Invertebrates: 2026FA Developmental Biology (BIO-350-MM01)

Animal Diversity

Developmental diversity from unique sets of developmental innovations

phylogeny

The evolutionary history and relationships among species, often depicted as a branching tree.

Parazoa/Porifera

No true tissues, no germ layers, asymmetric, irregular holoblastic cleavage, Have choanocytes and archeocytes

Choanocytes

Flagellated collar cells that drive water flow and capture food. Remarkably similar to choanoflagellates (one of the unicellular relatives of animals); the strongest piece of evidence for animal origins.

Archeocytes

Totipotent adult stem cells that can become any other cell type. Incredible regenerators; if you dissociate a sponge into single cells, they reaggregate into a fully functional sponge

Eumetazoa/Cnidarians

True tissues, diploblastic = ecto/endo derm layers in epithelial sheets with mesoglea in between, radial symmetry, oral/aboral axis, true gut and epidermis and neurons in nerve jet and life cycle polymorphism

Oral/Aboral signaling

Wnt - High orally for radiata / Low Orally for Bilaterians

Diploblastic

In Cnidarians, ectoderm and endoderm are organized into epithelial sheets (true tissues) with true gut, epidermis, and neurons in nerve jet

Mesoglea

Between endoderm and ectoderm, acellular jelly

Life cycle polymorphism

Alternation between asexual polyp and sexual medusa

Bilateria

True tissues, endo / ecto / meso, bilateral AP/DV axis, holo/mero cleavage, cephilization and colinear hox clusters and through gut and coelum. Two groups being Protostomes and Deuterostomes.

Cephalization

Creation of head

Through-gut

mouth + anus (separation of ingestion and excretion)

Coelom

(fluid-filled body cavity lined by mesoderm); space for organ development

Protostomes/Lophotrochozoa

Spiral cleavage, highly conserved, Autonomous specification, trochophore larvae, Cephalopods

Spiral cleavage

distinct clockwise/counterclockwise rotation of blastomeres at each division to make macromeres and micromeres. Highly conserved (few groups have broken out of this pattern); indicates very successful developmental strategy.

Autonomous specification

4d micromere (yes, it is larger than the 4D macromere) generates the entire endomesoderm, Easy to track cells and reproduce results.

Trochophore larva

free-swimming larval form with a ciliated band for swimming and feeding. Shared among many groups, morphologicalevidence of shared ancestry even though adults look different.

Cephalopods

Lose innovations, meroblastic cleavage, convergent evolution with vertebrates, skips larval stage.

Protostomes/Ecdysozoa

Variable cleavage patterns, molting, segmentation, drosophilia

Ecdysis (molting)

rigid cuticle is shed periodically to allow growth, hormonally controlled, Hard exoskeleton

Segmentation

Hox genes cause

Drosophila - Segment development

Hierarchical GRN that converts broad overlapping expression domains into discreet boundaries

Drosophila Segmentation

1.)AP axis gradients generated from maternally deposited mRNA

2.)gap genes covering broad regions (mutually repress)

3.) pair-rule genes expressed in every other segment

4.)segment polarity genes establishing AP boundary of each segment

5.)hox genes activated by the specific combo of all three sets of segmentation genes

Deuterostomes

Blastopore becomes anus (Mouth in protostomes), Radial cleavage, specify blastomeres conditionally, enterocoely(bud their coeloms from their gut cavity)

Chordata/Tunicates

Closest relatives, notochord, neural tube, segments, pharyngeal slits