Filastereans and Ichthyosporeans: Models to Understand the Origin of Metazoan Multicellularity
The origin of animals or metazoans from their unicellular ancestors is one of the most important evolutionary transitions in the history of life. To decipher the molecular mechanisms involved in this transition, it is crucial to understand both the early evolution of animals and their unicellular prehistory. Recent phylogenomic analyses have shown that there are at least three distinct unicellular or colonial lineages closely related to metazoans: choanoflagellates, ichthyosporeans and filastereans. However, until recently, choanoflagellates had been the only lineage for which an entire genome sequence was available. Moreover, the lack of transgenesis tools in any of these unicellular lineages had precluded the possibility of performing functional analyses. To better understand the unicellular prehistory of animals, we have recently obtained the genome sequences of both filastereans and ichthyosporeans. Analyses of their genomes identified many important genes for metazoan multicellularity and development, some of which are absent from the choanoflagellate genomes and thus were thought to be metazoan-specific. We have also established methods for transgenesis and gene silencing in ichthyosporeans. The combination of genomic information and molecular tools in filastereans and ichthyosporeans facilitate efficient functional analyses to understand how the key genes in the evolution of multicellularity were co-opted during the unicellular-tomulticellular transition that gave rise to metazoans. We propose that filastereans and ichthyosporeans are ideal model organisms for investigating the origin of metazoan multicellularity.