Phylogenetic relationships among Robertsonian karyomorphs of Graomys griseoflavus (Rodentia, Muridae) by mitochondrial cytochrome b DNA sequencing

摘要

Graomys griseoflavus (Waterhouse 1837) is a phyllotine murid rodent widely distributed in Argentina with Robertsonian autosomal polymorphisms, showing karyomorphs with diploid numbers equal to 42, 41, 38, 37, 36, 35 and 34 (Zambelli et al. 1994). In Graomysgriseoflavus, cytogenetic and molecular data supported the ancestrality of the 2n=42 karyomorph (Gardner and Patton 1976; Zambelli et al. 1994; Zambelli and Vidal-Rioja 1999) and it was proposed a chromosomal divergence pathway to explain the karyotype variability observed. Starting from 2n=42 karyomorph, two lines derived: one producing very low frequent 2n=41 individuals and the other, 2n=38 specimens. The 2n=38 karyomorphs were the consequence of two homozygous Robertsonian fusions (RF) that occurred in 2n=42 (RF15–17 and RF16–18) producing diploid number reduction. From 2n=38, the 2n=37, 36, 35 and 34 karyomorphs have appeared by a non-random downward sequence of Robertsonian fusions: RF1–6 and RF2–5 (Table 1; Zambelli et al. 1994).Table1. Robertsonian fusions found in each karyomorph of G. griseoflavus 2nRF1–6RF2–5RF15–17RF16–18Ht: heterozygousHm: homozygous–: absence of the RF42––––41Ht–––38––HmHm37Ht–HmHm36Hm–HmHm35HmHtHmHm34HmHmHmHmIn the area studied, the 2n=42 individuals inhabit the “Espinal” and “Western Chaco” phytogeographic regions located in central Argentina, while the 2n=38, 37, 36, 35, 34 complex (2n=38?34) mainly occupies the “Monte” region in the western-central area of the country. There are no significant geographic barriers separating different populations of Graomys, in fact, narrow overlapping zones occur in some regions (Theiler and Blanco 1996a; Tiranti 1998). In the laboratory, matings between 2n=42/41, 38/37, 38/36 and 37/37 karyomorphs resulted in F1 and F2 fertile progenies while matings between 2n=42/38, 42/37 and 42/36 individuals failed to breed or gave sterile hybrids heterozygous for RF15–17 and RF16–18 (Zambelli et al. 1994; Theiler and Blanco 1996a,b). Among the total wild animals sampled during 15 years (about 150) heterozygous specimens for these RFs were never reported. This finding was attributed to a possible relationship between the chromosomal rearrangements and the mechanism of reproductive isolation. We suggested that gametic cell precursors bearing the RF15–17 and 16–18 in heterozygous state fail to segregate during meiosis, affecting the fertility of the heterozygous individuals (Zambelli et al. 1994).Based on allozyme and reproductive behaviour analyses, Theiler and Blanco (1996b) and Theiler et al. (1999) revised the taxonomic status of Graomys and reassigned them to two sibling species: Graomys centralis for the 2n=42 specimens, and Graomys griseoflavus for the 2n=38?36 complex. In this revision these authors did not include the 2n=41, 35 and 34 specimens.The homozygous RF15–17 and RF16–18 are the common chromosomal feature in the 2n=38?34 complex, and their presence may be correlated with the Nucleolar Organizer Regions (NOR) pattern and satellite DNA organization. Analysis of NOR locations both by silver staining (Ag-NOR) and in situ hybridization revealed that the 2n=42 exhibit highly variable Ag-NOR patterns both in number and chromosome location, while 2n=38?34 karyomorphic group showed a single Ag-NOR pattern. The latter animals underwent two NOR deletions in reference to the 2n=42 karyomorphs, one of which would be the consequence of a Robertsonian fusion and the other would be produced by unequal crossing-over mechanism (Zambelli and Vidal-Rioja 1996). On the other hand, the Graomys chromosomal divergence has been correlated with molecular organization of two satellite DNA families (EG250 and Hpa3.2). When all karyomorphs were compared, a clear differentiation between 2n=42?41 and 2n=38?34 was found at the level of methylation pattern of the EG250 satellite, and the molecular organization of Hpa3.2 satellite, which is much more abundant in the 2n=38?34 group than in the 2n=42?41 (Zambelli and Vidal-Rioja 1999).Desp