Distribution and Dynamics of a Defensive Symbiosis in the Bugula neritina (Bryozoa) Sibling Species Complex

Geographical differences in selective pressure may shift the relationship outcome in mutualistic symbioses from positive to negative. The marine bryozoan Bugula neritina is a colonial invertebrate common in temperate waters worldwide. Evidence suggests that an uncultured vertically transmitted symbiont, “Candidatus Endobugula sertula,” hosted by B. neritina produces the polyketide bryostatins, which protect vulnerable larvae from predation. Studies of B. neritina along the Western Atlantic coast revealed a complex of two morphologically similar sibling species separated by an apparent biogeographic barrier: the Type S sibling species was found south of Cape Hatteras, North Carolina, while Type N was found to the north. Interestingly, the Type N colonies lacked E. sertula and defensive bryostatins; their documented distribution was consistent with traditional biogeographical paradigms of latitudinal variation in predation pressure. Upon further sampling of B. neritina populations, we found that both host types occur in wider distribution, with Type N colonies living south of Cape Hatteras, and Type S to the north. Distribution of the symbiont, however, was not restricted to Type S hosts. Genetic and microscopic evidence demonstrates the presence of the symbiont in some Type N colonies and larvae, and they are apparently endowed with defensive bryostatins. Molecular analysis of the symbiont from Type N colonies suggests an evolutionarily recent acquisition, which is remarkable for a symbiont thought to be transmitted vertically only. Furthermore, most Type S colonies found at higher latitudes lack the symbiont, indicating that this relationship may be more flexible than previously thought. Transplant and common-garden experiments further suggest that the endosymbiont’s geographical range is mediated by a combination of environmental impact on symbiont growth and a fitness cost on the host imposed by association with E. sertula. These results provide insight into possible mechanisms of regulation of context-dependent mutualisms

Saving up or settling down: Home ownership over the life cycle

In a Bewley model with endogenous price volatility, home ownership and mobility across locations and jobs, we assess the contribution of financial constraints, housing illiquidities and house price risk to home ownership over the life cycle. The model can explain the rise in home ownership and fall in mobility over the life cycle. While some households rent due to borrowing constraints in the mortgage market, factors that affect propensities to save and move, such as risky house values and transactions costs, are equally important determinants of the ownership rate. © 2013 The Authors

Latitudinal variation of a defensive symbiosis in the Bugula neritina (Bryozoa) sibling species complex.

Mutualistic relationships are beneficial for both partners and are often studied within a single environment. However, when the range of the partners is large, geographical differences in selective pressure may shift the relationship outcome from positive to negative. The marine bryozoan Bugula neritina is a colonial invertebrate common in temperate waters worldwide. It is the source of bioactive polyketide metabolites, the bryostatins. Evidence suggests that an uncultured vertically transmitted symbiont, "Candidatus Endobugula sertula", hosted by B. neritina produces the bryostatins, which protect the vulnerable larvae from predation. Studies of B. neritina along the North American Atlantic coast revealed a complex of two morphologically similar sibling species separated by an apparent biogeographic barrier: the Type S sibling species was found below Cape Hatteras, North Carolina, while Type N was found above. Interestingly, the Type N colonies lack "Ca. Endobugula sertula" and, subsequently, defensive bryostatins; their documented northern distribution was consistent with traditional biogeographical paradigms of latitudinal variation in predation pressure. Upon further sampling of B. neritina populations, we found that both host types occur in wider distribution, with Type N colonies living south of Cape Hatteras, and Type S to the north. Distribution of the symbiont, however, was not restricted to Type S hosts. Genetic and microscopic evidence demonstrates the presence of the symbiont in some Type N colonies, and larvae from these colonies are endowed with defensive bryostatins and contain "Ca. Endobugula sertula". Molecular analysis of the symbiont from Type N colonies suggests an evolutionarily recent acquisition, which is remarkable for a symbiont thought to be transmitted only vertically. Furthermore, most Type S colonies found at higher latitudes lack the symbiont, suggesting that this host-symbiont relationship is more flexible than previously thought. Our data suggest that the symbiont, but not the host, is restricted by biogeographical boundaries

<i>B. neritina</i> sibling species collected by haphazard sampling.

<p>Proportions at each site indicated by blue (Type N) and red (Type S) in charts.</p

Symbiotic status of sampled colonies.

<p>Proportions at each site indicated by black (symbiotic) and gray (aposymbiotic) in charts. Results from both haphazardly collected and targeted samples are included. Number of colonies sampled shown beside each graph.</p

Co-occurrence of Type N and Type S <i>B. neritina</i>.

<p>Colonies collected together in (A) Beaufort, NC, and (B) Oyster, VA. Blue arrows indicate Type N colonies; red indicate Type S.</p

<i>Bugula neritina</i> symbiont frequency along North American Atlantic coast.

<p>Symbiont occurrence was determined both for colonies collected haphazardly and in targeted sampling (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108783#s2" target="_blank">Materials and Methods</a>); not all haphazardly selected colonies were assayed for symbiont presence. Dashes indicate host phylotypes not found in a location during sampling.</p><p><i>Bugula neritina</i> symbiont frequency along North American Atlantic coast.</p

Percent identity of marker sequences among <i>B. neritina</i> sibling species.

<p>Types D and S show the greatest COI identity, but Type D differs from both Types S and N in symbiont markers.</p><p>Percent identity of marker sequences among <i>B. neritina</i> sibling species.</p