Evolution and ecology in modular organisms

Cheilostome bryozoans

Bryozoan zooidmodules interact with other units and are integrated both within themselves and in an inclusive whole. They originate from budding loci and differentiate as populations of cells. However, they are also inclusive of another level of modularity, in that organs differentiate as entities or parts of the zooids. Both levels of modularity can be individuated using key criteria of evolutionary developmental biology. Crucially, the fossil record of bryozoans sometimes preserves not only the ancestral zooid condition, but also different stages of polymorph differentiation among species. Inferences of homology among zooids and zooid parts enable me to compare morphological sequences of incipient polymorphism in deep time with analogous living colonies in which the feeding zooids and non-feeding zooid polymorphs co-occur. It's another virtue of working with colonial animals.

            Modular evolution leading to zooid polymorphism in cheilostomes has occurred repeatedly among lineages. My ongoing work with several collaborators shows that this  convergent evolution also occurs among at least three functionally and morphologically distinct parts of the body organization: (1) the zooid costal shield and the ascus, (2) the matrotrophic nutrition of larvae and the development of skeletal ovicells, and (3) the peculiar polymorphs called avicularia . Each occurrence profoundly affected the diversification of this clade, being correlated with patterns of increasing taxonomic richness. In all of these cases, evidence of predation by small invertebrate epibionts supports the view that “division of labor” manifest in the morphology of zooids likely evolved partly in response to a persistent, diffuse selective force. The calcified costal shield protects an otherwise exposed flexible membrane of the body cavity. Muscles acting on this membrane are necessary to protrude the feeding tentacles by a hydrostatic mechanism. The ascus is a water compensation sac that occurs in species where the membrane is reduced in area by the encroachment of this protective costal shield. Ovicells protect the developing larvae outside of the main body cavity. Avicularia differ from other zooids in their ability to trap and hold would-be predatory epibionts, often until their death.

            Both parcellation and integration are thought to be pathways of modular evolution. In derived modular structures, parcellation is usually described as a process of specialization of existent structures, and integration as a process of assembly. While the evidence of process is incomplete, polymorphic avicularia appear to have evolved through parcellation and vestigialization. Feeding zooids and nonfeeding polymorphic avicularia were derived from an ancestral form most similar to the feeding zooid. Species in a living bryozoan genus and a Cretaceous one also show a skeletal transformation series between feeding zooid and avicularium. However, modular integration has also occurred repeatedly in the evolution of bryozoan frontal shields and ovicells, from the integration of spines. These patterns suggest that overt structural modularity in bryozoans facilitates the generation of complexity and evolutionary flexibility, or evolvability. Yet conserved developmental pathways at some hierarchical level(s) retain elements of an entrenched zooid body plan in feeding zooids and polymorphs alike. This is a research direction that I believe is ripe for scientific innovation combining living and fossil perspectives.

            Going forward, I will look for model systems among groups of closely related cheilostome species that show different suites of polymorph expression, abundant Neogene fossil records, and readily accessible modern populations. The starting point for this project is a high quality cDNA library. The next step will be producing gene sequence tags, either (cheaply) one at a time or (efficiently but more expensively) by large scale EST sequencing. The tissue and organ targets of specific genes are tagged and visualized in preserved colonies to investigate similarities and differences during zooid polymorph development. I will carry out the morphological component of the project. The combined fossil-living approach we are planning requires a keen focus on aspects of skeletal morphology (hence, fossil patterns) that are correlated with the soft-tissue anatomy and the expression studies, for example variation in zooids and polymorphs related to feeding organ eversion versus trapping epibionts. The requirement for collecting appropriate fossil lineages with modern representatives (all showing degrees of polymorphism) will best be met by focusing on Neogene cheilostomes from depositional environments represented by multiple localities, for which ecologically similar habitats are also accessible today.

            Phylogenetic frameworks are certainly the best grounding for evolutionary developmental comparisons such as these, but a molecular phylogeny for bryozoans is in its infancy. Together with two molecular systematists and a morphological taxonomist, I plan to apply for NSF funding to perform a phylogenetic analysis of a circum-global bryozoan genus, Bugula, whose morphology and development are among the most studied of any cheilostome. Bugula species have several types of zooid polymorphs and are strong contenders for evo-devo model organisms. In the long term, one goal of this general research direction will be to explore the thesis that modular colonial organization both constrains and facilitates evolvability.