Laura I. Walker

Evolución de los filotinos (Rodentia, Muridae) en los Andes del Sur

La tribu Phyllotini es un grupo avanzado de 46 especies endemicas entre los Muridae de America del Sur que, estando adaptado a sus ambientes xericos, parece haber tenido el Altiplano Sur como area de diferenciacion original (Reig 1986). Esta hipotesis es puesta a prueba examinando la distribucion geografica y los datos citogeneticos de 35 especies, incluyendo la descripcion de bandas G de Andinomys edax y Euneomys chinchilloides y su comparacion con Phyllotis, bandas AgNOR y FISH para ADNr en siete especies, asi como datos de electroforesis de proteinas (118 electroalelos en 12 especies) y de secuencias del gen para citocromo b en el ADNmt (407 pb en 14 especies). Todos los cariotipos resultaron cromosomicamente distintos, excepto dos en los que no se conto con bandas G. Se encontro una asociacion general entre distribucion altiplanica de las especies con estados de caracteres ancestrales (cromosomas telocentricos, 2n altos, posiciones basales en las filogenias basadas en proteinas y secuencias de citocromo b), asi como entre distribucion no altiplanica y estados derivados. Se documentan aqui distintas combinaciones de brazos cromosomicos tipo Andinomys que se fusionarian para formar cromosomas metacentricos en Euneomys y Phyllotis, y tambien una disminucion del numero de NORes estructurales en Auliscomys respecto de Loxodontomys, rasgos derivados que demuestran una diversificacion hacia el sur y otra hacia el norte respectivamente, desde un centro en el Altiplano Sur

Cytogenetic and reproductive studies of two nominal subspecies of Phyllotis darwini and their experimental hybrids

Phyllotis darwini vaccarum and Phyllotis darwini darwini live contiguously in Central Chile, the former in the high Andes and the latter in the Central Valley and on the coast. All individuals examined of both kinds had 38 chromosomes of similar shape, but P. d. vaccarum always showed large paracentromeric C-bands on all chromosomes, whereas P. d. darwini showed none or only small ones. Fifty-six pairings between the two forms were made in the laboratory. They resulted in 23 hybrids produced by four P. d. vaccarum females and four P. d. darwini females. A higher incidence of aggression and mortality was observed in the cross-pairings, as well as a lower incidence of birth (14.3%), and a lower mean litter size (2.9), in comparison with 117 pairings within forms (65.0% and 4.2%, respectively). C-banded hybrid metaphases showed 19 chromosomes with large paracentromeric C-bands and 19 with none or only small ones. In the G-banded hybrid karyotypes, it was possible to identify eight chromosome pairs with total correspondence of band patterns, five pairs with correspondence only at their long arms, and 10 autosomes and the X chromosomes with no correspondence at all. Hybrids were completely sterile; adults had incomplete germinal lines and a lack of differentiated gametes in their gonads. These results, and the presence of individuals of both chromosomal forms in the same trap line with no traces of mixing, provide strong evidence of complete genetic isolation of the two forms; that is, they should be considered as separate species. Further considerations suggest the existence of gene flow between vaccarum and the other Andean forms living in northern Chile, and probably with those from southern Chile; the oldest species-group name available for all of these is xanthopygus .

Chromosome Divergences Among American Marsupials and the Australian Affinities of the American Dromiciops

To investigate the phylogenetic relationships of living marsupials, morphometric and G-banded chromosome analyses were made in the Chilean species Dromiciops gliroides (Microbiotheria) and Thylamys elegans (Didelphimorphia). Chromosome arm lengths and patterns of G-bands were compared in at least eight bone marrow metaphase spreads in six and nine specimens, respectively. They were contrasted with those published for another 11 American and Australian genera. Three of six autosomal pairs (A1, A3, and C2) were uniquely shared by Dromiciops and some Australian species, being different in shape and G-banded patterns from those with similar total sizes in Thylamys and other South American didelphoid karyotypes. Such chromosomal correspondences suggest the past occurrence of at least three pericentric inversions. A table of character states constructed from chromosomal G-band comparisons is presented, showing that cytogenetic data agree with Szalays (1982) hypothesis on the affinities of the South American Dromiciops with Australian marsupials.

Chromosomes of Octomys mimax and Octodontomys gliroides and Relationships of Octodontid Rodents

Octomys mimax possesses a chromosomal complement of 2n = 56, FN = 108, with heterochromatin restricted to the pericentric regions in all chromosomes except one autosomal pair and the X chromosome. One chromosome also has heterochromatin in a secondary constriction. C-bands of Octodontomys gliroides reveal the clear presence of pericentric heterochromatin on the X chromosome and six autosomal pairs. Lightly stained, pericentric C-bands also were present in five other autosomes. A satellite-bearing chromosome did not band. In both species, two active nucleolar organizing regions (AgNOR) were located at secondary constrictions. Based on chromosomal differences found among O. mimax and other octodontids, we hypothesize that the identical or similar 2n and FN observed for O. mimax , Aconaemys fuscus , and some other octodontid species is a result of parallel evolution.

Conservation of whole arms during chromosomal divergence of phyllotine rodents.

The assumption of simple fusion in a group showing a constant number of chromosome arms was tested by comparison of the G-band patterns of chromosomes of three Phyllotis species. The karyotypes, each of which has 40 chromosome arms, have a 2n of 38, 38, and 40 and are made up of mostly metacentric chromosomes. Operational concepts describing the amount of matching in G-band patterns are proposed, separating chromosomes or arms into those with total correspondence, partial correspondence, or unique cases. Seven chromosomes and 21 arms out of the total were identical in the three species, denoting a conservation of whole-arm band sequences in this group. A greater number of identical arms than of identical chromosomes were observed, giving some support to the simple fusion hypothesis. An unexpected chromosomal divergence was detected, including chromosomal variation in the C-banded sex chromosomes.

Tandem and centric fusions in the chromosomal evolution of the South American phyllotines of the genus Auliscomys (Rodentia, Cricetidae)

The karyotypes of three of the four extant species of the genus Auliscomys (A. micropus, living in central [2n = 32, NF = 34] and southern [2n = 34, NF = 36, 37] Chile; A. sublimis [2n = 28, NF = 32] and A. boliviensis [2n = 22, NF = 32], which inhabit the Andean Altiplano) were analyzed. Comparisons of G-, C-, and AgNOR-banded karyotypes showed that extensive conservation of entire chromosomes and chromosomal regions had occurred during the evolution of this genus, with centromeretelomere tandem fusions and centric fusions probably being the most frequent chromosome changes. A chromosomal phylogeny, based on the chromosome homoeologies detected and parsimonious analysis of the nature and distribution of the inferred chromosomal changes, is proposed. This hypothetical phylogeny assumes that the ancestral telocentric karyotype would have undergone three consecutive tandem fusions, first originating the 2n = 32 (NF = 34) karyomorph exhibited by present-day specimens of A. micropus captured in central Chile and then the 2n = 28 (NF = 32) karyotype of A. sublimis. Subsequent centric fusions involving the tandem-fusion products would presumably have generated the 2n = 22 (NF = 32) A. boliviensis karyotype. Assuming some conditions related to early geographic distribution, this chromosomal phylogeny is in agreement with a paleogeographic model, which explains the present distribution of living Auliscomys species mainly on the basis of geologic and climatic events.(ABSTRACT TRUNCATED AT 250 WORDS)

Similarities and differences among the chromosomes of the wild guinea pig Cavia tschudii and the domestic guinea pig Cavia porcellus (Rodentia, Caviidae)

Abstract Cavia tschudii Fitzinger, 1867 is a wild guinea pig species living in South America that according to the analysis of mitochondrial genes is the closest wild form of the domestic guinea pig. To investigate the genetic divergence between the wild and domestic species of guinea pigs from a cytogenetic perspective, we characterized and compared the C, G and AgNOR banded karyotypes of molecularly identified Cavia tschudii and Cavia porcellus Linnaeus, 1758 specimens for the first time. Both species showed 64 chromosomes of similar morphology, although C. tschudii had four medium size submetacentric pairs that were not observed in the C. porcellus karyotype. Differences in the C bands size and the mean number of AgNOR bands between the karyotypes of the two species were detected. Most of the two species chromosomes showed total G band correspondence, suggesting that they probably represent large syntenic blocks conserved over time. Partial G band correspondence detected among the four submetacentric chromosomes present only in the C. tschudii karyotype and their subtelocentric homologues in C. porcellus may be explained by the occurrence of four pericentric inversions that probably emerged and were fixed in the C. tschudii populations under domestication. The role of the chromosomal and genomic differences in the divergence of these two Cavia species is discussed.

Malformaciones Congénitas: Aspectos Generales y Genéticos

Los defectos del desarrollo se pueden deber a malformaciones congenitas, deformaciones o disrupciones. El 10% de las malformaciones se atribuyen a causas ambientales el 25% a factores geneticos y el 65% a factores desconocidos probablemente de orden multifactorial. Existe un periodo de mayor susceptibilidad frente a los teratogenos que corresponde a la etapa donde se estan formando la mayoria de los organos y sistemas. La ingestion de plantas teratogenicas puede dar lugar a anomalias congenitas en los fetos de animales. Los pesticidas como DDT, la contaminacion de las aguas por mercurio y los disruptores endocrinos afectan la embriogenesis de las distintas especies del reino animal. Tambien se consideran como factores causantes de malformaciones a los agentes ambientales infecciosos y a algunos medicamentos. Los agentes fisicos como los aumentos de temperatura, las condiciones de hipoxia y las radiaciones afectan a distintos organismos, desde los peces al ser humano. La genetica de las malformaciones ha sido dificil de establecer, principalmente porque la mayor parte de ellas se caracteriza por presentar manifestaciones fenotipicas diversas, que en muchos casos aparentemente no estan relacionadas y que son variables para los individuos afectados. Por otra parte, los estudios realizados indican que frecuentemente, en la determinacion genetica de las malformaciones participan varios genes y las interacciones de estos con el ambiente, aunque determinaciones monogenicas se han podido establecer para unos pocos casos. Ilustramos aqui estos dos tipos contrastantes de determinacion genetica, a traves de la descripcion de los factores geneticos que estarian involucrados en los defectos del tubo neural y en el sindrome de CHARGE, respectivamente.

Evolution of trans-Andean endemic fishes of the genus Cheirodon (Teleostei: Characidae) are associated with chromosomal rearrangements

BackgroundAmong Neotropical fishes, the family Characidae is highly diverse and speciose and its taxonomy is not completely resolved. In Chile, the family is represented by five species, all within the genus Cheirodon, of which C. pisciculus, C. galusdae, C. kiliani, and C. australe are endemic, while C. interruptus is introduced. We compared chromosomal markers in order to appreciate the taxonomy and evolution of these trans-Andean fishes.ResultsThe specimens were photographed in stereomicroscope to observe the ventral protrusive teeth and cusps for morphological analysis and species identification. All of the species analysed had equally diploid chromosome number 2n = 50, with karyotypes dominated by high number of acrocentric chromosomes as compared to those of other members of Cheirodontinae. The distribution of heterochromatin was inconspicuous and was similar in all the species. The number of active NORs (nucleolus organizer regions) was polymorphic, with the greater number of them in C. kiliani and C. galusdae. The location of 5S and 18S rDNA ranged in number and position, showing two sites in different chromosomes. The fluorescent in situ hybridization with telomeric probe did not reveal interstitial sites in all analysed species.ConclusionsThe comparative analysis of karyotypes and morphological markers revealed a biogeographic pattern of distribution, with the species that occur in the southern region forming one group and those in central and northern Chile forming another.