Abstract
The search for phylogenetic signal in morphological traits using geometric morphometrics represents a powerful approach to estimate the relative weights of convergence and shared evolutionary history in shaping organismal form. We assessed phylogenetic signal in the form of ventral and dorsal haptoral anchors of 14 species of Ligophorus occurring on grey mullets (Osteichthyes: Mugilidae) from the Mediterranean, the Black Sea and the Sea of Azov. The phylogenetic relationships among these species were mapped onto the morphospaces of shape and size of dorsal and ventral anchors and two different tests were applied to establish whether the spatial positions in the morphospace were dictated by chance. Overall significant phylogenetic signal was found in the data. Allometric effects on anchor shape were moderate or non-significant in the case of evolutionary allometry. Relatively phylogenetically distant species occurring on the same host differed markedly in anchor morphology indicating little influence of host species on anchor form. Our results suggest that common descent and shared evolutionary history play a major role in determining the shape and, to a lesser degree in the size of haptoral anchors in Ligophorus spp. The present approach allowed tracing paths of morphological evolution in anchor shape. Species with narrow anchors and long shafts were associated predominately with Liza saliens. This morphology was considered to be ancestral relative to anchors of species occurring on Liza haematocheila and M. cephalus possessing shorter shafts and longer roots. Evidence for phylogenetic signal was more compelling for the ventral anchors, than for the dorsal ones, which could reflect different functional roles in attachment to the gills. Although phylogeny and homoplasy may act differently in other monogeneans, the present study delivers a common framework to address effectively the relationships among morphology, phylogeny and other traits, such as host specificity or niche occupancy.
Highlights
Darwin’s view of species as evolving entities only detectable by gaps in morphological variation [1] established an explicit link between morphology and evolution
The Principal Component Analysis (PCA) based on the covariance matrix of landmark data of both ventral and dorsal anchors showed that a large proportion of the variation is contained in relatively few dimensions, with the first two PCs accounting for over a half of the total variance in the sample (Table 1)
This paper delivers a framework to study the evolution of attachment organs in monogeneans and paves the way for further studies addressing the relationships among morphology, phylogeny and other traits, such as host specificity or niche occupancy
Summary
Darwin’s view of species as evolving entities only detectable by gaps in morphological variation [1] established an explicit link between morphology and evolution. This inception has pervaded biological thought until today, to the point that it can be asserted that all post-Darwinian morphology has been, to a greater or lesser extent, evolutionary [2]. The similarity among forms of different species can be explained by inheritance from a common ancestor or by convergence where the form can arise more than once across taxa in response to similar ecological, adaptive, functional, and/or developmental pressures [3, 4] Both processes act concurrently and disentangling their roles has been until recently a daunting task. The current availability of phylogenetic tools, coupled with the development of geometric morphometrics methods that can examine morphological data as independent from the effect of phylogeny have greatly simplified this endeavour [5, 6]
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