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Independent Origins

Volvox; refered to as a colony by certain researchers

  • Multicellular organisms with differentiated cells originated multiple times
  • Red, green, and brown algae arose independently from each other
  • Ex:

VOLVOX - multicellular green algae with two differentiated cells

CHLAMYDOMONAS - unicellular, cell homologous (similar) to those of Volvox

  • Suggests that transition from unicellularity to multi- can result from changes in how existing genes are used (does not require origin of novel genes)

25.2 Multicellularity has Originated

Several Times in Eukaryotes

novel gene: gene that has never been identified as a gene before ("new")

Multicellular Colonies

The fact that organisms originated multiple times allows us to compare and contrast how independent groups were originated

  • 1st multicellular forms (occur early in fossil record and also remain common today)

  • Colonies: collections of cells that are connected to one another but show little or no cellular differentiation (specialization)

  • Found in eukaryotic lineages whose members have rigid cell walls and some who do not
  • The former remain attached by shared cell walls
  • Latter held together by proteins that connect to each other

Steps in the Origin of Multicellular Animals

Multicellular Volvox compared to unicellular Chlamydomonas

  • Protist groups closely related to animals serve as information to help track origin
  • Origin of multicellularity required evolution of cells - adhering and signaling each other
  • Research - Identifying specific proteins that played roles in origin

True: Origin resulted from co-opting genes used for a different purpose (or converting existing genes to have different uses)

False: Origin resulted from evolution of a "toolkit" made of novel genes

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