Phenotypic plasticity in haptoral structures of Ligophorus cephali (Monogenea: Dactylogyridae) on the flathead mullet (Mugil cephalus): a geometric morphometric approach

Abstract

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

Evolutionary morphology in shape and size of haptoral anchors in 14 Ligophorus spp. (Monogenea: Dactylogyridae)

“…Empirical evidence from L . cephali indicates a tighter control of the shape and size in ventral anchors perhaps because they seem to be responsible for firmer attachment [31, 32]. Thus the differences observed could be explained in terms of different functional roles in attachment to the gills [27].…”

“…(Geographical details of all localities are given in S1 Table). Gills were examined for parasites as per Rodríguez-González et al [32]. …”

“…The bars were not studied because they are more difficult to observe flat and more prone to distortion during fixation and mounting. We used photographs and drawings only for ventral and dorsal anchors of partly digested individuals following Rodríguez-González et al [32]. Any anchor showing apparent deformation, tear or rupture (about 2–3% of the initial sample) was excluded from the study.…”

“…We digitized 8 landmarks in 2D covering the anchor surface selected and recorded in each anchor using tpsDig version 2.17 [38] representing homologous points (see Figure 1 in Rodríguez-González et al [32]). Generalized Procrustes analysis in MorphoJ was employed to obtain a matrix of shape coordinates (S1 Dataset) from which all information related to position, scale and orientation were removed [39].…”

“…Geometric morphometrics has already been applied to Ligophorus spp. to explore the correlation between phenotypic variation in attachment organs and factors such as phylogeny, to elucidate mechanisms determining phenotypic buffering, character displacement, as well as in species discrimination [18, 29–32]. …”