Background:

The evolutionary pattern of dinophytes is complex and requires comprehensive taxonomic studies addressing the species' development and morphology, ecology, evolution, and genetics.

Questions:

How recent was the transition of D. baltica from Mexico to freshwater based on a calibrated phylogeny tree of the Kryptoperidiniaceae?

Studied species and data description:

Durinskia baltica, Kryptoperidiniaceae and the known fossil record of dinotoms (endosymbiont diatom-host dinophytes).

Study sites and dates:

Xochimilco, Mexico City. 2010 to 2019.

Methods:

Phylogenies were constructed with Bayesian inferences and host sequences were calibrated with a relaxed clock method.

Results:

We obtained genetic sequences of the 18S rDNA sector in the host and the rbcL sector in the endosymbiont. The Nitzschia-type endosymbiont is genetically distinct from endosymbionts of other dinotomes with the same diatom type. In contrast, considering the known freshwater species of Durinskia, the genetic relationship between hosts is robust and of recent origin. The antiquity of the Mexican sequence coincides with the geological origin of the Xochimilco basin, suggesting particularities in the evolutionary history of dinotomes linked to the geographic regions they inhabit.

Conclusions:

Our evolutionary scenario with a calibrated phylogeny indicates that the transition of Durinskia baltica to freshwater in the Xochimilco basin is geologically contextualized. The historical environmental change of Lake Xochimilco with respect to salinity corresponds with an initial colonization of D. baltica in a brackish environment and its subsequent adaptation to freshwater.

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