Botryllus schlosseri

The native range of Botryllus schlosseri is the north eastern Atlantic Ocean, the Mediterranean Sea and the North Sea.^[1] Its range has spread over the last 100 years to a nearly worldwide extent. Ranging in the western Atlantic Ocean from the Bay of Fundy to North Carolina, it is regarded as an invasive species and is "the most common colonial tunicate in North America."^[7]

Biology

Botryllus schlosseri is used as a model organism. Clones have been maintained in continuous laboratory culture for several decades, with new adults developing from buds that form from the body wall of existing adults. Under typical culture conditions, asexual reproduction occurs on an approximately two week cycle, during which a new bud will grow and begin to actively feed, while the adult it emerged from regresses and is eventually re-absorbed.^[8][9]

When sexually productive, these Botryllus are known to produce,"yellowish-white or pale orange tadpole larva" exhibiting an oval outline.^[10]

Colonial tunicates are the only chordates that are able to reproduce both sexually and asexually.^[11][12][13] B. schlosseri is a sequential (protogynous) hermaphrodite, and in a colony, eggs are ovulated about two days before the peak of sperm emission.^[14] Thus self-fertilization is avoided, and cross-fertilization is favored. Although avoided, self-fertilization is still possible in B. schlosseri. Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23.1% vs. 1.6%).^[14] Also a significantly lower percentage of larvae derived from self-fertilized eggs metamorphose (51.5% vs 87.2%), and the growth of the colonies derived from their metamorphosis is significantly slower. These observations suggest that self-fertilization leads to inbreeding depression associated with developmental deficits likely arising from expression of deleterious recessive mutations.^[15]

Genome

The genome has been sequenced. It is 580 megabases in length organised into 16

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