Green algae phylogenomics

NAISS 2024/5-186


NAISS Medium Compute

Principal Investigator:

Charlie Cornwallis


Lunds universitet

Start Date:


End Date:


Primary Classification:

10615: Evolutionary Biology



Multicellularity has evolved independently ca. 25 times in the past 3.5 billion years, but it has also been lost. To understand mechanisms involved in the transition to and from multicellularity we need to establish the direction of evolution among uni- and- multicellular species. The volvocine algae are often depicted as an enigmatic example of a stepping-stone evolutionary process where unicellular organisms have given rise to more complex multicellular forms. Starting with the ancestral unicellular Chlamydomonas reinhardtii evolution proceeds to produce small simple multicellular taxa such as Tetrabaena socialis (four undifferentiated cells), then larger simple taxa such as Pandrina morum (16 undifferentiated cells), up to Volvox spp., that have several thousand cells and a division of labor (cell differentiation into germ and soma). Recent molecular phylogenies, however, provide preliminary evidence that this description of multicellular evolution is over-simplified for a number of reasons. Firstly, unicellular and multicellular species come out as polyphyletic in an 18S gene tree, indicating that multicellularity has evolved independently several times even within this young clade. Secondly, Volvox spp. are polyphyletic, indicating that division of labor either evolved independently on multiple occasions, or was lost in several lineages. Finally, examining groups that are more distantly related to the volvocines indicates that the most recent common ancestor of all volvocines (including C. reinhardtii and closely related unicellular species to Volvox spp.) encompasses lineages that are multicellular such as Paulschulzia pseudovolvox and Tetraspora spp. This suggests that the origins of multicellularity lie much deeper within the order Chlamydomonadales, and has subsequently been lost multiple times. The current interpretation that few genetic changes are required for the evolution of multicellularity may therefore be incorrect as the unicellular species that were previously multicellular have been used in comparative genomic studies. The available phylogenies for Chlamydomonadales, including more targeted volvocine trees, are based on sparse data: Limited numbers of genes have been used to construct trees and relatively few species have been genotyped. Because mechanistic inquiries into the transition between uni- and- multicellularity hinge on the relationships among species being well established, it is critical to reconstruct a robust phylogeny of the group. Through our project, we will reconstruct the phylogeny of the volvocine algae using powerful Bayesian and Maximum Likelihood methods on NGS data to resolve the evolutionary history of this important group.