Some of the most widely accepted theories of animal evolution hinge on the assumption that the feeding larvae of different groups of marine invertebrates are the result of a single evolutionary event. I received a three-year NSF research grant to reconstruct the evolutionary trees for the polychaete annelid groups Terebellida and Sabellida based on DNA sequences from several different genes, so that evolutionary patterns of change in feeding and non-feeding larval development can be traced in these worms. Our results to date suggest that non-feeding development is the ancestral larval state in these worms and that feeding larvae have evolved independently at least twice.
- McHugh, D. (1998) Evolution of larval developmental modes in terebellidan and sabellidan polychaete annelids. American Zoologist 38: 138A (abstract)
- McHugh, D. and Rouse, G. (1998) Life history evolution of marine invertebrates: new views from phylogenetic systematics. Trends in Ecology and Evolution 13: 182-186
Hydrothermal vents, which occur along deep-ocean spreading centers (see map
), can be separated by tens or thousands of meters and may remain volcanically active for as short as one year or as long as a hundred years. Bacteria convert sulphides in the mineral-laden fluid spewing from the vents into energy that is accessible to a unique assemblage of vent animals. For populations of animals inhabiting the patchy, unpredictable environment of deep-sea hydrothermal vents, a big mystery is whether, how, and when these animals colonize new vent habitats. For example, the reproductive biology of the polychaete annelid Paralvinella pandorae indicates that this species bypasses any dispersal stage and broods its embryos – so how does it establish new populations at new vent sites?
To address this question, I have been working with Dr. Verena Tunnicliffe at the University of Victoria, Canada and two students, Andy Hock '00 and Neil Rekhi '00, to reconstruct the biogeographic patterns among populations of Paralvinella pandorae in the northeast Pacific. Dr. Tunnicliffe has provided us with frozen samples of worms from over 15 vent populations. Andy and Neil sequenced DNA from a rapidly evolving gene (cytochrome oxidase I) for individuals from five different vent populations. Preliminary results based on their analyses of these data indicate that gene flow, and therefore dispersal, is occurring between populations that are separated by distances of approx. 50 km, but there is no evidence of gene flow between these populations and others that are approx. 225 km away. So, the genetic data indicate that the worms are dispersing, at least to relatively close vents, but their life history includes no dispersal stage. Judging from the known direction and speed of deep ocean currents around the vent populations, it appears that this pattern may be the result of oceanographic rather than biological factors. This project is being expanded to include DNA sequence data from individuals representing all the available population samples, and a more thorough analysis of the biogeography of this species is being undertaken.
- Tunnicliffe, V., MacArthur, A., and McHugh, D. (1998) Historical biogeography of hydrothermal vent fauna. Advances in Marine Biology 34: 353-442
I have used molecular data to reconstruct the evolutionary history of the annelids and other worm-like animals for which a poor fossil record exists. My analyses of DNA sequences from one gene, elongation factor-1a, showed that the Annelida we traditionally define is not a natural group that includes all the descendents from the original annelid ancestor. In fact, my data indicate that the deep-sea hot vent tube worms and the unsegmented, mud-dwelling echiurids – both groups that have been considered as separate, independent evolutionary lineages – are actually derived annelids with bizarre body forms. These results alter our views on the appearance of numerous independent worm-like body plans during the early Paleozoic era; the phylum-level diversity of the Cambrian period is not as great as we once thought. The results also question some basic assumptions regarding the evolution of segmentation in invertebrates: segmentation has been lost in the echiurids.
I am continuing to use molecular techniques to reconstruct the evolutionary tree for animal groups for which morphological analyses have not yielded good resolution of relationships. For example, I recently collaborated with a Belgian colleague, Dr. Igor Eeckhaut, on a project that uses molecular data from two genes (18S rRNA and elongation factor-1a) to address the evolutionary relationships of a group of parasitic marine worms, the myzostomids. Our analyses clearly demonstrate that myzostomids are not annelids, as previously hypothesized, and instead support a close relationship between myzostomids and flatworms. These results suggest that the common ancestor of myzostomids, flatworms, and trochozoans (which includes the annelids and molluscs (clams, snails, squid, etc.) was a segmented worm-like organism.
In another project, a molecular evolutionary analysis of deep-branch relationships among annelids, molluscs, and arthropods is underway. These major animal groups evolved 600 million years ago and are now represented by over a million species on earth. In addition to analyses of DNA sequence data, higher level molecular characters were explored by Courtney Bellomo '00 for this project. She analyzed the copy number and non-coding regions of a gene, elongation factor-1a, to determine whether they provide information that will be useful in reconstructing evolutionary relationships among these animal groups.
- Eeckhaut, I., McHugh, D., Mardulyn, P. Tiedemann, R., Monteyne, D., Jangoux, M., and Milinkovitch, M.C. (2000) Myzostomida: a link between trochozoans and flatworms? Proceedings of the Royal Society, London B 267: 1383-1392
- McHugh, D. (2000) Molecular phylogeny of the Annelida. Canadian Journal of Zoology 78: 1873-1884
- McHugh, D. (1999) Phylogeny of the Annelida: Siddall et al. (1998) rebutted. Cladistics 15: 85-89
- McHugh, D. (1999) Annelida. In Encyclopedia of Reproduction (E. Knobil and J.D. Neill, Eds.). Academic Press, pp. 219-223
- Westheide, W., McHugh, D., Purschke, G., and Rouse, G. (1999) Systematization of the Annelida. Hydrobiologia 402:291-307