Male differentiation patterns in two polyphenic sister species of the genus Onthophagus Latreille, 1802 (Coleoptera: Scarabaeidae): a geometric morphometric approach

Abstract

Male differentiation patterns in two polyphenic sister species of the genus Onthophagus Latreille, 1802 (Coleoptera: Scarabaeidae): a geometric morphometric approach Abstract This paper focuses on morphological (both shape and size) differences that quite similar polyphenic sister species evolve during divergence processes. Traits were analysed using a geometrical morphometric approach, which has the ability to evidence also very subtle differences in shape. As a case study, we considered males of the dung beetle sister species pair Onthophagus taurus and Onthophagus illyricus (Coleoptera, Scarabaeidae); these species represent a typical example of polyphenic trait expression concerning the facultative development of horns and considerable body size differences. External shape morphology failed to discriminate O. taurus from O. illyricus, whereas the reproductive system shape showed significant interspecific discrimination power. However, the head of O. taurus was significantly larger than that of O. illyricus and the reverse was true for the elytra. The two species also showed different allometric values of the head with respect to body size. This complex pattern of interspecific morphological divergence is discussed in the light of the differential trait divergence rate hypothesis. In both species, differences between major and minor forms concern the overall shape of head and pronotum: we suggest that such different forms, which likely reflect morphological readjustment to accommodate horns of considerable bulk and disproportionate length, may be nevertheless advantageously used by the two male morphs in their alternative reproductive tactics. Male genitalia sizes were virtually constant with respect to body size; however, the ratio between phallotheca and body size was significantly higher in minor males, in keeping with the hypothesis of a higher investment in genitalia borne by this morph.

Trade-off between horns and other functional traits in two Onthophagus species (Scarabaeidae, Coleoptera)

“…This result matches with the habitat selection of O. fracticornis, which has been described as rather stenoecious, being restricted to pastures in mountain areas (Avila and Pascual 1988;Baum 1989;Gangloff 1991;Petrovitz 1956;Sowig 1995) and with recent genetic studies showing that different O. fracticornis populations share little genetic flow, which is indicative of a low dispersal capacity and of a certain degree of habitat specialisation (Macagno et al 2011a;Pizzo et al 2011). By contrast, genetic studies have detected consistent gene flow across populations of O. taurus (Pizzo et al 2006a), as a result of a higher mobility and a better tolerance to environmental conditions.…”

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

“…Such a head modification (in particular, clypeus stretching in response to horn development) might have evolved as a balancing static system compensating barycentre backing due to horn protrusion in major males. Similar patterns of head shape modification in response to horn expression are likely operating in other Onthophagus species: a morph dependent variation of head shape was also highlighted in Onthophagus taurus and Onthophagus illyricus (Pizzo et al. 2006a), where major males develop a pair of cephalic horns.…”

“…Eight landmarks on the frontal side of the head (Fig. 1) were digitized by using TpsDig 2.10 (Rohlf 2006): they were all positioned on the left hand side to prevent bias possibly caused by asymmetry (Pizzo et al. 2006a,b, 2008) and chosen for their relative ease of identification, homology between the two sexes and the morphs, and ability to capture the general head shape, excluding the portion from which the horn originates.…”

“…2006a,b, 2008) and chosen for their relative ease of identification, homology between the two sexes and the morphs, and ability to capture the general head shape, excluding the portion from which the horn originates. To evaluate the confidence of the landmark configuration, a repeatability test was conducted by digitalizing the same set of landmarks on the same specimen 10 times: the landmark configuration was accepted only if the ratio between variance on the same specimen and variance of the total sample [variance = Σ(Procrustes distances) 2 /( N –1), where N is the number of objects considered in each set of measures] was less than or equal to 0.05 (Pizzo et al. 2006a, 2006b, 2008).…”

Identification of cryptic species and morphotypes in male Euglossa: morphometric analysis of forewings (Hymenoptera: Euglossini)

“…Importantly, such identification seems to conform to those based on molecular markers (Francoy et al 2012;Oleksa and Tofilski 2015). However, in bees, the use of geometric morphometrics for the discrimination of cryptic species has remained uninvestigated, in spite of its promising use in other insect taxa Pizzo et al 2006;GurgelGoncalves et al 2011;Muñoz-Muñoz et al 2011;Mitrovski-Bogdanović et al 2013). Overall, the application of geometric morphometrics to assess intraspecific morphological differentiation and intra-sexual plasticity is still a largely unexplored field in bees (Danforth and Desjardins 1999).…”