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Ramírez-Sánchez MM, E. De Luna y C. Cramer. 2016

Geometric and traditional morphometrics for the assessment of character state identity: multivariate statistical analyses of character variation in the water mite genus Arrenurus (Acari, Hydrachnidia, Arrenuridae). Zoological Journal of the Linnean Society 177: 720–749.

Geometric and traditional morphometric approaches are tested to describe and reveal taxonomic characters and character states in variation of shape and size of idiosoma morphology in the Arrenuridae. Patterns of variation of idiosoma and glandularia features of males from 11 Mexican species of Arrenurus (Megaluracarus) and two species of subgenus Dadayella were explored with five landmark configurations and three sets of interlandmark distances. Separate principal component analyses (PCA) and canonical variate analyses (CVA) were performed for each data set. The eight multivariate analyses of variance among 13 a priori groups (species) detected significantly different morphometric variants, which were interpreted as different taxonomic character states. Patterns of character state similarity among species were examined with unweighted-pair grouping method using averages (UPGMA) cluster analyses on Mahalanobis distances. Analyses of five landmark configurations revealed important taxonomical variation in the anterior idiosoma outline (10 character states), the outline of the posterior region or cauda (13 states), the distribution of postocularia, and the second and third pairs of dorsoglandularia (nine states), the fourth pair of dorsoglandularia (three states), and ventroglandularia on the posterior side of idiosoma (nine character states). Multivariate analyses of three sets of distance measurements also resulted in the detection of potential taxonomic characters related to idiosoma size (12 character states), postocularia and dorsoglandularia (13 states), and ventroglandularia (nine character states). Morphometric analyses of distances and shapes provide a formal basis for the interpretation of taxonomic characters, and for the discovery of character states. These characters should be investigated further in a wider sample of species for the phylogenetic systematics of these water mites. In the meantime, idiosoma regions and structures were tested for congruence in a phylogenetic analysis, and were proposed as homologous among the species sampled.

 Additional keywords: cauda shape – character homology – glandularia – idiosoma shape – idiosoma size – Megaluracarus – taxonomic characters.
Figure 7. Shape variation in the dorsal view of the cauda outline (Data set 2). Scatterplot of
canonical variates scores (Root 1 versus Root 2), deformation grids of shape changes
relative to the mean shape are shown for both axes (A). Overall pattern of shape similarity
among 11 Megaluracarus species and two Dadayella species based on Mahalanobis
distances computed from the canonical variates analysis (B). All species resulted
significantly different from each other and therefore 13 character states were discovered in
the cauda outline. Branches in the UPGMA phenogram are labeled according to
discrimination order defined by the CVs. Symbols in the plot and in the phenogram are as
in Figure 6.
Figure. 14. Single most parsimonious tree selected in our phylogenetic analysis of the
combined morphometric data with TNT (score 95.8119). The character matrix included
continuous values of three distance sets and five landmark configurations. Only one shape
character, the cauda outline (Data set 2), is optimised in this tree. Landmark configurations
of the cauda shape at terminal nodes are those as observed in each species. The numbers on
each configuration indicate shapes that are significantly different, as evaluated by the CVA
and MANOVA for landmark Data set 2, as illustrated and labelled in Fig. 7B. In all
hypothetical landmark configurations of the cauda shape at internal nodes, deformation
vectors at each point indicate displacements relative to the ancestral shape as optimised
with TNT.