lunes, junio 25, 2007

Vinchucas Esperanzadas

Las posibilidades de cambios cromosómicos pueden variar dependiendo de los linajes. En le caso de la vinchuca del género Mepraia, los cromosomas presentan kinetocoros difusos, esto da la posibilidad de que fragmentos cromosómicos acéntricos generados durante las divisiones celulares conformen agregados al huso y puedan migrar autónomamente a los polos durante la divisiones celulares siguientes. Para grupos que presentan sistemas cromosómicos holokinéticos, la producción de nuevos cromosomas durante la evolución puede deberse a este tipo de procesos. En el caso de Mepraia un cambio de este tipo pudo haber originado diversificación parapátrida y expansión a ambientes más áridos en la variante resultante, además de cambios en la expresión de la morfología alar en el macho. Para conocer este estudio haz click aquí...


martes, junio 05, 2007

Chromosomic Speciation and Hopefull Monsters

Another interesting dimension of “monstrosity” and evolutions occurs as chromosomic speciation, where the occurrence of chances in chromosome number, hybridization etc drive the change and diversification of a group. These kinds of changes may lead to reproductive incompatibilities and isolation without a need of geographic barriers. For example, nearly 50% of all existing angiosperm species are estimated to have arisen by ancient polyploidy, more of them by allopolyploidy (doubling of the chromosome complement in a hybrid between two previously existing species) than by autopolyploidy (multiplication of the chromosome complement of a single species).
Between humans and chimpanzees, as is well known, there is a difference of just ~1.24% of their DNA sequences, but studies have confirmed that a substantial number of chromosomal rearrangements have occurred, nine chromosomes exhibit pericentric inversions between these species, and human chromosome 2 shows a fusion of two acrocentric chromosomes present in chimpanzees (chromosomes 12 and 13) and other great apes (chromosomes 11 and 12 in gorillas and orangutans)!. Furthermore, some authors has been proposed that protein evolution would happened more than 2.2 times faster in chromosomes that had undergone structural rearrangements compared with colinear chromosomes. Recent studies have suggested that gene gain and loss in the Y-chromosomes may contribute significantly to the divergence between humans and chimpanzees. Initial comparisons of the human and chimpanzee Y-chromosomes indicate that chimpanzees have a disproportionate loss of Y-chromosome genes, which may have implications for sex-specific as well as reproductive traits.
Now a case in invertebrates: in mosquitoes (Diptera, Culicide), in the genus Anopheles, (nearly 500 known sp.), there are no fewer than 170 cryptic taxa belonging to 30 complexes of closely related sps. Most siblings sps are outcomes of recent speciation processes detected by paracentric inversions (mostly involving the X chromosome), as well as ribosomal DNA sequences. The chromosome rearrangements act as mechanisms of crossover suppression for reorganized regulatory units of gene expression. Inside a cluster of seven sister sps (morphologically nearly indistinguishable but that differ in genetic and ecological attributes), lets talk about two most studied (because are vectors for malaria): Anopheles arabiensis and A. quadriannulatus and . Chromosome inversion patterns (in the X-chromosomes) indicate that A. arabiensis gave rise to A. quadriannulatus. (ANAGENTICALLY!). From this one the group diversified in several other sps and extended through Africa. It has been proposed that fixed X-chromosome inversions differentiated the sister species, this differentiation included habitat and host use diversification as incipient reproductive isolation factors between these sps and has been suggested as the preliminar isolation factors involved in the chromosomic speciation process.

Chromosome rearrangements like inversions, fussions or fissions are strong genetic reproductive barriers because they reduce recombination between heterokaryotypes and facilitate divergence, Thus “monstrosities” such as a chromosome change are key processes in the diversification of parapatric or sympatric populations. In spite of the relevance of cytogenetic processes, until today's day the degree of chromosomal differentiation necessary to consider two organisms as belonging to different species is unknown... Too much of faith in allmigthy structural DNA sequence Maybe?...



Ayala, FJ & Coluzzi M (2005) Chromosome speciation: Humans, Drosophila, and mosquitoes. PNAS,102: 6535–6542.

Forsdyke, DR (2003) William Bateson, Richard Goldschmidt, and non-genetic modes of speciation. Journal of Biological Systems, 11, 341-350.

Navarro, A & Barton NH (2003) Chromosomal Speciation and Molecular Divergence--Accelerated Evolution in Rearranged Chromosomes. Science, 300: 321-324.

Perry, GH, Tito, RY, & Verrelli, BC, (2007) The Evolutionary History of Human and Chimpanzee Y-Chromosome Gene Loss. MBE Advance Access published on March 1, 2007, DOI 10.1093/molbev/msm002. Mol Biol Evol 24: 853-859.