....Fig 1: Rachel Collin

Dr Collin describes the work which received the unanimous support of the adjudicators...

For my dissertation research I used a phylogenetic approach to examine the evolution of mode of development in calyptraeid gastropods. This work can be broken down into four major parts:

1. A review of the available data on mode of development in calyptraeids.
Collin, R. 2003. World-wide patterns of development in calyptraeid gastropods. Marine Ecology Progress Series 247: 103-122.

This includes a compilation of egg size, hatching size, size at settlement and presence or absence of nurse eggs. This data was then used to test the hypotheses that (1) egg-size distribution is bimodal, (2) mode of development is correlated with latitude, (3) mode of development is correlated with female body size, (4) hatching size is correlated with egg size, and (5) time to hatching is correlated with size at hatching. Analysis of these data shows that egg size is not bimodally distributed and that mode of development correlates with egg size in species without nurse eggs. There is a relationship between mode of development and latitude, but no developmental variables are associated with body size. There is an unexpected relationship between nurse eggs and latitude: direct developing species in the southern hemisphere have nurse eggs while those in the northern hemisphere do not.

2. Phylogenetic analyses of calyptraeids.
Collin, R. 2003. The utility of morphological characters in gastropod phylogenetics: An example from the Calyptraeidae. Biological Journal of the Linnean Society 78: 541-593.
Collin, R. Phylogenetic relationships among calyptraeid gastropods and their implications for the biogeography of speciation. Systematic Biology (conditionally accepted March 2003).

This includes a phylogenetic analysis of the Calyptraeidae based on fragments of mitochondrial 16S and cytochrome oxidase c subunit I, and nuclear 28S genes. 70 species of Crepidula, 11 Crucibulum and 9 Calyptraea and 5 species in other, lesser calyptraeid genera were included in this analysis. Monophyletic Calyptraeidae, Crucibulum, and a clade of Crepidula s.s. cannot be rejected. Calyptraea is clearly polyphyletic as is Crepidula s.l. The monophyletic genus (or subgenus) Bostrycapulus nests within a clade comprised of Crucibulum, some Calyptraea and Crepipatella. Western Pacific Crepidula species and some Calyptraea appear as a poorly resolved grade at the base of the tree. Shell morphologies are strikingly conservative in some groups such as Bostrycapulus and Crepipatella where quite divergent taxa cannot be distinguished on the basis of shell characters. Shell morphologies are shockingly convergent in some other species and appear to be associated with substrate specialization. As a result of this convergence, phylogenetic analyses based on 116 morphological characters shows little agreement with the topology obtained from the DNA sequence data.

3. Comparative analysis of the evolution of development in calyptraeids.
Collin, R. The loss of complex characters, phylogenetic effects, and the evolution of development in a family of marine gastropods. Evolution (submitted January 2003).

A comparative analysis of mode of development in the Calyptraeidae was undertaken to answer the following questions: (1) is the loss of feeding larvae irreversible? (2) Is there a phylogenetic effect on the evolution of mode of development? and (3) Do embryos of direct developing species lose the structures necessary for larval feeding and swimming and if so is the degree of embryonic modification correlated with the genetic distance between species? There are three areas of the phylogeny in which it appears that planktotrophic development may have re-evolved from species with direct development. In all of these cases they appear to be derived from species with nurse eggs. There is little support to the idea that the loss of feeding larvae is irreversible and little support for phylogenetic effects on these characters. Finally, there is a correlation between the genetic distance and developmental modification between sister species. Taken together these results suggest that there is a window of opportunity after the evolution of direct development before which the larval characters are lost, during which reversals to planktotrophic development could occur.

4. Examination of the effects of mode of development on population structure
Collin, R. 2001. The effects of mode of development on phylogoegraphy and population structure of North Atlantic Crepidula (Gastropoda: Calyptraeidae). Molecular Ecology 10: 2249-2262.

I demonstrate that mode of development is associated with levels of gene flow in Crepidula species from
the east coast of North America. In the direct developing species the among-population variance accounts for about 80% of the total genetic variance. Among-population variance accounts for 0-20% of the total genetic variance in planktotrophic species. In addition, species range endpoints cluster at well-known biogeographic boundaries and do not depend on mode of development.

Taken together this work begins to lay a foundation for understanding the evolution of development at several hierarchical levels. By compiling a large dataset on the development of species in a large monophyletic group I have explicitly tested commonly help beliefs about the evolution of development. These results, combined with a hypothesis of character evolution within the calyptraeidae, give strong support to the idea that variation in developmental characters reflects adaptation to current conditions rather than historical effects. Work that I am currently conducting at the Smithsonian Tropical Research Institute in Panama on within-species variation in developmental characters will produce a full picture of evolution of development in this gastropod family. I hope that this approach will lead to better understanding of the evolutionary causes and consequences of mode of development.


Rachel Collin