The tempo and mode of cryptobiosis evolution within tardigrades are still unknown, but it is clear that comprehending this process is the key to better understand evolutionary history and ecology of this phylum, and the process of animal terrestrialization, given the ubiquitous continental distribution of tardigrades. Our study was mainly aimed at timing tardigrade radiation and key events in tardigrade evolution. Given the existence of two lineages of continental tardigrades, i.e. Eutardigrada and Echiniscidae, we decided to implement a relaxed molecular clock based approach to attempt to derive a minimal time interval for tardigrade terrestrialization. Data from new and Genbank partial 18S and 28S rDNA sequences of 41 specimens, belonging to 31 species from 26 genera, representing all known tardigrade orders and superfamilies, were acquired. Phylogenetic inference was achieved using Bayesian mixture models that have allowed the most accurate estimates of evolutionary rates. The following molecular clock analyses were then based on 3 tardigrade calibration points derived from fossil records and improved by the addition of 5 new calibrations points pertaining to Arthropoda, Priapulida and Kinoryncha, emerged from paleontological studies. To clarify the evolutionary history of cryptobiosis, and to evaluate whether its origin might have played a role in the process of tardigrade terrestrialization, a maximum-likelihood based ancestral character state reconstruction was used. Molecular phylogeny analyses yielded a robustly supported and well-resolved evolutionary tree for all considered tardigrade taxa. Our results suggest the origin of the tardigrade stem group in the Ediacaran age (~620 Million years ago -Mya), with the two major extant tardigrade lineages (Eutardigrada and Heterotardigrada) split quite recently ~443 Mya. Ancestral character state reconstruction indicated a probability of ~30% for cryptobiosis to have been present in the last common ancestor of Heterotardigrada and Eutardigrada. Hence, it is most likely that this adaptive trait evolved twice independently within this phylum. Cryptobiosis most likely emerged in the time interval between 443 and 359 Mya, in the stem Eutardigrada lineage that existed for ~84 My. This is in accordance with available fossil evidence suggesting terrestrial ecosystems first flourished in the Silurian. The second independent origin (within Heterotardigrada) was within the Echiniscoidea clade and can be dated to an interval included between 238 and 141 Mya (Middle Triassic to early Cretaceous). In both cases, existence of a link between cryptobiosis and terrestrialization was clear. All the eutardigrade superfamilies originated within a short-time span of ~257-200 Mya, suggesting an early Mesozoic eutardigrade radiation. Moreover, within each parachelan superfamily a great mid Jurassic to Early Cretaceous process of diversification was recorded.
|Data di pubblicazione:||2012|
|Autori:||Marchioro, T.; Rota-Stabelli, O.; Rebecchi, L.; Guidetti, R.; Pisani, D.|
|Titolo:||Dating tardigrade evolution and early terrestrialization events.|
|Appare nelle tipologie:||Abstract in Atti di Convegno|
File in questo prodotto:
I documenti presenti in Iris Unimore sono rilasciati con licenza Creative Commons Attribuzione - Non commerciale - Non opere derivate 3.0 Italia, salvo diversa indicazione.
In caso di violazione di copyright, contattare Supporto Iris