EVOLUTION OF DEFENSE: TRADEOFFS AND CONVERGENCE IN BRYOZOA

Life has developed a wide array of phenotypic defenses against predators and parasites. Investment in defense is costly and is associated with a variety of tradeoffs. These tradeoffs may be allocational (investing in defense reduces growth rate) or functional (a structure modified for defense may have poor performance for other functions). The strength of these tradeoffs may shape the evolution of defense, and our ability to predict defensive strategies.

Bryozoans are colonial, sessile invertebrates that exhibit a range of defenses, both at the colony-level and the individual-level. The repeated convergent evolution of defensive strategies in this group allows us to investigate 1) the importance of allocational tradeoffs in defensive evolution; 2) whether the adaptive landscape can predict convergent defensive morphologies; and 3) if increased division of labor (DOL) reduces functional tradeoffs.

To investigate allocational tradeoffs between different defenses, we ran several multi-state Markov models and found that loss rate of an expensive defense (avicularia) was higher in the absence of other defenses. Instead of an allocation tradeoff between defenses, this highlights different life-history strategies in bryozoans; heavy investment in defense vs limited defenses and investment in growth.

The predictability of convergent defensive morphologies (avicularium mandible shape) was investigated using geometric morphometrics and simulated evolution. We found that the magnitude of morphological convergence in the empirical data was entirely possible under Brownian motion (BM) and did not require an adaptive explanation. However, the actual convergent morphologies were unlikely under BM, suggesting that adaptation may still be an important driver of convergent defenses.

Some bryozoan species have extremely high levels of defensive DOL. While we predicted that increased DOL would reduce functional tradeoffs and result in the absence of intermediate (potentially suboptimal) defensive morphologies. However, colonies with high defensive DOL did not show increased clustering at optima or loss of intermediate forms. Thus, high morphological DOL does not necessarily ease functional tradeoffs.

Overall, our results reveal the nuanced impact of allocational and functional tradeoffs on the evolution of defense in a colonial organism.