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| Experimental analysis of inducible morphology
Questions relating to the interaction between an organism's genetic background and environmental factors are fundamental in evolutionary biology. Phenotypic plasticity - the conditional expression of characters depending on environmental cues - is generally accepted to be associated with spatially and/or temporally variable environments 1,2. In assessing the adaptive significance and fitness consequences of a novel inducible morphology in zooids of the bryozoan Electra pilosa, we developed further the experimental and analytical methodologies that have proved reliable and successful in previous studies 3-6. The primary objective of this project was to ascertain whether plasticity for the trait confers increased fitness in this abundant epifaunal species. We developed a model laboratory system for the study of a complex inducible phenotypic plasticity in the encrusting cosmopolitan marine bryozoan, Electra pilosa. This species is widespread, abundant and ecologically especially important as an epibiont of seaweeds on European shores. We have identified the inductive mechanism underlying a discrete (all-or-nothing) phenotypic plasticity - extended zooidal spine formation - which apparently is unique in being a response not to a biotic environmental component (i.e. contact with predators or competitor species), but to an abiotic factor, wave-related abrasion 3. Extended spines, at least in clusters, probably protect feeding lophophores from mechanical disturbances and/or damage. However, neither this assumption nor the hypothesis that the plastic expression of the trait is in itself adaptive (= "Adaptive Plasticity Hypothesis") have been tested experimentally. Most studies of phenotypic plasticity are confined either to laboratory or field experimentation, and investigations of the adaptive aspects of plasticity by reciprocal experiments or reciprocal transplants are still greatly lacking 7. Further developments of this project would involve experimental testing of adaptive effects of both the extended spines and plasticity for their expression, in the field and the laboratory. Also, fundamental characteristics of the spine formation response, such as the time/intensity threshold for the inductive cue, remain to be investigated. References
Inducible Morphology (Elongate Spines) in the Marine Bryozoan Electra pilosa Bryozoans are colonial organisms that form an important component of marine fouling communities in all of the world's oceans. Bryozoan colonies are made up of asexually produced modules called zooids, which typically consist of a calcareous exoskeleton and a feeding structure called the polypide. Colonies may comprise hundreds to thousands of zooids. To date, our work has focused mainly on Electra pilosa, an apparently cosmopolitan species which typically inhabits ephemeral substrata in the intertidal and shallow sublittoral, and is probably the ecologically most successful bryozoan species in British waters.
Modular organisms like E. pilosa frequently evolve pronounced phenotypic plasticity in response to the ecological challenges resulting from passive dispersal of offspring into unpredictable habitats, and temporal variability of the environment colonized by the immobile adult stage. This bryozoan is potentially long-lived and reproduces by means of a long-term planktonic larval stage called the cyphonautes. During the larval development period, prior to settlement and metamorphosis to the benthic form, the larvae are presumed to be dispersed considerable distances from the parental habitat. E. pilosa colonies on wave-exposed shores differ morphologically from those found on sheltered shores (Figure above) in possessing numerous long-spined zooids (Figure below).
Our recent work demonstrates that spine formation in E. pilosa is environmentally inducible. Unlike other examples of inducible morphology, which involve responses to biotic factors (such as predators or potential space competitors), this inducible morphology in E. pilosa is apparently unique in being triggered by an abiotic factor, wave-related abrasion by macroalgae (Bayer et al. 1997). The elongate spines do, however, also have a fortuitous anti-predator effect in discouraging predation by the specialist nudibranch molluscs Adalaria proxima and Polycera quadrilineata. The inducible spines of E. pilosa appear to constitute an adaptation for the protection of feeding polypides (Figure below) in high-energy environments, and plasticity for the trait presumably is of adaptive value in this organism which exploits a diverse range of physically unpredictable habitats. Although a number of traits in this species clearly are subject to considerable phenotypic plasticity, other attributes apparently are highly deterministic, heritable and genotype-specific. E. pilosa displays pronounced among-genotype variation in colony growth rate, and our work shows that much of this variation is due to proximate factors which affect growth rate and covary with genotype (Bayer & Todd 1996). Recently, we also have been able to show the first evidence of senescence at the zooid level in E. pilosa (Bayer & Todd 1997): the feeding polypide with each zooid is repeatedly 'rejuvenated' by complete tissue breakdown and regeneration in a cyclical manner. We showed that individual zooids deteriorate systematically over time, as indicated by their decreasing polypide life spans and increasing polypide regeneration times. None the less, whole-organism (colony) senescence does not appear to occur in this species. References (for full list and abstracts see M. Bayer Homepage:)
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