The predicted link between telomeres/telomerase and the lifespan is not universally valid
Most research on telomere length and telomerase activity has been conducted on determinate growers such as mammalian or bird species, and it collectively proposes that (1) telomerase activity and telomere restoration both contribute to cell proliferation and regenerative potential; (2) telomerase is not active in all life stages or tissue types, which causes telomere shortening and replicative senescence in the majority of somatic cells; for this reason, telomerase activity acts as a determinant of organismal development, an anticancer mechanism, and a marker of organismal aging; (3) telomerase activity and telomere length in proliferative cells of adults can be used to predict lifespan (Cong et al. 2002; Blasco 2007; Haussmann et al. 2007; Tan et al. 2012; Hatakeyama et al. 2016; Whittemore et al. 2019). However, from a broad perspective, the connection between the length of telomeres and predicted lifespan or the reason of telomerase activity in somatic tissues of numerous animal species are not clear. Numerous studies have found wide variation in telomere and telomerase dynamics in relation to ectothermy/endothermy (note that ectothermy is frequently associated with indeterminate growth), reproductive mode, the organism’s regenerative potential, and environmental conditions (Gomes et al. 2010; Sköld et al. 2011; Sauer et al. 2021; Smith et al. 2021). For example, telomerase activity and telomere length have been studied in sea urchins, which are indeterminate growers, in the context of the long-lived Strongylocentrotus franciscanus (live over 100 years) and the short-lived Lytechinus variegatus (with an estimated lifespan of only 3-4 years). While both species exhibit telomerase activity in their somatic tissues, L. variegatus possessed longer telomeres (average length of approximately 21 kb) compared to those ofS. franciscanus (average telomere length of approximately 5.5 kb). Moreover, no differences in telomere length were observed between the young and old individuals. This suggests that sea urchins do not utilize telomerase repression as a mechanism to suppress neoplastic transformation (as a decline of telomerase is hypothesized to prevent tumor development, see below), and the continuous telomerase activity in somatic tissues of the sea urchins is explained by the indeterminate growth of these organisms throughout their lifespan (Francis et al. 2006). Similarly, somatic telomerase activity is proposed for all species with indeterminate growth, including fungal mycelium, meristematic plant tissue, various invertebrate species, reptiles, amphibia, and fish (Klapper et al. 1998a, b; Gomes et al. 2010; Gruber et al. 2014). Furthermore, numerous studies have shown that telomerase activity is unrelated to telomere length, as reviewed in Smith et al. (2021), or that it remains constant or increases with age in animal species with both determinate and indeterminate growth (Francis et al. 2006; Hartmann et al. 2009; Anchelin et al. 2011; Hoelzl et al. 2016; Korandová and Frydrychová 2016; Koubová et al. 2021a; Sauer et al. 2021). Finally, telomerase and telomere length can be regulated by environmental factors such as stress exposure or seasonal effects (Entringer et al. 2011; Young et al. 2013; Rollings et al. 2014; Mu et al. 2015; Koubová et al. 2021b; Smith et al. 2021).