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. 2022 Feb 10;119(7):e2113589119. doi: 10.1073/pnas.2113589119

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Copyright © 2022 the Author(s). Published by PNAS.

This open access article is distributed under Creative Commons Attribution License 4.0 (CC BY).

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Fig. 3. — Comparison of the number of putative orthologous genes on chromosomes containing the ArSSP1 and ArSSP2 genes in A. rubens and chromosomes containing the SS-type precursor gene in the cephalochordate B. floridae and the ASTC-type precursor gene in the mollusk P. maximus. (A) ArChr18 (contains SS2 gene) compared with B. floridae chromosome 14 (BfChr14; contains SST gene) = 252 putative orthologs. (B) A. rubens chromosome 18 (ArChr18; contains SS2 gene) compared with P. maximus chromosome 16 (PmChr16; contains ASTC gene) = 258 putative orthologs. (C) B. floridae chromosome 14 (BfChr14; contains SST gene) compared with P. maximus chromosome 16 (PmChr16; contains ASTC gene) = 283 putative orthologs. (D) A. rubens chromosome 6 (ArChr6; contains SS1 gene) compared with B. floridae chromosome 14 (BfChr14; contains SST gene) = 33 putative orthologs. (E) A. rubens chromosome 6 (ArChr6; contains SS1 gene) compared with P. maximus chromosome 16 (PmChr16; contains ASTC gene) = 21 putative orthologs. (F). A. rubens chromosome 18 (ArChr18; contains SS2 gene) compared with A. rubens chromosome 6 (ArChr6; contains SS1 gene) = 44 putative orthologs. This intraspecies chromosomal analysis provides a comparator for the interspecies chromosomal analysis in A–E. Chromosome lengths and gene positions can be inferred from markers at 1- and 5-Megabase (M) intervals. The large numbers of putative orthologs identified in the comparisons in A (252), B (258), and C (283) provide evidence that the A. rubens SS2 precursor gene, B. floridae SS precursor gene, and P. maximus ASTC precursor gene are located on chromosomes that are extensively derived from the urbilaterian chromosome that contained a gene that is the common ancestor of bilaterian SS/ASTC-type precursor genes. Conversely, the smaller number of putative orthologs in D (33) and E (21) indicates that the ArSSP1 gene is located on a chromosome that is to a much lesser extent derived from the urbilaterian chromosome that contained a gene that is the common ancestor of bilaterian SS/ASTC-type precursor genes.