INTEGRATING DEVELOPMENTAL EVOLUTIONARY PATTERNS AND MECHANISMS: A CASE STUDY USING THE GASTROPOD RADULA

Determining the connection between ontogeny and phylogeny continues to be a major theme in biology. However, few studies have combined dissection of pattern and process that lead to transformation of complex morphological structures. Here we examine the patterns and processes of shape change in a model system-the gastropod radula. This system is a simple one having only two processes: initial secretion and postsecretional movement of teeth. However, it produces a tremendous amount of shape variability and fusion patterns. To determine both pattern and mechanism of shape change in an evolutionary context, we use three complementary approaches and datasets. First, we use a phylogenetic hypothesis to determine the polarity of developmental events. Second, we perform a morphometric analysis of shape change using relative warp analysis that allows us to locate and compare the direction and magnitude of ontogenetic and phylogenetic shape divergence. These comparisons are the basis for testing hyptheses of heterochrony and heterotopy, and we show how our results do not conform to expectations of pure heterochrony. The rejection of heterochrony as a hypothesis is based on empirically demonstrating (1) initial shape differs in each taxon; (2) a single dimension of shape variability does not simultaneously describe ontogenetic and evolutionary shape changes; and (3) a significantly different shape and size covariance between taxa. This rejection is probably based on spatial changes in initial conditions and not spatial changes caused by the process itself. Finally, we construct a mechanistic model that explains how shape change happens based on the sequence of events during ontogeny. By using the parameters in the model as characters in the phylogenetic dataset, we show that different parts of the system have arisen at different times and become co-opted into the process. By integrating our analyses together we show that spatial process parameters can be responsible for our nonspatial patterns and that different ontogenetic processes can create similar end morphologies.

Recent introduction of the dominant tunicate, Pyura praeputialis (Urochordata, Pyuridae) to Antofagasta, Chile

The large sessile tunicate Pyura stolonifera (Pleurogona: Stolibranchiata: Pyuridae), has been regarded as a complex taxon with disjointed distributions, including Australia (Pyura stolonifera praeputialis), South Africa (Pyura stolonifera stolonifera) and South America (Chile, Antofagasta: Pyura sp., the 'piure de Antofagasta'), and has been cited under at least five taxonomic combinations. The 'piure de Antofagasta' is a competitively dominant species in rocky intertidal habitats and shows a limited geographical range (60-70 km) exclusively inside the Bay of Antofagasta. Using cytochrome oxidase I (COI) mitochondrial sequence data from Pyura specimens of the three taxa we tested whether the Chilean taxon represents: (i) a Gondwana relict; (ii) a more recently divergent species; or (iii) a recently introduced species. The results suggest that the Chilean taxon is a recent introduction to Chile from Australian populations and that Pyura stolonifera praeputialis, from Australia, and the 'piure de Antofagasta' are geographical populations of a single species: Pyura praeputialis; whereas the South African taxon represents a second species: Pyura stolonifera.

Reconnecting cell and animal lineages: what do cell lineages tell us about the evolution and development of Spiralia?

Cell lineage studies in the clade Eutrochozoa, and especially the Spiralia, remains a rich and relatively untapped source for understanding broad evolutionary developmental problems; including (1) the utility of cell timing formation for phylogenetic hypotheses; (2) the evolution of cell timing changes and its relation to heterochronic patterns; (3) stereotypy or lack thereof in rates of change of cell growth during evolution and its relation to both evolutionary history and current usage; and (4) how mosaic cleavage timing variation may be expected to differ from other groups. A compilation of available cell timing information was made from previous studies where each division was explicitly followed and the total number of cells followed was greater than 24. From that compilation, we performed a series of heuristic and quantitative analyses, including a phylogenetic analysis using cell timing data as characters and analyses of timing variation across all taxa. Our results show that: (1) cell lineage data reconstructs a phylogenetic hypothesis that has similarities, especially among the Mollusca. to the patterns found in morphological and molecular analyses; (2) the mesentoblast (4d) is a unique cell compared to other cell in that it speeds up and slows down relative to other cells in taxa with both unequal and equal cell sizes; (3) some cells that form in the same quartet at the same point in the cell lineage hierarchy have much lower variations than analogous other cells, arguing for architectural constraint or stabilizing selection acting on those cells; and (4) although variation in cell timing generally increases during development, timing of formation of progeny cells in the first quartet has lower variation than the parent cells, arguing that some regulation-like behavior might be present.

Molluscan engrailed expression, serial organization, and shell evolution

Whether the serial features found in some molluscs are ancestral or derived is considered controversial. Here, in situ hybridization and antibody studies show iterated engrailed-gene expression in transverse rows of ectodermal cells bounding plate field development and spicule formation in the chiton, Lepidochitona cavema, as well as in cells surrounding the valves and in the early development of the shell hinge in the clam, Transennella tantilla. Ectodermal expression of engrailed is associated with skeletogenesis across a range of bilaterian phyla, suggesting a single evolutionary origin of invertebrate skeletons. The shared ancestry of bilaterian-invertebrate skeletons may help explain the sudden appearance of shelly fossils in the Cambrian. Our interpretation departs from the consideration of canonical metameres or segments as units of evolutionary analysis. In this interpretation, the shared ancestry of engrailed-gene function in the terminal/posterior addition of serially repeated elements during development explains the iterative expression of engrailed genes in a range of metazoan body plans.

Oxygen and evolutionary patterns in the sea: onshore/offshore trends and recent recruitment of deep-sea faunas

Over the last 15 years a striking pattern of diversification has been documented in the fossil record of benthic marine invertebrates. Higher taxa (orders) tend to originate onshore, diversify offshore, and retreat into deep-water environments. Previous studies attribute this macroevolutionary pattern to a variety of causes, foremost among them the role of nearshore disturbance in providing opportunities for the evolution of novel forms accorded ordinal rank. Our analysis of the post-Paleozoic record of ordinal first appearances indicates that the onshore preference of ordinal origination occurred only in the Mesozoic prior to the Turonian stage of the Cretaceous, a period characterized by relatively frequent anoxic/dysoxic bottom conditions in deeper marine environments. Later, in the Cretaceous and Cenozoic, ordinal origination of benthic organisms did not occur exclusively, or even preferentially, in onshore environments. This change in environmental pattern of ordinal origination roughly correlates with Late Cretaceous: (i) decline in anoxia/dysoxia in offshore benthic environments; (ii) extinction of faunas associated with dysoxic conditions; (iii) increase in bioturbation with the expansion of deep burrowing forms into offshore environments; and (iv) offshore expansion of bryozoan diversity. We also advance a separate argument that the Cenomanian/Turonian and latest Paleocene global events eliminated much of the deep-water benthos. This requires a more recent origin of modern vent and deep-sea faunas, from shallower water refugia, than the Paleozoic or early Mesozoic origin of these faunas suggested by other workers.

A deep water patellogastropod from oligocene water-logged wood of Washington State, USA (Acmaeoidea: Pectinodonta)

Late Oligocene concretions from the shores of the Strait of Juan de Fuca, Washington State, USA contain limpets that represent the earliest record of the taxon Pectinodonta. Assignment of these specimens to this taxon is based on scanning electron microscopy of shell microstructure and their intimate association with fossil wood. Shell microstructures in the Pectinodonta consist of an outer calcitic homogeneous/simple prismatic layer followed by a calcitic foliated layer, an aragonitic comarginal crossed lamellar layer, myostracum, and an aragonitic cone complex crossed lamellar layer. A size class analysis of pectinodontids from different pieces of fossil and Recent wood suggests that few cohorts are found on any single piece of wood, and we speculate about possible factors that may produce this pattern. Lastly, we describe Pectinodonta palaeoxylodia Lindberg & Hedegaard new species and associated taxa from this water-logged wood community.

The First Historical Extinction of a Marine Invertebrate in an Ocean Basin: The Demise of the Eelgrass Limpet Lottia alveus

Lottia alveus, a gastropod limpet once found only on the blades of the eelgrass Zostera marina from Labrador to New York in the western Atlantic Ocean, is the first marine invertebrate known to have become extinct in an ocean basin in historical time. The last known specimens were collected in 1929, immediately prior to the catastrophic decline of Zostera in the early 1930s in the North Atlantic Ocean. The brackish water refugium of Zostera throughout the decline was apparently outside of this gastropod's physiological range, and the limpet became extinct. Few marine invertebrates have habits as specialized and ranges and tolerances as narrow as did L. alveus. The fact that most marine invertebrates have large effective population sizes may account for their relative invulnerability to extinction.