Origins of multicellular complexity: Volvox and the volvocine algae

Abstract

The collection of evolutionary transformations known as the 'major transitions' or 'transitions in individuality' resulted in changes in the units of evolution and in the hierarchical structure of cellular life. Volvox and related algae have become an important model system for the major transition from unicellular to multicellular life, which touches on several fundamental questions in evolutionary biology. The Third International Volvox Conference was held at the University of Cambridge in August 2015 to discuss recent advances in the biology and evolution of this group of algae. Here, I highlight the benefits of integrating phylogenetic comparative methods and experimental evolution with detailed studies of developmental genetics in a model system with substantial genetic and genomic resources. I summarize recent research on Volvox and its relatives and comment on its implications for the genomic changes underlying major evolutionary transitions, evolution and development of complex traits, evolution of sex and sexes, evolution of cellular differentiation and the biophysics of motility. Finally, I outline challenges and suggest future directions for research into the biology and evolution of the volvocine algae.

Positive size–speed relationships in gametes and vegetative cells of Chlamydomonas reinhardtii ; implications for the evolution of sperm

“…We confirm with very large samples that the gametes of C. reinhardtii are isogamous, that vegetative cells from different mating types are also monomorphic in size, and we show for the first time that the mating types of both cell types are monomorphic in swimming speed again using a very large sample. This is important verification of C. reinhardtii in its position as closely representing the putative ancestral state for the evolution of anisogamy (Herron and Michod ; Hallmann ; Herron ), as well as a suitable model for the evolution of mating type associated size and speed differences.…”

Multicellular Features of Phytoplankton

“…In populations of the freshwater alga Chlorella as well as Chlamydomonas, a unicellular Volvocine alga, clumping occurred in response to predation as a defense mechanism (Sathe and Durand, 2016;Kapsetaki et al, 2017). Volvocine algae have been studied as a model system for the transition from unicellular to multicellular life (Kirk, 1999;Herron, 2016). This group includes species of clumping unicells such as Chlamydomonas, and the complex multicellular species of Volvox exhibiting division of labor into non-reproductive cells (Kirk, 1999;Hanschen et al, 2014;Herron, 2016).…”

“…Volvocine algae have been studied as a model system for the transition from unicellular to multicellular life (Kirk, 1999;Herron, 2016). This group includes species of clumping unicells such as Chlamydomonas, and the complex multicellular species of Volvox exhibiting division of labor into non-reproductive cells (Kirk, 1999;Hanschen et al, 2014;Herron, 2016). Genetic analysis of Volvocine algae revealed the key role of extracellular matrix in the transition to multicellularity (Merchant et al, 2007;Prochnik et al, 2010).…”

Volvox: A simple algal model for embryogenesis, morphogenesis and cellular differentiation

“…CRISPR-mediated gene targeting has been reported for Chlamydomonas (Shin et al, 2016) and could probably be adapted for use in Volvox and other volvocine species. Additional tools and resources developed in recent years will make tackling some of the above questions easier (Herron, 2016; Umen and Olson, 2012). Foremost among these are genome sequences that provide a foundation for molecular genetics and genomics approaches that were not available during the heyday of Volvox forward genetics in the 1970s-2000s.…”