Craniogenetic studies in Sus scrofa: With emphasis on the 'orbitosphenoid' problem.

The orbitosphenoid is a skeletal element of the endocranium of extant mammals. However, it has also been described in many of their fossil ancestors. Craniogenetic studies show that it is composed of two types of bone: first, the cartilaginous ala orbitalis and parts of the trabecular plate are transformed by endochondral ossification; second, so-called 'appositional bone' ('Zuwachsknochen') arises directly from the perichondrium of the two optic pilae and spreads in all directions and overlays the remaining cartilage and the endochondral ossifications. For some time, both bone types can be distinguished microscopically, but later in craniogenesis they fuse completely to become the presphenoid sensu lato of the osteocranium. We interpret the 'appositional bone' as neomorphic mode to reinforce the endocranial bone structures, which are the ossification of the delicate cartilaginous template of the chondrocranium. We studied the ossifications of the presphenoidal skull region in a series of ontogenetic stages of the pig Sus scrofa. We applied conventional histology as well as stained and unstained µCT scans. We can show the above-mentioned modes of ossification, and we can demonstrate the contribution of 'appositional bone' well into neonatal and infantile stages. The ossifications of the presphenoid (including the orbitosphenoid) are very slender in therapsids and early mammaliaforms as previously described by other authors. In mammaliaforms, they tend to become thicker and closely connected with the frontal bone, which may be due to the contribution of neomorphic appositional bone. We assume that thereby the presphenoid sensu lato becomes an enforcement of the orbital pillars.

Convergent vomeronasal system reduction in mammals coincides with convergent losses of calcium signalling and odorant-degrading genes.

The vomeronasal system (VNS) serves crucial functions for detecting olfactory clues often related to social and sexual behaviour. Intriguingly, two of the main components of the VNS, the vomeronasal organ (VNO) and the accessory olfactory bulb, are regressed in aquatic mammals, several bats and primates, likely due to adaptations to different ecological niches. To detect genomic changes that are associated with the convergent reduction of the VNS, we performed the first systematic screen for convergently inactivated protein-coding genes associated with convergent VNS reduction, considering 106 mammalian genomes. Extending previous studies, our results support that Trpc2, a cation channel that is important for calcium signalling in the VNO, is a predictive molecular marker for the presence of a VNS. Our screen also detected the convergent inactivation of the calcium-binding protein S100z, the aldehyde oxidase Aox2 that is involved in odorant degradation, and the uncharacterized Mslnl gene that is expressed in the VNO and olfactory epithelium. Furthermore, we found that Trpc2 and S100z or Aox2 are also inactivated in otters and Phocid seals for which no morphological data about the VNS are available yet. This predicts a VNS reduction in these semi-aquatic mammals. By examining the genomes of 115 species in total, our study provides a detailed picture of how the convergent reduction of the VNS coincides with gene inactivation in placental mammals. These inactivated genes provide experimental targets for studying the evolution and biological significance of the olfactory system under different environmental conditions.