STUDIES ON THE CYTOLOGY OF YEASTS - VI. Lethal Chromosomal Mutations in Yeasts

Abstract

1. The  fate  of  the  cells  originating  as  a  result  of  mitotic  aberrations  in  the  distillery  yeast  has  been  investigated.  Yeasts  appear  to  be  more  favourable  for  a  study  of  the  chromosome  lethals,  since  changes  in  a  growing  population  could  be  observed  at  very  short  intervals.
2. Some  recent  publications  are  critically  reviewed  and  it  is  shown  that  the  cause  for  confusion  is  the  result  of  a  lack  of  appreciation  of  the  important  differences  in  the  cytological  behaviour  of  aerobically  growing  cultures  from  those  in  active  fermentation.
3. Abnormal  segregations  of  chromosomes  in  the  hexaploid,  pentaploid  and  triploid  cells  are  described.  Amitosis,  lagging  of  chromosomes,  micronuclei  formation  and  pycnosis  appear  to  be  more  frequent  in  the  hexa- and  pentaploids  than  in  triploids.  A  segregation  of  chromosomes  without  splitting  occurs  in  all  the  three  forms.
4. Regular  mitoses  occur  in  the  diploid  immediately  after  its  origin  by  mitotic  irregularities  in  the  various  stable  and  unstable  polyploid  cells.  Since  stable  diploids  could  not  be  isolated  by  serial  dilution  plates  it  appears  that  they  transform  themselves  into  tetraploids  by  "somatic  doubling".
5. Possession  of  two  or  four  chromosomes  does  not  mean  that  these  should  be  viable  combinations.  When  cells  apparently  having  chromosome  numbers  suggestive  of  viability  show  micronuclei  formation,  it  appears  that  different  pairs  of  chromosomes  in  the  original  tetraploid  may  not  have  identical  gene  sequences.
6. A  pictographic  summary  of  the  mitotic  irregularities  and  the  fate  of  the  various  types  originating  as  a  result  of  such  aberrations  is  given.
7. Mitotic  aberrations  appear  to  be  under  genetic  control  Cells  with  unbalanced  chromosomal  complements  attempt  to  produce  the  original  four  chromosome  condition  by  irregular  segregation  of  the  unsplit  chromosomes.  This  may  in  all  probability  be  a  regulating  and  stabilising  mechanism  to  compensate  the  lethal  effects  of  mitotic  abnormalities.  A  similar  mechanism  probably  operates  in  endopolyploid  cells  also.
8. If  tetraploidy  itself  is  the  result  of  a  gene  mutation,  the  same  mutant  allele  in  the  diploid  originating  from  a  tetraploid  should  produce  a  somatic  doubling
9. When  purity  of  gametes  is  meaningless  in  polyploids,  the  peculiar  cytological  behaviour  of  the  auto-tetraploid  distillery  yeast  emphasizes  the  need  for  caution  in  offering  radical  theories  to  explain  Some  curious  segregation  observed  in  hybridization  experiments.   therein  year  after  year.