Felix J. Tapia

Ene 14

Demostrada una gran divergencia entre los cromosomas Y del hombre y el chimpancé

Publicado el 14-01-2010 en Academia, Actualidad, Biología y Medicina, Ciencia pura, Genética y sus manipulaciones, Noticia | Sin comentario

La revista Nature presenta un trabajo multicéntrico liderizado por David C. Page, con la secuenciación completa de la región que determina la masculinidad en el cromosoma Y (MSY) del chimpancé. La comparación de la MSY del chimpancé con la del hombre demuestra una divergencia evolutiva importante con mas genes y secuencias estructurales para el primero.

¿Quiere decir esto que el chimpancé es mas evolucionado que el hombre sexualmente? No. Todo parece indicar que esta variabilidad de un 30% se debe a cuatro factores sinergísticos: el papel de la MSY en la producción de esperma, las recombinanciones ectópicas dentro de la MSY, la ausencia de recombinaciones meióticas que promueven “aventones” o mudanzas genéticas y las diferentes conductas en el apareo sexual.

Dicho en otras palabras, los primos del hombre modificaron muchos mas sus MSY en los últimos 6 millones de años, para procrearse en forma efectiva, cualidad que los humanos resolvieron en forma mas simple.

El cromosoma Y de humanos y chimpancés difiere en un 30%

La gran diferencia indica que el mecanismo genético masculino ha evolucionado mucho en los últimos seis millones de años

MALEN RUIZ DE ELVIRA - Madrid
ELPAIS.com - Sociedad – 13-01-2010

En 2003 se completó la secuencia genética del cromosoma Y humano, el que determina el sexo masculino, pero hasta ahora no se había hecho lo propio con el del chimpancé, la especie más próxima evolutivamente a la humana. La comparación de ambos cromosomas Y ha sorprendido a los científicos, porque sus regiones específicamente masculinas (alrededor del 95% del total) son notablemente distintas, tanto en su estructura como en los genes que contienen. Mientras que el genoma completo de ambas especies coincide en un 98,8%, el del cromosoma Y difiere en más de un 30%. Además, en el del chimpancé hay muchos menos genes que en el del ser humano, tanto porque ha perdido genes respecto al ancestro común como porque el humano los ha ganado.

Dado que ambas especies se separaron hace seis millones de años, este resultado indica que ha habido una evolución muy rápida en el cromosoma Y humano, explican en la revista Nature los científicos, liderados por David C. Page, del Instituto Whitehead del MIT (EE UU). Entre los factores que han contribuido a esta “extraordinaria divergencia”, señalan los autores, están el papel predominante de esta región del cromosoma en la producción del esperma, las diferencias en el comportamiento sexual de ambas especies (en el chimpancé varios machos copulan con la misma hembra en un mismo ciclo ovulatorio, por que el esperma funciona en un marco de competencia) y diferentes mecanismos de recombinación.

El cromosoma Y, foco de atención especial desde su descubrimiento, ha resultado ser muy especial, tanto en su comportamiento genético como en sus características genómicas. Los dos cromosomas sexuales, el X y el Y, se originaron hace centenares de millones de años, a partir de un cromosoma ancestral no sexual, durante la evolución de los distintos sexos en los seres vivos. En la actualidad, la secuencia de pares de bases del Y es tres veces más corta que la del X. Por eso se ha supuesto que es un cromosoma degenerado, que ha ido perdiendo la carga genética no relacionada con la determinación sexual y que en el futuro podría incluso llegar a desaparecer.

Su secuenciación en 2003 demostró, sin embargo, que es muy complejo y que se renueva constantemente. El nuevo estudio remacha la insuficiencia de las teorías de degeneración decelerada.

Resumen del artículo:

Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content

Jennifer F. Hughes, Helen Skaletsky, Tatyana Pyntikova, Tina A. Graves, Saskia K. M. van Daalen, Patrick J. Minx, Robert S. Fulton, Sean D. McGrath, Devin P. Locke, Cynthia Friedman, Barbara J. Trask, Elaine R. Mardis, Wesley C. Warren, Sjoerd Repping, Steve Rozen, Richard K. Wilson, David C. Page

Nature advance online publication 13 January 2010 | doi:10.1038/nature08700

The human Y chromosome began to evolve from an autosome hundreds of millions of years ago, acquiring a sex-determining function and undergoing a series of inversions that suppressed crossing over with the X chromosome1, 2. Little is known about the recent evolution of the Y chromosome because only the human Y chromosome has been fully sequenced. Prevailing theories hold that Y chromosomes evolve by gene loss, the pace of which slows over time, eventually leading to a paucity of genes, and stasis3, 4. These theories have been buttressed by partial sequence data from newly emergent plant and animal Y chromosomes5, 6, 7, 8, but they have not been tested in older, highly evolved Y chromosomes such as that of humans. Here we finished sequencing of the male-specific region of the Y chromosome (MSY) in our closest living relative, the chimpanzee, achieving levels of accuracy and completion previously reached for the human MSY. By comparing the MSYs of the two species we show that they differ radically in sequence structure and gene content, indicating rapid evolution during the past 6?million years. The chimpanzee MSY contains twice as many massive palindromes as the human MSY, yet it has lost large fractions of the MSY protein-coding genes and gene families present in the last common ancestor. We suggest that the extraordinary divergence of the chimpanzee and human MSYs was driven by four synergistic factors: the prominent role of the MSY in sperm production, ‘genetic hitchhiking’ effects in the absence of meiotic crossing over, frequent ectopic recombination within the MSY, and species differences in mating behaviour. Although genetic decay may be the principal dynamic in the evolution of newly emergent Y chromosomes, wholesale renovation is the paramount theme in the continuing evolution of chimpanzee, human and perhaps other older MSYs.

The fickle Y chromosome. Chimp genome reveals rapid rate of change
nature.com
13/01/2010
Lizzie Buchen

The male sex chromosome, long dismissed as the underachieving runt of the genome, has now been fully sequenced in a common chimpanzee. And comparison with its human counterpart — the only other Y chromosome to have been sequenced in such detail — reveals a rate of change that puts the rest of the genome to shame.

The common chimp (Pan troglodytes) and human Y chromosomes are “horrendously different from each other”, says David Page of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, who led the work. “It looks like there’s been a dramatic renovation or reinvention of the Y chromosome in the chimpanzee and human lineages.”

Sex chromosomes evolved some 200 million–300 million years ago, but the chimpanzee and human lineages diverged only 6 million–7 million years ago. Comparisons of the chimp and human genomes suggested that not much has changed between the species since1.

But those analyses excluded the Y chromosome, much of the genetic sequence of which is made up of palindromes and elaborate mirrored sets of bases that elude standard whole-genome sequencing techniques. Portions of the chimp Y chromosome were sequenced a few years ago2,3, but the full landscape is only now available, after Page and his team precisely sequenced large segments of the chromosome, then stitched them together. They report their findings in a paper published online in Nature on 13 January4.

As the earlier studies had suggested, many of the stark changes between the chimp and human Y chromosomes are due to gene loss in the chimp and gene gain in the human. Page’s team found that the chimp Y chromosome has only two-thirds as many distinct genes or gene families as the human Y chromosome and only 47% as many protein-coding elements as humans. The remainder of the chimp and human genomes are thought to differ in gene number by less than 1%.

Even more striking than the gene loss is the rearrangement of large portions of the chromosome. More than 30% of the chimp Y chromosome lacks an alignable counterpart on the human Y chromosome, and vice versa, whereas this is true for less than 2% of the remainder of the genome.

Even the portions that do line up have undergone erratic relocation. In the only other chromosome to have been sequenced to the same degree of completeness in both species, chromosome 21, the authors found much less rearrangement.

“If you’re marching along the human chromosome 21, you might as well be marching along the chimp chromosome 21. It’s like an unbroken piece of glass,” says Page. “But the relationship between the human and chimp Y chromosomes has been blown to pieces.”

The rapid evolution of the Y chromosome is not a total surprise, because the Y chromosome has no partner during cell division and so largely avoids the exchange of DNA that occurs between partnered chromosomes and keeps modifications in check. “It’s expected that they are going to be more different than the rest of the genome, but the extent of it is pretty amazing,” says geneticist Christine Disteche at the University of Washington in Seattle.

The Y chromosome is also prone to change because most of its characterized genes are involved in producing sperm, which are at the frontline of reproductive fitness, particularly in chimps; receptive females will often mate with many males in one session, so the male with the most virile sperm has the highest likelihood of success.

“The Y is full of surprises,” Page says. “When we sequenced the chimp genome people thought we’d understand why we have language and write poetry. But one of the most dramatic differences turns out to be sperm production.”

References
1. The Chimpanzee Sequencing and Analysis Consortium . Nature 437, 69-87 (2005).
2. Hughes, J. F. et al. Nature 437, 100-103 (2005).
3. Kuroki, Y. et al. Nature Genet. 38, 158-167 (2006).
4. Hughes, J. F. et al. Nature doi:10.1038/nature08700 (2010).