Regensburg, Univ., Diss., 2003. / Computerdatei im Fernzugriff.

Regensburg, Univ., Diss., 2002. / Computerdatei im Fernzugriff.

Regensburg, Univ., Diss., 2002. / Computerdatei im Fernzugriff.

Regensburg, Universiẗat, Diss., 2003.

Regensburg, Universiẗat, Diss., 2003.

Regensburg, Universiẗat, Diss., 2002.

Univ., Diss.--Regensburg, 1998.

Thesis--Cologne. / In Periodical Room.

Regensburg, Univ., Diss., 2005.

The evolution of multicellularity is an example of an evolutionary transition in individuality, in which a group of lower-level biological units (cells, in this case) emerges as a higher-level unit (the multicellular organism) with its own fitness, heritability and individuality. The volvocine green algae are a model system for the transition to multicellularity and for the evolution of cellular differentiation. Some of the developmental changes that collectively make up this transition have occurred more than once in the volvocine lineage; others have reverted from derived to ancestral states. The transition from cells to multicellular organisms began over 200 million years ago in this lineage, and the subsequent changes have been sporadic, with several important changes occurring early in the transition and some body plans remaining largely unchanged over long evolutionary time scales. Two suites of characters that differ among species within the genus Volvox have each evolved convergently or in parallel in lineages that diverged at least 175 million years ago. This complex history suggests that other origins of multicellularity may have involved important roles for cooperation, conflict and conflict mediation; parallel evolution of some traits; sporadic rather than constant change; and long-term coexistence of forms with different levels of complexity. Data from one species, Pleodorina starrii, support motility as a major selective pressure driving the the origins of cellular differentiation. Optimization of the proportion of soma in this species appears to be prevented by a constraint that prevents independent change in colonies with different numbers of cells. Finally, P. starrii presents an exceptionally high level of phenotypic variability, suggesting that the genotype-phenotype map has not completely shifted from the cell to the colony and that the transition to a new, higher-level individual in this species is incomplete.