Ancestral character-state reconstruction of baculum in Primates: why a critical literature review is crucial.

"ANCESTRAL CHARACTER-STATE RECONSTRUCTION OF BACULUM IN PRIMATES: WHY A CRITICAL LITERATURE REVIEW IS CRUCIAL" FEDERICA SPANI1 , GABRIELE GENTILE2 , MASSIMILIANO SCALICI1 , MONICA CAROSI1 1 Department of Sciences, Roma Tre University 2 Department of Biology, University of Rome Tor Vergata Selection pressures rapidly shape the anatomy of biological structures, usually affecting both their occurrence and form. This is the case of structures involved in copulatory system, as for example the penile bone (i.e. baculum), either detectable in the distal end of penis or covering its entire length. Recently, Schultz et al. (2016) reviewed this neglected topic attempting to map baculum evolution on mammalian phylogeny, using data from the literature. Among all species included in the analyses, there were also some primate species (N=102). However, they were not able to set the ancestral state of this character for the Primate order, probably due to both a low number of primate species included into the analysis, and several arbitrary choices in assessing some occurrence data. We accomplished a complete literature research and critical revision, collecting data about the occurrence of penile bone in 306 primate species. The resulting presence/absence binary matrix was mapped onto the most complete primate phylogeny by Springer et al. (2012) and counting 367 primate species. Starting from this species list, we matched our species list with tip labels of primate phylogeny by performing two analyses (1) including (N=367), and (2) excluding (N=255) taxa with missing data. By doing so we aimed at verifying whether a higher number of species could modify results about the ancestral state reconstruction of the character. No significative differences between analyses were found, rather a lower phylogenetic signal for the ancestral state of baculum. Main results show 8 independent losses of baculum, 2 of which need to be deepened: 1) in the Atelinae subfamily baculum losses observed in Lagothrix and Ateles genera might be interpreted as a phylogenetic parallelism (homoplasy); 2) in the genus Tarsius, only T. syrichta (Linnaeus, 1758) is reported to have a baculum, whereas remaining species apparently lost it, contrary to Schultz et al. (2016) who missed T. syrichta’s baculum presence, and therefore considered an absence of penile bone for the entire genus. Based on data analysed it appeared that the ancestor of primates had a baculum, which therefore is a symplesiomorphic trait for the entire order. Our study may finally clarify uncertainty reported by Schultz and colleagues, however further analyses including more primate species might help to disentangle both evolution and function of this neglected bone.