Telomerase and alternative lengthening of telomeres coexist in the regenerating zebrafish caudal fins

Telomeres are essential for chromosome protection and genomic stability, and telomerase function is critical for organ homeostasis. Zebrafish is a useful vertebrate model for understanding cellular and molecular mechanisms of regeneration. The regeneration capacity of the caudal fin of wild-type zebrafish is not affected by repetitive amputation, but the behaviour of telomeres during this process has not yet been studied. Here, we characterize the regeneration process in a telomerase-deficient zebrafish model, and study the regenerative capacity after repetitive amputations at different ages. We find that the regenerative efficiency decreases with aging in all genotypes but telomere length is maintained even in telomerase-deficient fish. Our data indicate that telomere length can be maintained by the regenerating cells through the recombination-mediated Alternative Lengthening of Telomeres (ALT) pathway, which likely supports high rates of cell proliferation during the caudal fin regeneration process.