Animal Origins Lecture 2 PDF

Organizing Invertebrate Life Lecture #2  Loads of single celled Eukaryotes Eukaryote tree of life (after Burki et al., 2019) Green = Clade inc. plants and algae Red = Clade inc. animals and fungi Opisthokonta Clade includes animals and fungi, plus several g...

Organizing Invertebrate Life Lecture #2  Loads of single celled Eukaryotes Eukaryote tree of life (after Burki et al., 2019) Green = Clade inc. plants and algae Red = Clade inc. animals and fungi Opisthokonta Clade includes animals and fungi, plus several groups of single-celled eukaryotes Share cells with single posterior flagellum and chitin synthesis Holozoa Clade includes animals and about 300 single-celled species All holozoic feeders Share unique cellular and genomic traits linked to animal multicellularity Choanoflagellatea Most closely related unicellular group to animals Morphologically similar to choanocytes of sponges Relationship supported by molecular data Found in marine, brackish, and freshwater environments Including hypoxic and hypersaline waters, and sea ice Free-living heterotrophs, filter-feeding on bacteria and small eukaryotes Can be solitary or colonial Can reproduce via sexual and asexual reproduction Metazoa Animals most likely derived from single celled eukaryotes resembling choanoflagellates Relationships between the five major clades is contentious BUT understanding these relationships is key to understanding animal evolution The Metazoa: Ctenophora - Comb-jellies Porifera - Sponges Placozoa - Blob-like animals composed of aggregations of cells Cnidaria - Jellyfish, anemones, and corals Bilateria – Bilaterally symmetrical animals MORPHOLOGY vs. MOLECULES  BUT, even molecular analyses differ! Anatomical and genetic clues are often obscured due to the hundreds of millions of years of evolution since the first animal lineages originated. Nielsen, et al. (2019): Morphology + Molecules Implications of Ctenophores as SOM Either: 1) Porifera lost the morphological features that Ctenophora share with Placozoa and Cnidaria OR 2) All shared features evolved independently Fundamental features include: the ectoderm & endoderm, extracellular digestion, and many critical eumetazoan genes Ctenophora first hypothesis *Gains (homoplasies) in red, Losses in blue Problematica The Ctenophore Problem Morphological data previously placed them as the sister taxon to Cnidaria Some molecular analyses place it in a more basal position Basal to ALL other metazoan clades Basal to all eumetazoan clades Porifera first hypothesis Metazoa Little evidence to refute the sister clade of Bilateria + Cnidaria BUT recent work suggests Placozoa may be sister group to Cnidaria Problematica The Cnadaria-Placozoa Problem Laumer et al. (2019) suggests Cnidaria and Placozoa are sister taxa Cnidaria + Placozoa clade would then be sister clade to Bilateria BUT sample size of placozoans small But wait… What if the Placozoa are the outgroup? This hypothesis proposes that the animal kingdom began as a colony of single celled organisms that received the initial stimulus for differentiation from gravity: Cells at the bottom of the colony would have been in contact with the hard substrate, while the cells on top would have faced a different environment. According to this hypothesis, these separate stimuli would have led to the evolution of the first two somatic cell types, and subsequently to the formation of an upper and a lower epithelium, as seen today in Placozoa. This hypothesis, the fact that placozoans are the most simple free-living extant animals and have the smallest nuclear (and the largest mitochondrial) genomes of all animals, have been brought forward as support for Placozoa as the SOM.  So, which one is right??? Anatomical and genetic clues are often obscured due to the hundreds of millions of years of evolution since the first animal lineages originated. Many schemes for Metazoan relationships Summary of fifteen studies and the phyla/subphyla included Black box = taxon included Green: Annelida, Arthropoda and Brachiopoda included in all Red: Cephalopoda rarely included (2!) Many schemes for Metazoan relationships Likewise, lots of different terms have been used to name taxon groups Branching Level: Many schemes for Metazoan relationships Radiata likely outdated! Does Eumetazoa exist, if Porifera isn’t the SOM?!? Branching Level: Many schemes for Metazoan relationships 5 major clades well-supported, though relationships still contentious Branching Level: Many schemes for Metazoan relationships Within the Bilateria: Protostomia, unlike Deuterostomia, not universally recognized Branching Level: Phylogeny and divergence times of the major deuterostome taxa * Or outgroup to Bilateria Peterson et al (2016) Problematica The Xenacoelomorpma Problem Morphological and molecular data identify phylum Xenacoelomorpha as outgroup to all other Bilateria Lack a traditional gut and body cavity So, which one is right??? Ecdysozoa Laumer et al (2019) Howard et al (2022) Phylogenomics + fossil data Spiralia/Lophotrochozoa The Spiralia/Lophotrochozoa Problematica Sister clade to the Ecdysozoa Spiralia = Protostomes that undergo spiral cleavage Lophotrochozoa = Protostomes that have either a lophophore or a trochophore larval form Rouphozoa Phyla Gastrotricha and Platyhelminthes All lack lophophores, most lack trochophores, all have spiral cleavage Gnathifera Phyla Gnathostomulida, Rotifera, Micrognathozoa and Chaetognatha All lack lophophores, some lack trochophores, some lack spiral cleavage Laumer et al (2019)