Chromosomal origins of caveolin. (A) Samples across animal diversity and in two unicellular relatives of animals were used to determine the ALG identities of chromosomes on which caveolins were present. (B) Caveolins are present on BCnS ALG Eb-bearing chromosomes in all but two species observed within the Parahoxozoa. In sponges, caveolins are present on ALG N in species from two anciently diverged sponge clades. The BCnS ALG identity of S. rosetta chromosome 10, on which caveolin is present, does not have a significant ALG identity. (C–E) Possible models for the chromosomal origins of caveolin. (C and D) Assuming that caveolin was present in the choanozoan ancestor, it must have been lost in the ctenophore lineage. Caveolin’s chromosomal provenance is parsimoniously equally likely to have been ALG N, followed by translocation to ALG Eb in the parahoxozoan ancestor (C), or ALG Eb in the ancestral myriazoan genome, followed by translocation to ALG N in the common ancestor of Demospongiae and Homoscleromorpha sponges. (E) Another potential explanation of caveolin’s provenance that accounts for its presence in both animals and choanoflagellates is HGT. This could have occurred in either direction after acquisition of the gene (i.e., from choanoflagellates to animals, or from animals to choanoflagellates). BCnS, bilaterian–cnidarian–sponge; HGT, horizontal gene transfer.