Animals and Their Unicellular Ancestors

Abstract

Animals belong to the Opisthokonta, one of the major divisions of the eukaryotic Tree of Life. This supergroup also includes other well‐known groups such as fungi and choanoflagellates, in addition to some newly discovered unicellular taxa as the Ichthyosporea or the Filasterea. To unveil the origin of animal multicellularity, it is vital to understand the evolution of their single‐celled relatives, as they might hold key genetic clues that might help us understand how the unicellular ancestors of animals became animals. Our current knowledge of unicellular animal relatives, their specific phylogenetic relationships and the role they might play in future research is being improved, thanks to molecular data. Key Concepts: Animals are members of the opisthokonts. The Opisthokonta clade includes animals, fungi and several unicellular lineages. Choanoflagellates are the closest animal unicellular relatives. Ichthyosporea and Filasterea lineages are most closely related to animals than to fungi. The opisthokonts include several multicellular types. The opisthokont is a key evolutionary window to understand the unicellular to multicellular transition.

An epithelial tissue in Dictyostelium challenges the traditional origin of metazoan multicellularity

“…Consistent with this idea, colony formation and morphology are influenced by presence of a specific bacterial prey species 47. Another group of metazoan relatives, the ichthyosporeans, also transitions between a unicellular amoeboid state and simple spherical colonies 48, 49. The cellular slime mold Fonticula alba exhibits facultative multicellularity very similar to Dictyostelium , despite being more closely related to fungi than to social amoebae 39.…”

Environmental Survey Meta-analysis Reveals Hidden Diversity among Unicellular Opisthokonts

“…Recent phylogenetic and phylogenomic analyses have shown that the opisthokonts also include several unicellular lineages (Steenkamp et al 2006). These include the nucleariids (Amaral-Zettler et al 2001), Fonticula alba (Brown et al 2009), the filastereans (Shalchian-Tabrizi et al 2008), the ichthyosporeans (also known as DRIPs or mesomycetozoeans) (Ragan et al 1996), Corallochytrium limacisporum (Raghu-Kumar 1987), and the choanoflagellates (Carr et al 2008) (for a review see Paps and Ruiz-Trillo 2010). Although we have some knowledge of the diversity of metazoans, fungi, and choanoflagellates, our current understanding of the evolutionary diversity of the other opisthokont lineages is very poor.…”

Novel Diversity of Deeply Branching Holomycota and Unicellular Holozoans Revealed by Metabarcoding in Middle Paraná River, Argentina

“…Holozoa is the group that contains Metazoa and their closest unicellular relatives, namely Choanoflagellata, Filasterea, Ichthyosporea, and Plurimorfea, the latter with an uncertain phylogenetic position in relation to Ichthyosporea (Paps and Ruiz-Trillo, 2010;Torruella et al, 2015;Hehenberger et al, 2017). Holozoa also contains some exclusively environmental clades, mainly within choanoflagellates and ichthyosporeans (del Campo and Ruiz-Trillo, 2013).…”

“…This lack of knowledge of eukaryotic diversity also occurs in Opisthokonta, the superclade that comprises Metazoa, Fungi and several unicellular lineages (Ruiz-Trillo et al, 2008;Paps and Ruiz-Trillo, 2010;Torruella et al, 2012). These unicellular opisthokonts are understudied compared to their multicellular relatives, even though they are essential for better understanding key evolutionary transitions, including those that took place at the origins of both animals and fungi (Ruiz-Trillo et al, 2007;Richter and King, 2013;Brunet and King, 2017;Richards et al, 2017;Sebé-Pedrós et al, 2017).…”