Horn polyphenism and related head shape variation in a single-horned dung beetle:Onthophagus(Palaeonthophagus)fracticornis(Coleoptera: Scarabaeidae)

Abstract

Horn polyphenism and related head shape variation in a single-horned dung beetle: Onthophagus (Palaeonthophagus) fracticornis (Coleoptera: Scarabaeidae) Abstract Horns of Onthophagus beetles are typical examples of phenotypically plastic traits: they are expressed as a function of environmental (nutritional) stimuli, and their reaction norm (i.e. the full set of horn lengths expressed as a response to different degrees of nutritional states) can be either linear or threshold-dependent. Horned males of Onthophagus (Palaeonthophagus) fracticornis (Preyssler, 1790) bear a single triangular cephalic protrusion of vertex carina, which has received phylogenetic support as the most primitive horn shape in the genus. Inter-and intra-sexual patterns of horn expression were studied in O. fracticornis by means of static allometries while associated variations in head shape were assessed using geometric morphometric techniques. The relation between log-transformed measurements of body size and vertex carina supported an isometric scaling in females. On the contrary, a sigmoidal model described better the horn length-body size allometry in males, with a switch point between alternative morphs at a pronotum width of 3.88 mm. Sigmoidal static allometries of horns in Onthophagus populations arise from a threshold-dependent developmental process of horn growth. This process underlies the expression of both plesiomorphic and apomorphic horn shapes in the genus. Given that the single-horn model has been identified as primitive, we propose that such a developmental process giving rise to it may be evolutionarily ancient as well. Horn expression was accompanied by a deformation of the head which makes minor and major morphs appear even more different. Therefore, in this species both horn and head shape expression contribute to male dimorphism.

The First Morphometric Study of the Horn Morphological Pattern in a Geotrupidae: The Case of the Dor BeetleCeratophyus rossiiJekel, 1865

“…in Onthophagus (Emlen, 2001;Emlen et al, 2005;Macagno et al, 2009;Pizzo et al, 2012); Xylotrupes (Rowland, 2003) and Calchosoma (Kawano, 1995)): when these patterns are accompanied by non-linear allometry and by a clear bimodality in horn length frequency distributions, they can reveal a threshold-dependent mechanism of horn expression, with a developmental switch point separating major and minor morphs within males. However, in Ceratophyus rossi males, even if larger males seem to be not simply expanded versions of the smallest ones, no such bimodality in horn length or in pronotum width frequency distributions was found.…”

“…This may be due to the larger number of parameters available in a sigmoidal with respect to in a linear model . Thus, to determine if the two models differed significantly in how well they fitted the data, the residuals from the two regressions types were also compared by means of an Extra-Sum-of -Squares F-test Macagno et al, 2009). However, some authors have expressed concerns in the use of r 2 values to compare the goodness of fit between linear and non-linear models, even when corrected for the number of parameters taken into account (as r 2 values cannot properly evaluate non-linear models) (Spiess and Neumeyer, 2010); for this reason, the corrected Akaike's information criterion (AICc) (Akaike, 1973;Motulski and Christopoulos, 2003;Knell, 2009) was used to select the most appropriate models, in addition to the r 2 values comparison.…”

“…The developmental mechanisms of horns and their intra and interspecific expression patterns are well studied especially in Scarabaeidae: within this family, due to its large number of species and its morphological variability, Onthophagus is the most studied genus (Emlen, 1997;Emlen and Nijhout, 1999;Emlen et al, 2005;Moczek, 2011;Palestrini et al, 2000;Pizzo et al, 2006aPizzo et al, , b, 2012Macagno et al, 2009Macagno et al, , 2011a. Despite the wealth of studies on the Scarabaeidae, the horn expression pattern of the Geotrupidae, to our knowledge, has never been investigated.…”

Interspecific shape divergence in Aphodiini dung beetles: the case of Amidorus obscurus and A. immaturus (Coleoptera: Scarabaeoidea)

“…In the subfamily Scarabaeinae, the tribe Onthophagini (2,500 taxa or species; Scholtz et al 2009;Philips 2011), for instance, displays a tremendous diversity of horn structures, with many species developing horns or horn-like structures of some kind that represent evolutionary modifications of the same original ancestral structure (Moczek 2005(Moczek , 2008(Moczek , 2011. In many species of the genus Onthophagus, there are large major males producing horns and smaller minor males that are hornless (a case of male polyphenism; Emlen et al 2005;Macagno et al 2009;Tomkins and Moczek 2009;Knell 2011). Phylogenetically close species with similar horns may diverge in static allometries, males of each species developing horns at different body sizes (Emlen et al 2005;Lukasik et al 2006;Moczek 2002;Tomkins and Simmons 2002).…”

Seasonal constraints on the mandible allometry of Lucanus cervus (Coleoptera: Lucanidae)

“…These images were used to measure the width of the pronotum, used as a proxy for body size (Emlen, 1997;Moczek et al, 2002;Lagarde et al, 2005;Hongo, 2007;Macagno et al, 2009), and mandible length. Mandible length was measured as indicated in Fig.…”