Mammalian+chromosome+evolution

Group 7

Name of group: Group secretary plus email: Negar Afshar Group members plus email: Vian Rajabzadeh, Dylan Smith, Marina Georgiou and Katie Emelianova. Chromosomal changes have resulted in the evolution of humans from primates.. ....other primates, they are primates, however, they have also resulted in genetic diseases such as Down Syndrome and Parkinsons disease. We aim to investigate whether these changes in humans can result in evolution or not by following Johannes Wienberg’s research on the evolution of eutherian chromosomes, which outlines the chromosomal changes that occurred over the evolution of humans from Afrotheria, the processes by which these occurred and the mechanisms behind the changes. To understand whether or not chromosomal changes in human disease are evolution, we must first have a look at the history of human genome and the evolution of human chromosomes. The role of chromosomal rearrangements in evolution is particularly well supported by several models. These models suggested that chromosomal changes are strong genetic barriers because they reduce recombination in heterokaryotypes. Such strong barriers would facilitate divergence in the rearranged region during the time when the diverging populations are in parapatry, i.e., have limited gene flow. Moreover Navarro A. & Barton H.N. (2003) supported that: “the accumulation of incompatibilities facilitated by chromosomal differences generates genetic barriers of growing strength that, eventually, produce complete reproductive isolation and, therefore, speciation”. This accumulation of positively selected alleles in chromosomes presenting rearrangements underlies the key prediction of this and similar models of chromosomal speciation: that positive selection should be stronger across chromosomes carrying rearrangements than in colinear chromosomes (Navarro A. & Barton H.N., 2003). ....good Within-population polymorphism for rearrangements has rarely been described in mammals. Human chromosomes 1, 4, 5, 9, 12, 15, 16, 17, and 18 are separated from their chimpanzee's homologs by pericentric inversions. Moreover human chromosome 2 is the result of the tandem fusion of the Great ape chromosomes 11 and 12, during the transition between early hominoids and humans (Navarro A. & Barton H.N., 2003). Furthermore, Wienberg (2004:657) has explored the genomic organization of eutherian chromosomes, which show a general “simple architecture” and there have been few changes. ....no figure Figure. 1 This karyotype highlights the differences of a suggested Afrotherian (the oldest clade of Eutheria) karyotype to modern hominoids.
 * Chromosomal changes in human disease; is this evolution? **

The Afrotherian karyotype differs only by a few arrangements compared to the human karyotype, although seemingly few changes in karotype have incurred quite large scale speciation and phenotypic changes. This represents changes at the genome scale, however Wienberg (2004) also explores chromosomal change at the individual chromosome level through chromosome painting. This involves fluorescent probes, which attach to chromosomes of similar mass and size (Wienberg., 2004:661). The technique which provided further insight to chromosomal change was fluorescent in situ hybridization with bacterial artificial chromosomes, which allowed a more sensitive analysis and was able to detect changes on a smaller scale on chromosomes. For example Wienberg (2004:661) state that “recent sequence analyses of the human genome numerous duplications of segments up to 100kb or more were identified”. He concludes the identification of these duplications occurring on modern day humans have a similar sequence to those in early hominoids ....how do they know? . This reflects the small changes in genome structure contributing greatly to speciation and the evolution of humans (Wienberg., 2004:661).

These increasingly sensitive techniques allowed the analysis of evolutionary derived chromosome- break-points, which are breaks in chromosomes that have led to karyotype changes resulting in evolution. These breakpoints are located on weak sequence lengths composed of low copy repeats. This analysis allows the tracking of evolution of apes and the location of preserved sequences (Wienberg., 2004:661). Carvalho //et al// (2010:1765) propose that “the study of genomic disorders has uncovered the essential role played by the genomic architecture, especially low copy repeats”. They further discuss the presence of LCR ....what is this? increases the chance of duplications, deletions, gene conversions and inversions, which contribute to many modern genetic diseases. In this specific research Carvalho //et al// (2010:1766) focuses on the “proximal 17p chromosome”, which represents an area of high low copy repeats. Low copy repeats (LCR’s) arise from duplication of parts of the genome. The proliferation of LCR’s is a result of the instability caused on the chromosome because of LCR’s, which give rise to non-allelic homologous recombination (NAHR), which is the recombination of highly homologous sequences that are not on alleles. These LCR’s have been preserved in genomes, and mutations that have led to increased diversity in karyotype have also been known to cause detrimental mutations in humans (Carvalho //et al//., 2010) An example from Carvalho //et al// (2010:1765) of gene duplication that has caused significant change in primate evolution, and moreover, has caused related detrimental effects in humans is the Opsin array, which consists of 2 genes: // OPN1LW // and // OPN1MW //, which are long wave sensitive and middle wave sensitive respectively. Gene duplications as a result of LCR’s led to the pigments present in primates to become more diverse and develop trichromatic vision. The Opsin locus is still in an LCR hotspot today, and as a result, further rearrangements resulted in a high frequency of male humans with defective colour vision, most frequently in differentiation between red and green colours (Carvalho //et al//., 2010) . In conclusion it can be said that, the changes in chromosomes from the early ancestral karotype have shown a slow rate of change. With regard to Wienberg (2004) there is an increase in aberrations as we increase proximity to the divergence of humans. Therefore chromosomal diseases can lead to human evolution but in long term, as the present individuals with chromosomal disorders are sterile ....usually, not always, and not totally. References: Carvalho, M.B.C, Zhang, F, Lupski, J.R.,2010. Genomic disorders: A window into human gene and genome evolution. //PNAS//. 107:1765-1771 Navarro A. & Barton H.N., 2003. Chromosomal Speciation and Molecular Divergence-Accelerated Evolution in Rearranged Chromosomes. //Science.// **300**. Available at: [|www.sciencemag.org] [Accessed at: 15th November 2010] Wienberg. W.,2004. The evolution of eutherian chromosomes. //Current// //Opinion in Genetics and Development//. 14:657-666 ....massively better