Genomics comparisons provide new insights into the evolution of karyotype and body patterns in Anguilliformes species.

Anguilliformes species not only possess distinctive appearance (such as body elongation and absence of pectoral and/or pelvic fins), but also display diversity in chromosome number, supporting them as a suitable model for studying karyotype evolution and related molecular mechanisms of evolutionary body patterns. However, the ancestral chromosomes and evolutionary chromosomal reorganization in various eels have not been reported yet. The most regulatory or related genes of their distinctive appearance are still unknown. Here, we predicted an eel-based ancestral chromosome karyotype for the first time, and revealed multiple chromosomal fusion and fission events that reduced the ancestral chromosome number from haploid n = 21 to the commonly extant n = 19 within the Anguilla lineage. Moreover, we carried out a genome-wide comparison of two significant gene families including homeobox (Hox) and T-box (tbx), revealing genomic loss of some Hox genes (such as HoxB9β and HoxD13α) and variation of certain tbx gene (i.e., tbx5) may be responsible for the evolutionary development of pectoral fins. Interestingly, loss of certain secretory calcium-binding phosphoprotein (SCPP) genes was identified in various eel genomes, which possibly contribute to the common reduction of scales. Overall, our current findings provide new insights into evolutionary karyotype and body pattern evolution across diverse Anguilliformes species.