DIVERSITY, POPULATION STRUCTURE, AND HOLOBIONT ORGANIZATION OF BOTRYLLID ASCIDIANS WITH A FOCUS ON BOTRYLLUS SCHLOSSERI IN RELATION TO ENVIRONMENTAL PARAMETERS ALONG TURKISH COASTS

Date

2026-1-22

Author

Tohumcu, Begüm Ece

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Botryllid ascidians, especially Botryllus schlosseri, are used as model organisms for studies on immune system evolution, developmental biology, and whole-body regeneration. Ecologically, botryllid ascidians, as sessile filter feeders, serve as efficient bioindicators in coastal ecosystems and exist as holobionts that harbor diverse microbial communities within. In addition to their ecological and evolutionary significance, detailed information regarding botryllid diversity and the genetic composition of B. schlosseri populations in Turkish coastal waters has remained limited locally. In this thesis context, the diversity of botryllid ascidians and the population structure of B. schlosseri along the Turkish coastline, including the Black Sea, Sea of Marmara, Aegean Sea, and Eastern Mediterranean, were studied. Specimens were located in between 2023 and 2024 from 26 stations selected as representative of diverse hydrographic conditions and microhabitats. Phenotypic variability was evaluated through morphotype analyses, whereas mitochondrial COI sequences and microsatellite markers were employed to assess genetic diversity and population structure. Host-associated microbial communities were characterized through 16S and 18S rRNA amplicon sequencing, and environmental variables were incorporated to define their holobiont system components. The findings indicate significant environmental variability among basins and microhabitats. Eight species (Botrylloides niger, Botrylloides israeliensis, Botryllus humilis, Botryllus schlosseri, Botryllus gaiae, Botrylloides sp. (Saudi Arabia, ON053356)), Symplegma brakenhielmi, and Polyclinum constellatum) were reported, along with an additional botryllid lineage identified at the Konacık station that could not be aligned with existing reference databases. B. schlosseri was the sole botryllid species exhibiting a complete north-south distribution, albeit with an uneven presence. The research additionally documents the first record of B. gaiae in Turkish coastal waters. Population analyses revealed significant genetic diversity in B. schlosseri, characterized by 83 distinct COI haplotypes and pronounced spatial heterogeneity. A total of 68 morphotypes were recorded, and the variation among morphotypes was predominantly independent of mitochondrial genetic structure. Holobiont analyses revealed distinct differences between tissue-associated and seawater-derived microbial communities, identified multiple potential bacterial symbionts, and uncovered a highly specialized tissue-associated eukaryotic component predominantly comprised of the obligate parasite Lankesteria. This thesis serves as the first comprehensive assessment of botryllid ascidian diversity across Turkish waters by synthesizing environmental, genetic, morphological, and microbial data, thereby providing essential baseline information for further phylogenetic, ecological, and taxonomic research.

Subject Keywords

Botryllus schlosseri, population genetics, diversity, eDNA, Turkish coastlines

URI

https://hdl.handle.net/11511/119000

Collections

Graduate School of Marine Sciences, Thesis