Primitive forms of meiosis: the possible evolution of meiosis

Meiosis is a basic process of most eukaryotes, as it forms with conjugation the basis of sexual reproduction. As sex seems to be present in the vast majority of eukaryotes, the origin of meiosis is presently unknown. Protists having optional or alternative sexual and asexual cycles seem to be the best targets for research on the evolution of meiosis. While the budding yeast Saccharomyces cerevisiae shows an elaborate and well-known meiotic process, the fission yeast Schizosaccharomyces pombe, has a much simpler meiosis, which may show some of the most primitive features of meiotic mechanisms. The present availability of whole genome sequences of many bacteria and some protists is revealing that eukaryotic sexual reproduction has recruited some prokaryotic processes for its own development. Some of these processes are analyzed and the basic role of chromosome linearity and telomere constitution in the development of meiosis is underlined.

Comparative morphologic placental types in Dasypodidae (Chaetophractus villosus, Cabassous chacoensis, Tolypeutes matacus and Dasypus hybridus)

Information about the morphology of placentas in armadillos is scarce, except for D. novemcinctus. A comparative study of morphologic placental types in armadillos is important in order to have a comprehensive view of the peculiar reproductive physiology in this family. The aim of this paper is to perform a comparative analysis of the morphological features of the placenta in Chaetophractus villosus, Cabassous chacoensis, Tolypeutes matacus and Dasypus hybridus in order to classify them in accordance with Grosser (1909). The placentas were studied macroscopically and histologically (light microscopy in 1 micron thick sections and electron microscopy for fine structure). The macroscopic study in the 4 studied species showed a similar pear-shaped placenta homogeneously villosus in almost all the surface. The histological analysis showed that the 4 studied species had a hemochorial type of placenta. This type of placenta was also found in D. novemcinctus (Dasypodidae), but it is different from those described for other xenarthrans. Hemochorial types of placenta have also been described in more modern mammals. Despite the many primitive features of the armadillos and the different anatomical and physiological features between the genuses of dasypodids, all the studied species share this structural type of placenta.

Recombination nodules, synaptonemal complexes and heterochromatin in the hemipteran Triatoma infestans

Se ha realizdo un estudio cuantitativo de la morfología, número y localización de los nódulos de recombinación (NR) en espermatocitos de Triatoma infestans, y se han correlacionado estos datos con las configuraciones quiasmáticas en diacinesis y en matafase 1. Además se han estudiado las relaciones cuantitativas entre cantidad de heterocromatina y longitud de complejo sinaptonémico (CS). En T. Infestans los três autosomas mayores tienen bloques grandes de heterocromatina, C, que corresponden a 61,4; 64,1 y 35,7% de sus longitudes mitóticas. Los CS correspondientes son proporcionalmente menores a los CS de los otros autosomas. Las ecuaciones de regresión lineal de longitud de CS en función de las longitudes mitóticas muestran una clara diferencia entre los bivalentes que contienen heterocromatina y aquéllos sin ella. Por ello, se propone que la heterocromatina está sub-representada en los CS. Los nódulos de recombinación son esféricos y siempre uno por bivalente. Su localización no es al azar. En los CS 1 y 2 hay una proporción más alta en las regiones mediales; y en el CS 10 hay una proporción alta cerca de los extremos. Los quiasmas de 20 células en diacinesis y 20 células en metafase 1 se clasificaron en intersticiales o terminales. No hay diferencias estadísticas significativas entre las medias de quiasmas terminales en diacinesis y en metafase 1. La distribución de localizaciones quiasmáticas en los bivalentes es similar a la de los nódulos de recombinación. En conclusión, no hay evidencia de un proceso de terminalización quiasmática en esta espécie