The signs of adaptive mutations identified in the chloroplast genome of the algae endosymbiont of Baikal sponge.

Background: Monitoring and investigating the ecosystem of the great lakes provide a thorough background when forecasting the ecosystem dynamics at a greater scale. Nowadays, changes in the Baikal lake biota require a deeper investigation of their molecular mechanisms. Understanding these mechanisms is especially important, as the endemic Baikal sponge disease may cause a degradation of the littoral ecosystem of the lake. Methods: The chloroplast genome fragment for the algae endosymbiont of the Baikal sponge was assembled from metagenomic sequencing data. The distributions of the polymorphic sites were obtained separately for the genome fragments from healthy, diseased and dead sponge tissues. Results: The distribution of polymorphic sites allows for the detection of the signs of extensive mutations in the chloroplasts isolated from the diseased sponge tissues. Additionally, the comparative analysis of chloroplast genome sequences suggests that the symbiotic algae from Baikal sponge is close to the Choricystis genus of unicellular algae. Conclusions: Mutations observed in the chloroplast genome could be interpreted as signs of rapid adaptation processes in the symbiotic algae. The development of sponge disease is still expanding in Baikal, but an optimistic prognoses regarding a development of the disease is nevertheless considered.