Gerardo R. Theiler

Patterns of evolution in Graomys griseoflavus (Rodentia, Muridae). IV. A case of rapid speciation

The South American group of rodents known as Graomys griseoflavus comprises two sibling species differing only in diploid chromosomal complement: G. griseoflavus (2n = 36, 37 and 38) and G. centralis (2n=42). Reproductive barriers comprising postzygotic as well as precopulatory mechanisms prevent gene exchange between these species. We have studied genetic polymorphism in two populations of G. centralis and four of G. griseoflavus by means of gel electrophoresis of enzymes and other proteins giving information on a total of 30 loci. Values of interspecific genetic identity were similar to those obtained for conspecific populations, suggesting that fixation of Robertsonian fusions would have occurred without significant bottlenecks. It would also indicate that the cladogenetic process must be relatively recent. FIS values showed no evidence of inbreeding. Fixation indices (FST) for the ancestral species showed a tendency to form demes with very low gene flow among them, while in the derived species such tendency was not shown. However, because of the characteristics of the region they occupy, populations are of moderate size, and genic flow is low. Lack of correlation between gene flow levels and geographical distance between population pairs would indicate a recent and fast colonization of its distribution areas by the derived species. It is possible that fixation of Robertsonian fusions occurred in a marginal deme of the ancestral species, e.g. in a parapatric geographical context.

Patterns of Evolution in Graomys griseoflavus (Rodentia: Muridae): II. Reproductive Isolation between Cytotypes

Graomys griseoflavus is a South American murid rodent exhibiting marked chromosomal polymorphism. Breeding experiments between individuals from six populations, four showing diploid complements of 36, 37, or 38 and two having 2n = 42, were conducted. Interpopulation crosses showed that 2n = 36–38 animals were interfertile, indicating that these cytotypes belong to a single species complex. This species complex is isolated re-productively from the 2n = 42 chromosomal race. Chromosomal speciation may have been attained through multiple sequential Robertsonian fusions. Isolation is asymmetric; matings between 2n = 42 males and 2n = 36–38 females produced hybrid offspring, whereas reciprocal crosses were non viable. These results are compatible with Kaneshiros hypothesis. Hybrid males were sterile, backcrosses were productive in ca. 23% of matings between female hybrids and males of the parental populations, and reciprocal backcrosses were unsuccessful. Results indicate that the 2n = 42 cytotype is a separate sibling species from those of the 2n = 36–38 complex.

A contribution to the subgeneric systematics of Oligoryzomys (Rodentia, Muridae) from Argentina by means of PCR–RFLP patterns of mitochondrial DNA

Subgeneric recognition of rodents of the genus Oligoryzomys from Argentina is specially difficult because morphologic and morphometric criteria to identify species are poorly defined. In order to contribute to the unequivocal identification and geographic distribution of Oligoryzomys species, we studied the restriction fragment length polymorphism (PCR-RFLP) patterns of the D-loop region of the mitochondrial DNA in specimens collected from different regions of Argentina. PCR amplification products of 59 individuals were digested with five restriction enzymes. Patterns obtained were used to produce two phylogenetic consensus trees, one obtained with PAUP and the other with MIX analyses. All specimens from the Patagonia (southern region) clustered together and would correspond to O. longicaudatus. Individuals from the North grouped in four clusters (two of them comprising one individual), supported by high bootstrap values. The one including the 82% of the rodents from the northern region would correspond to the species O. chacoensis.

Association of cellular and molecular alterations in Leydig cells with apoptotic changes in germ cells from testes of Graomys griseoflavus×Graomys centralis male hybrids.

Spermatogenesis is disrupted in Graomys griseoflavus×Graomys centralis male hybrids. This study was aimed to determine whether morphological alterations in Leydig cells from hybrids accompany the arrest of spermatogenesis and cell death of germ cells and whether apoptotic pathways are also involved in the response of these interstitial cells. We used three groups of 1-, 2- and 3-month-old male animals: (1) G. centralis, (2) G. griseoflavus and (3) hybrids obtained by crossing G. griseoflavus females with G. centralis males. Testicular ultrastructure was analyzed by transmission electron microscopy. TUNEL was studied using an in situ cell death detection kit and the expression of apoptotic molecules by immunohistochemistry. The data confirmed arrest of spermatogenesis and intense apoptotic processes of germ cells in hybrids. These animals also showed ultrastructural alterations in the Leydig cells. Fas, FasL and calbindin D28k overexpression without an increase in DNA fragmentation was detected in the Leydig cells from hybrids. In conclusion, the sterility of Graomys hybrids occurs with ultrastructural changes in germ and Leydig cells. The enhancement of Fas and FasL is not associated with cell death in the Leydig cells. Probably the apoptosis in these interstitial cells is inhibited by the high expression of the antiapoptotic molecule calbindin D28k.