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Y-­‐chromosome  evolu.on  in  humans

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Y-­‐chromosome  evolu.on  in  humans  

 

Ana  María  Vázquez  Mochales                            Gene4cs  Degree.  Faculty  of  Biosciences.  Universitat  Autònoma  de  Barcelona  (UAB)  

The   human   Y-­‐chromosome   has   increasingly   reduced   its   size   through   its   evolu4onary   history,   currently   being   the   shortest   of   the   23   human   chromosomes.  This  raises  ques4ons  as  to  whether  it  will  con4nue  to  shorten,  and  if  so,  down  to  which  size?  Did  it  already  stop  losing  genes,  or   will  it  eventually  disappear  completely?  

Sex  chromosomes  

Chromosomes  that  determine   the  sex  of  an  organism.  

 

Chromosome  degenera.on   Process  of  gene  loss.  

 

Heterochroma.c  

Formed  by  heterochroma4n,   a  4ghtly  packed  form  of  DNA,   typically  gene4cally  inac4ve   and  containing  repe44ve   sequences  and  few  genes.  

 

Euchroma.c  

Formed  by  euchroma4n,  a   lightly  packed  form  of  DNA,   typically  rich  in  gene  

concentra4on  and  gene4cally   ac4ve.  

 

Recombina.on  

Breaking  and  rejoining  of  DNA   strands  to  form  a  new  

combina4on  of  gene4c   informa4on.  

 

Gene  conversion   Non-­‐reciprocal   recombina4on.  

 

Chromosomal  transposi.on   Movement  of  DNA  from  one   loca4on  to  another.  

 Chromosomal  inversion   Rearrangement  in  which  a   segment  of  a  chromosome  is   reversed  end  to  end.  

 Proto-­‐sex  chromosomes   New  pair  of  chromosomes   that  recently  acquired  a  sex-­‐

determining  func4on  but  that   otherwise  contains  iden4cal   genes.  

 

Beneficial  muta.on  

Muta4on  that  increases  the   survivorship  or  fecundity  

(fitness)  of  its  carrier.  

 

Deleterious  muta.on  

Muta4on  that  decreases  the   survivorship  or  fecundity  

(fitness)  of  its  carrier.  

 

Muller’s  ratchet  

Irreversible  accumula4on  of   deleterious  muta4ons  in  a  

non-­‐recombining  popula4on.  

 

Gene.c  hitchhiking  

Fixa4on  of  a  deleterious   muta4on  that  is  linked  to  a   beneficial  allele.  

 

Ruby  in  the  rubbish  model   Elimina4on  of  a  beneficial   muta4on  that  is  linked  to  a   deleterious  allele.  

Objec4ves   Methodology  

Results  

Conclusions  

Without  neglec4ng  that  the  X-­‐chromosome  is  equally  relevant,  this  study  will  focus   on  Y-­‐chromosome.  

 

!  Which  are  the  processes  underlying  Y-­‐chromosome  degenera4on?  

 

!  Compara4ve  study  of  a  variety  of  diploid  species  

Study  based  on  scien4fic  literature  research  using  Pubmed  tool  (NCBI)  and  Google   Scholar.    

 

Keywords:  sex-­‐chromosomes,  Y-­‐chromosome,  evolu7on  and  degenera7on.  

 

Selec4on   of   principal   interest   papers   through   abstracts.   Mind-­‐blowing   diversity.  

Exercise  of  synthesis  and  concre4on.  

OPEN  QUESTIONS      

   

  What  remains  is  a  sex-­‐determina4on  system,  regardless  of  how  it  is  accomplished.  

Further   compara4ve   analysis   of   Y-­‐chromosomes   at   different   ages   of   differen4a4on   will   give   major   insights   into   the   characteris4cs   and   the   evolu4onary  forces  that  act  on  sex  chromosomes.    

References    

1

 

Bachtrog  D.  2013.  Y-­‐chromosome  evolu4on:  emerging  insights  into  processes  of  Y-­‐chromosome  degenera4on.  Nat  Rev  Genet  14:  113–124.  

2    Navarro-­‐Costa  P.  2012.  Sex,  rebellion  and  decadence:  The  scandalous  evolu4onary  history  of  the  human  Y-­‐chromosome.  Biochim  Biophys  Acta.  1822:  1851-­‐63.  

Structural  classifica.on  and  func.onal  aspects  of  Y-­‐chromosomes  

Figure  1.B  DNA  units  sharing  more  than  99%  sequence  iden4ty  (amplicons)  are  represented   by  an  iden4cal  colour  code.  Gray  blocks  denote  single-­‐copy  genes  (taken  from  Ref.  2).  

Forma.on  of  human  sex  chromosomes  

Figure  2  Graphic  representa4on  of  the  poten4al  path  leading  to   heteromorphic  sex  chromosomes  (taken  from  Ref.  1).  

Figure  3  Gene  decay  caused  by  a  shuhng  down  of  X-­‐Y  crossing  over  can  be  

consequence  either  of  an  accumula4on  of  deleterious  muta4ons  (Muller’s  ratchet   and  gene4c  hitchhinking)  or  of  a  lower  rate  of  adapta4on  (ruby  in  the  rubbish)  

(taken  from  Ref.  1).  

Figure  5  Comparison  between  a  simple  model  of  constant  gene  decay  and  

ex4nc4on  of  the  Y-­‐chromosome  (red  line),  theore4cal  approxima4ons  (green  line)   and  computer  simula4ons  (*)      (taken  from  Ref.  1)  .    

Figure  4  Neo-­‐sex  chromosomes  in  Drosophila  species.  Neo-­‐sex  chromosomes  (in   green)  and  ancestral  sex  chromosomes  (in  grey).  0  million  years  is  set  as  the  

4me  of  their  origin  (taken  from  Ref.1).    

Popula.on  processes  underlying  Y-­‐chromosome  degenera.on    

Forma.on  of  Drosophila  neo-­‐sex  chromosomes  

Evolu.onary  dynamics  of  Y-­‐chromosome  degenera.on  

!  Why  not  all  homomorphic  sex  chromosomes  stop  recombining  with  each  other  and  thus  become  heteromorphic  over  long  evolu4onary   periods?  

!  If  gene4c  diversity  is  one  of  the  cornerstones  of  evolu4on,  how  can  a  chromosome  largely  devoid  of  recombina4on  with  a  meio4c  partner   survive?  

!  Can  any  predic4ons  be  made  about  the  evolu4onary  fate  of  the  Y-­‐chromosome?  

1  

2   3  

4  

5  

Figure  1.A  Y-­‐chromosomes  in  different  species  (taken  from  Ref.  1).  

A   B  

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