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Coloniality has evolved once in Stolidobranch Ascidians
Liyun Zeng1, Molly W Jacobs, Billie J Swalla
1Department of Biology and Center for Developmental Biology, University of Washington Box 351800, Seattle, WA 98195-1800, USA.
Integrative and Comparative Biology
|June 16, 2011
Summary
Coloniality in ascidians likely evolved only once from a solitary ancestor within the Stolidobranchia order. This study used molecular data to analyze evolutionary relationships and coloniality in ascidian families.
Area of Science:
- Marine Biology
- Evolutionary Biology
- Genomics
Background:
- Ascidians (sea squirts) display diverse life histories, including solitary and colonial forms.
- The order Stolidobranchia contains both solitary and colonial species, with intermediate forms also observed.
- Understanding the evolution of coloniality is key to ascidian diversity.
Purpose of the Study:
- To investigate the number of independent evolutionary events of coloniality within the ascidian order Stolidobranchia.
- To clarify the phylogenetic relationships among families within the Stolidobranchia, specifically Molgulidae, Styelidae, and Pyuridae.
- To determine the ancestral life history state (solitary vs. colonial) of the Stolidobranchia.
Main Methods:
- Phylogenetic analyses were conducted using full-length 18S ribosomal DNA (rDNA) and partial cytochrome oxidase B gene sequences.
- Molecular data were analyzed using Maximum Likelihood (ML) and Bayesian inference methods.
- Taxonomic sampling included species from the families Molgulidae, Styelidae, and Pyuridae.
Main Results:
- Phylogenetic analyses confirm that the orders Phlebobranchia and Stolidobranchia are sister groups.
- The family Molgulidae is monophyletic and proposed for elevation to subordinal rank.
- Styelidae and Pyuridae were resolved as distinct monophyletic groups, differing from some previous studies.
- A single clade within Styelidae includes both colonial and solitary species, indicating coloniality evolved once within this family.
- These findings suggest that the ancestor of Stolidobranchia was solitary.
Conclusions:
- Coloniality within the Stolidobranchia appears to have evolved only a single time.
- The family Styelidae contains a clade demonstrating the evolution of coloniality from solitary ancestors.
- Further research on other ascidian orders (Aplousobranchia, Phlebobranchia) is needed to ascertain the total number of coloniality origins in Ascidiacea.
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