species with the Seasonal Forest, indicating different ranges of transition between these two forest formations (MARQUES et al., 2005).Sampling effort was carried out in fragments of a native vegetation characteristic of the region.The altitude varies from 400 to 789 m asl.
Specimens were collected from August 2008 to May 2010.During this period, one expedition in each locality was conducted: Giruá (Aug/08), Floriano Peixoto (Sep/08), Mato Castelhano (Oct/08), Erechim (Jan/09), Getúlio Vargas (Mar/09), and Faxinalzinho (May/10).Standard Tomahawk traps of one size (12×12×25 cm) mounted for four consecutive nights were used, totaling a sampling effort of 1,200 trap-nights (200 at each site).Traps were baited with a mixture of peanut butter, mashed bananas and sardines applied to a slice of maize.The captured animals were identiied in ield, and later taken to the laboratory, where they were submitted to cytogenetic analysis in accordance with the standard protocol of Ford and Hamerton (1956).The animals were captured with the consent of environmental agencies (ICMBio), under the permanent and personal license number 15224-2 (J.R.M.).The method of Ford and Hamerton (1956) for mitotic bone marrow preparation was followed.Cytogenetic data were obtained through analysis of the optical microscope slides, settling the diploid number of chromosomes by counting the metaphases, and the best were selected for photograph and assembly karyotype.
All individuals of A. montensis were captured in traps placed on the ground, in areas of understory vegetation, border areas and within the native forest.The karyotype shown by the species was 2n = 24 and FN = 44.The irst pair of chromosomes is composed of two large submetacentrics; the second pair is composed of two large metacentrics; pairs 3-6 are composed of medium-sized metacentrics; pairs 7-9 are composed of small metacentrics; pair 10 is composed of two small acrocentrics, and the pair 11 of two tiny metacentrics.
X chromosome is a small acrocentric and Y is a tiny acrocentric (Figure 2).Individuals of A. paranaensis were captured in traps placed on the ground, inside the forest, in areas of low vegetation and inner edge of native forest.The karyotype shown by the species was 2n = 44 and FN = 46.The irst pair of chromosomes is composed of two large acrocentrics; pairs of chromosomes 2-12 are composed of medium-sized acrocentrics; pairs 13-20 are composed of small acrocentrics; pair 21 is composed of two tiny metacentrics.The X chromosome is a medium-sized acrocentric and Y is a small acrocentric (Figure 2).

Discussion
The data obtained in this study are in accordance with the already known distribution patterns of the genus Akodon described by D' Elía (2003) and Pardiñas et al. (2003), where A. paranaensis and A. montensis might occur in sympatry.Akodon montensis has a wide distribution and it is one of the dominant Sigmodontinae species in primary or secondary forest, in the Rio Grande do Sul state.It has been identiied as one of the dominant species of rodent communities in other fragments of the northern Rio Grande do Sul state (GALIANO et al., 2007;2013;KUBIAK et al., 2009).Both Akodon species are found in high altitude regions, especially A. montensis, found at an altitude above 700 meters in some regions along its distribution (GEISE et al., 2004;GONÇALVES et al., 2007;GALIANO et al., 2014).
These two species belong to the cursor group (SMITH; PATTON, 2007), which shows an extensive karyotypic diversity, ranging from 2n = 14-15 in Akodon cursor Winge, 1887, to 2n = 44 in Akodon mystax Hershkovitz, 1998, Akodon paranaensis and Akodon reigi González, Langguth & Oliveira, 1998.For species such as A. paranaensis and A. reigi, which are considered morphologically cryptic, the karyotype may be a tool in determining the correct species.However, these species show no distinction between their karyotypes in the conventional cytogenetic analysis, as performed in this study.In the results, individuals identiied with the karyotype 2n = 44 were assumed to be A. paranaensis due to the geographical location of the sampling points, since, according to González et al. (1998), A. reigi is distributed only among the southeastern portion of the Rio Grande do Sul state.This species occurs in eastern Uruguay and extreme southern Brazil (Rio Grande do Sul state) (MUSSER; CARLETON, 2005), and was never registered in other regions of this state.The identiication of A. paranaensis was based only on the karyotype and geographical information, and no morphological analysis was performed.
Besides these two species of Akodon found in this study, the region may show the occurrence of A. serrensis Thomas, 1902and A. azarae Fischer, 1829(BONVICINO et al., 2008).The fact that only A. montensis and A. paranaensis were captured might be an indication that these species are the only ones with conirmed occurrence in the northern portion of this state, as reported in other studies carried out in the region (GALIANO et al., 2007;2013;KUBIAK et al., 2009).Accurate information on the distribution of the genus Akodon in the Riograndense Plateau is still scarce, making it necessary to increase the number of specimens karyotyped, with the aim of generating increasingly reliable data, and analyzing the patterns of distribution of the species in the region, given the taxonomic dificulties of identiication.Despite the lack of broad revisionary treatments of Akodon, the number of recognized species within the genus has experienced a steady increase in the last years (GONÇALVES et al., 2007).This inventory contributes to the knowledge of the distribution of Akodon species in the region of transition between the Araucaria Forest and the Seasonal Forest.

FIGURE 1 :
FIGURE 1: Sample localities in the northern Rio Grande do Sul state, Brazil's Southern region.

FIGURE 2 :
FIGURE 2: Conventional karyotype colored with Giemsa obtained from a sampled male of Akodon montensis (A); female of Akodon montensis (B); male of Akodon paranaensis (C); female of Akodon paranaensis (D), in the northern Rio Grande do Sul state, Brazil's Southern region.