Uma análise cladista sobre a origem evolucionária do homem

The Evolutionary Origin of Man Can Be Traced in the Layers of Defunct Ancestral Alpha Satellites Flanking the Active Centromeres of Human Chromosomes

Valery A. Shepelev1, Alexander A. Alexandrov1, Yuri B. Yurov2, Ivan A. Alexandrov2*

1 Institute of Molecular Genetics, Russian Academy of Sciences, Moscow, Russia, 2 Mental Health Research Centre, Russian Academy of Medical Sciences, Moscow, Russia

Abstract

Alpha satellite domains that currently function as centromeres of human chromosomes are flanked by layers of older alpha satellite, thought to contain dead centromeres of primate progenitors, which lost their function and the ability to homogenize satellite repeats, upon appearance of a new centromere. Using cladistic analysis of alpha satellite monomers, we elucidated complete layer patterns on chromosomes 8, 17, and X and related them to each other and to primate alpha satellites. We show that discrete and chronologically ordered alpha satellite layers are partially symmetrical around an active centromere and their succession is partially shared in non-homologous chromosomes. The layer structure forms a visual representation of the human evolutionary lineage with layers corresponding to ancestors of living primates and to entirely fossil taxa. Surprisingly, phylogenetic comparisons suggest that alpha satellite arrays went through periods of unusual hypermutability after they became “dead” centromeres. The layer structure supports a model of centromere evolution where new variants of a satellite repeat expanded periodically in the genome by rounds of inter-chromosomal transfer/amplification. Each wave of expansion covered all or many chromosomes and corresponded to a new primate taxon. Complete elucidation of the alpha satellite phylogenetic record would give a unique opportunity to number and locate the positions of major extinct taxa in relation to human ancestors shared with extant primates. If applicable to other satellites in non-primate taxa, analysis of centromeric layers could become an invaluable tool for phylogenetic studies.

Author Summary

The primate centromere evolves by amplification of alpha satellite sequences in its inner core, which expands and moves the peripheral sequences sideways, forming layers of different age in the “pericentromeric” area. The expanding centromere model poses two main questions: (1) whether the succession of layers is symmetrical on both sides of the centromere, and (2) whether different chromosomes share the same layers. We have analyzed and dated the layers on both sides of human chromosomes 8, 17, and X and shown that they were largely symmetrical on one chromosome and largely shared and arranged similarly in non-homologous chromosomes. The layer pattern revealed that genome-wide waves of expansion of new satellite variants have occurred repeatedly in the human evolutionary lineage. The layers which are likely to be the relic centromeres of our common ancestors with primate taxa follow each other in chronological order. The two layers that do not match any living primate indicate the two completely extinct ancestral taxa aged 26–40 and 18–23 million years. These could be Propliopithecidae (Cathopitecus and Egyptopithecus) and Pliopithecidae (Proconsul), aged 33–35 and 17–27 million years, respectively. The possibility to reveal and date extinct ancestors makes the analysis of satellite layers a unique tool for the reconstruction of primate phylogeny.

Citation: Shepelev VA, Alexandrov AA, Yurov YB, Alexandrov IA (2009) The Evolutionary Origin of Man Can Be Traced in the Layers of Defunct Ancestral Alpha Satellites Flanking the Active Centromeres of Human Chromosomes. PLoS Genet 5(9): e1000641. doi:10.1371/journal.pgen.1000641

Editor: M. Katharine Rudd, Emory University School of Medicine, United States of America

Received: March 2, 2009; Accepted: August 11, 2009; Published: September 11, 2009

Copyright: © 2009 Shepelev et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: The authors received funding from their institutions. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

* E-mail: ivanalx@hotmail.com

+++++

Free PDF here/PDF gratuito do artigo aqui.