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Session Schedule & Abstracts

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Wednesday 29th June, 2016

Morphological Integration & Modularity 2

Room: Salon H   4:30 pm–5:45 pm

Moderator(s): Cobb SN, Felice RN
MIM2-1  4:30 pm  Morphological modules within the avian skull evolve at different rates. Felice RN*, University College London; Goswami A, University College London
Abstract: Modularity and integration are key features of biological systems, describing the degree to which individual components of organisms are interrelated with one another. Although these concepts are closely associated with evolvability and constraint, their macroevolutionary patterns have been investigated in very few vertebrate clades. We evaluated modularity and integration in a phylogenetic context and quantify phylogenetic signal and evolutionary rate in individual modules, using the avian skull as a focal system. Previous studies have found the avian skull to be generally integrated, more so than has been observed in mammals, but these analyses have been either phylogenetically restricted or based on limited shape data (i.e., using 2D data and/or excluding major cranial regions). Recent advances in data acquisition and analytical techniques for high-dimensional data allow for a more robust consideration of phenotypic evolution in the avian skull. Using 3D surface scans of a broad sample of Neornithines, we quantified cranial morphology using a high density of anatomical landmarks and semilandmarks. Covariance ratio analyses supported significant modularity between the braincase and face, with greater independence between these two regions than between random groupings of traits. Moreover, the facial skeleton exhibits higher evolutionary rate and higher morphological disparity than the braincase, whereas the level of within-module integration is significantly higher in the braincase. These results support the hypothesis that high integration constrains morphological evolution and suggest that the modular nature of the avian cranium facilitates individual subregions to respond to selective pressures and functional constraints in independent ways. This study also illustrates the utility of phenomic data for studying patterns of morphological evolution and serves to further our understanding of how modularity has influenced the evolution of morphological variation.

MIM2-2  4:45 pm  The timing of cranial lateral line morphogenesis and its implications for ontogeny of sensory function. Webb JF*, University of Rhode Island
Abstract: The pattern and timing of the ontogeny of the vertebrate sensory systems make critical contributions to an organism’s ability to ultimately formulate appropriate behaviors. The mechanosensory lateral line system of fishes detects unidirectional or oscillatory water flows that arise from predators, prey, environmental flows, and obstacles. The cranial lateral line system of bony fishes goes through dramatic changes during the larval stage and then through metamorphosis to the juvenile stage. After the initial establishment of an array of neuromast receptor organs on the skin in young larvae, neuromasts may increase in number and size and change shape. Then, typically at metamorphosis, a subset of neuromasts becomes enclosed in pored lateral line canals in a conserved subset of dermatocranial bones, changing these receptors from velocimeters to accelerometers. In growing juveniles, the neuromasts continue to increase in size, and the relative pore size and canal diameter determine adult canal phenotype, which vary among taxa. Both the absolute and relative timing of each of these morphological changes (which speaks to modularity and integration in the dermatocranium and placode-derived sensory organs) is likely to have important implications for the ontogeny of flow sensing capabilities with respect to fish size, swimming behavior, and transitions in life history. This paper will compare the timing of lateral line development among fishes as structurally and ecologically diverse as zebrafish (Danio rerio), cichlids (Aulonocara stuartgranti, Tramitichromis sp.), butterflyfishes (Chaetodon ocellatus), and gobies (Elacatinus lori, E. colini), and will consider implications for the ontogeny of lateral line function and lateral line-mediated behaviors. Supported by NSF grants 0843307 and 1459546 to JFW and the University of Rhode Island.

MIM2-3  5:00 pm  Constraint and convergent evolution of diprotodonty in therian mammals. Cobb SN*, Hull York Medical School, University of York; Morris PJR, Hull York Medical School, University of Hull; Cox PG, Hull York Medical School, University of York
Abstract: Despite their high dietary and taxonomic diversity, living and extinct rodents share a conservative dentition consisting of a pair of enlarged and continually growing upper and lower incisors (diprotodonty), separated from a reduced posterior dentition by a large diastema. Diprotodonty has also independently evolved in a phylogenetically diverse range of non-rodent therian mammals, including hyraces, lagomorphs and individual species of marsupials and primates. Here we examine whether the independent evolution of diprotodonty across therian mammals is limited to the dentition, or if it constrains the disparity of the whole masticatory system. Three-dimensional landmarks were collected from CTs of the cranium and mandible of non-rodent diprotodont species, species representing the main extant rodent families and non-diprotodont outgroup taxa. Geometric morphometrics methods were used to examine the convergence between the rodent and non-rodent specimens. The taxa in this study samples large phylogenetic distances, however in both the cranium and mandible morphospaces the diprotodont taxa group very tightly together. Within the rodents, taxa from the main groupings based on masticatory musculature (hystricomorphs, myomorphs, protrogomorphs and scuiromorphs) form discrete groupings in cranial results. Partial least squares (PLS) show a high level of covariation between cranium and mandible in all taxa. The findings of the study clearly demonstrate that convergent evolution of morphology in diprotodont mammals is not restricted to the dentition, but is also found in the cranium and mandible and their pattern of covariation. This indicates that there are strong functional constraints on the masticatory system associated with diprotodonty.

MIM2-4  5:15 pm  A maximum likelihood approach to assessing modularity with 3-D morphometric data. Goswami A*, University College London; Finarelli J, University College Dublin
Abstract: Presently, identification of phenotypic modules can be accomplished through exploratory (e.g., cluster analysis) or confirmatory (e.g., RV coefficient analysis, Covariance Ratio analysis) approaches. Confirmatory approaches are more robust, but suffer from an inability to compare across different model parameterizations. For example, both a two-module neurocranial/facial model and a six-module model have been significantly supported for the therian mammalian skull using RV coefficient analysis. Here, we present an approach to analyzing modules with morphometric data in which model log-likelihoods of trait correlation matrices are compared using the finite-sample corrected Akaike Information Criterion, allowing for discrimination of hypotheses across different model structures. We validated this method on correlation matrices simulated to have no modularity and simple (2-module) and complex (6-module) patterns of integration, testing these simulated matrices against 31 model structures, including no modules, and various partitions of 2, 3, 4, 6, and 8 modules. We also assessed the effect of different degrees of integration magnitude, varying the average within- and across module correlations. In all cases, the correct model structure was recovered with greater than 0.99 posterior probability. We then focused on a published dataset of 61 landmarks measured on 181 macaque skulls (Macaca fuscata), spanning five age cohorts. Results for both coordinate and multidimensional vector correlations clearly supported a complex 6-module model for all cohorts, with separate estimates of within- and inter-module correlations as the best fit for all datasets, demonstrating that this pattern of integration is stable through the postweaning ontogeny of macaques. Subsampling analyses of the best-sampled data set (M1 erupted) further demonstrates the ability to recover identical model structures and estimates of within- and among-module correlations with severely degraded sample sizes.

MIM2-5  5:30 pm  Modularity or integration or both? 3D analysis of 21 genera of frogs demonstrates phylogenetic conservatism in skulls and lability in limbs. Vidal-Garcia M.*, The Australian National University; Keogh J. S., The Australian National University
Abstract: Quantifying morphological diversification across taxa can provide valuable insight into evolutionary processes, yet the its complexities can make it difficult to identify appropriate units for evaluation. One of the challenges in this field is distinguishing between the morphological integration and modularity hypotheses, where morphological evolution of different structures is explained either by co-variation between them, or by independent evolution respectively. Here we used a 3D geometric morphometric approach with x-ray micro CT scan data of the skull and bones of fore-limbs and hind-limbs of representative species from all 21 genera of the ancient Australo-Papuan myobatrachid frogs, and analysed their shape both as a set of distinct modules and as a multi-modular integrative structure. We then tested three questions: (i) is morphological disparity similar between the two major subfamilies, (ii) do skulls and limbs show different levels of integration, and (iii) is shape variation correlated with locomotion, burrowing behavior, and ecology. We found that morphological disparity was similar in both species-rich subfamilies. Skull shape diversity was phylogenetically conserved, whereas limb shape was more associated with ecology, particularly in fossorial species. Morphological differences between different limb bones were highly correlated, depicting high morphological integration. In contrast, overall limb and skull shape displayed semi-independence in morphological evolution, favouring the modularity hypothesis. Our results show how form can be correlated with function, with the evolution of limb shape being driven by selective pressures imposed by the environment and functional requirements of locomotion and behaviour. Our results also illustrate how morphological evolution can display varying degrees of independence across different modules, and that quantifying this is crucial in order to make accurate predictions of complex evolutionary processes.

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