We present Procrustean Approach to Cophylogeny (PACo), a novel statistical tool to test for congruence between phylogenetic trees, or between phylogenetic distance matrices of associated taxa. Unlike previous tests, PACo evaluates the dependence of one phylogeny upon the other. This makes it especially appropriate to test the classical coevolutionary model that assumes that parasites that spend part of their life in or on their hosts track the phylogeny of their hosts. The new method does not require fully resolved phylogenies and allows for multiple host-parasite associations. PACo produces a Procrustes superimposition plot enabling a graphical assessment of the fit of the parasite phylogeny onto the host phylogeny and a goodness-of-fit statistic, whose significance is established by randomization of the host-parasite association data. The contribution of each individual host-parasite association to the global fit is measured by means of jackknife estimation of their respective squared residuals and confidence intervals associated to each host-parasite link. We carried out different simulations to evaluate the performance of PACo in terms of Type I and Type II errors with respect to two similar published tests. In most instances, PACo performed at least as well as the other tests and showed higher overall statistical power. In addition, the jackknife estimation of squared residuals enabled more elaborate validations about the nature of individual links than the ParaFitLink1 test of the program ParaFit. In order to demonstrate how it can be used in real biological situations, we applied PACo to two published studies using a script written in the public-domain statistical software R.

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Evaluating phenotypic plasticity in attachment organs of parasites can provide information on the capacity to colonize new hosts and illuminate evolutionary processes driving host specificity.We analysed the variability in shape and size of the dorsal and ventral anchors of Ligophorus cephali from Mugil cephalus by means of geometric morphometrics and multivariate statistics. We also assessed the morphological integration between anchors and between the roots and points in order to gain insight into their functional morphology. Dorsal and ventral anchors showed a similar gradient of overall shape variation, but the amount of localized changes was much higher in the former. Statistical models describing variations in shape and size revealed clear differences between anchors. The dorsal anchor/bar complex seems more mobile than the ventral one in Ligophorus, and these differences may reflect different functional roles in attachment to the gills. The lower residual variation associated with the ventral anchor models suggests a tighter control of their shape and size, perhaps because these anchors seem to be responsible for firmer attachment and their size and shape would allow more effective responses to characteristics of the microenvironment within the individual host. Despite these putative functional differences, the high level of morphological integration indicates a concerted action between anchors. In addition, we found a slight, although significant, morphological integration between roots and points in both anchors, which suggests that a large fraction of the observed phenotypic variation does not compromise the functional role of anchors as levers. Given the low level of genetic variation in our sample, it is likely that much of the morphological variation reflects host-driven plastic responses. This supports the hypothesis of monogenean specificity through host-switching and rapid speciation. The present study demonstrates the potential of geometric morphometrics to provide new and previously unexplored insights into the functional morphology of attachment and evolutionary processes of host-parasite coevolution.4

An important question in the study of phenotypic evolution is whether characters are independent of each other or behave and evolve as integrated modules. Morphological integration and modularity provide a powerful framework for the analysis of the evolution of morphological traits. We used geometric morphometrics and phylogenetically independent contrasts (PIC) to test four different modularity hypotheses in the haptoral anchors of 14 monogenean species of Ligophorus. Integration between the modular units identified was further evaluated with two-block partial least squares analysis. Roots and points represented two modules in the dorsal and ventral anchors, but modularity was not statistically supported when parasite phylogeny was accounted for, which may indicate convergent evolution related to host characteristics and gill morphology. In contrast, PIC revealed medial and lateral modules in ventral anchors only. Moreover, we found evidence for ventral and dorsal anchor pairs forming two modules, supporting the notion that they play different functional roles. Integration between all identified modules was strong. We conclude that there is modular structure in the anchors of Ligophorus spp., accounted by adaptive and phylogenetic factors acting at different levels, and ventral and dorsal anchors evolve as integrated modules with specific roles in attachment.

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