Partial melanism as a frequent phenotypic variation of Philodryas chamissonis (Squamata: Dipsadidae) (Wiegmann, 1835) in the O’Higgins Region, Chile

Unusual differences in coloration and design patterns have been described in some snake species around the world. These descriptions usually consider morphological abnormalities observed in single individuals, but not in a subgroup of the species, and few studies have evaluated the distribution of snake species subgroups with similar coloration and design patterns. In this study, we describe in central Chile the distribution of a subgroup, non-geographically isolated, of the endemic snake Philodryas chamissonis, presenting a similar design pattern named partial melanism, phenotypic variation showing a greater expression in the O’Higgins region, without records at the north of Maipo River.


INTRODUCTION
Chilean fauna is characterized by possessing a high level of endemism (Smith-Ramírez, 2004), especially in the Reptilia class, despite having a low diversity of native species (135 taxa; Ruiz de Gamboa (2016)) compared to countries with similar geoclimatic conditions (Greene & Jaksic, 1992); 49% of this taxa are found only in Chilean territory (Demangel, 2016).
This extensive latitudinal distribution, facilitates the development of genetic variability as an evolutionary mechanism of improvement in adaptive capacity and acclimatization to the different environmental gradients throughout the territory, and historical geographical barriers, in the case of low mobility species such as reptiles, which generate important cases of genetic divergence (Avise, 2000). In fact, Pleistocene glaciations, as the last one occurring 23,000 -17,000 years ago, covered much of the continental territory across the Los Andes mountain formation, even forming a constant ice cap between mountain range and the coastal region from 42° S latitude (McCulloch et al., 2000). In this climatic scenario, and subsequent glacial retraction, the rivers that crossed our territory towards the coast (e.g. Aconcagua, Maipo, Cachapoal and Tinguiririca) might have presented considerably broader flows than currently, constituting important barriers for the dispersion of the species, and favoring the mechanisms of vicarious evolution with formation of new evolutionary units (Lamborot & Eaton, 1997;Lamborot et al., 2003). These effects, during the quaternary glaciations might have contracted the distribution of the species, later to generated a postglacial expansion which could have reduced genetic diversity (Hewitt, 2000), which together with prevailing low temperatures, had to model the genetic distribution and adaptability of ectothermic species such as P. chamissonis (Sallaberry-Pincheira et al., 2011). Sallaberry-Pincheira et al. (2011) previously analyzed the genetic divergence of P. chamissonis using two mitochondrial DNA markers throughout its current distribution. Results confirmed the existence of four haplogroups, consistent with the different geographical and latitudinal patterns of Chile, and confirming the Maipo River as the historical geographical barrier separating two haplogroups named central and southern clades. This last clade would include the administrative territory of the O'Higgins region and the rest of the southern distribution territory, extending to Galvarino (Araucania region), considering therefore the specimens of this region, with characteristics or patterns of particular genetic expression that potentially may affect a phenotypic expression, different from the specimens that live from the Maipo River towards the north.
Philodryas chamissonis occupies a wide variety of habitats, from the extremely dry Atacama Desert to the cold Valdivian temperate rain forests (Donoso-Barros, 1966), and is morphologically described as a medium-sized colubrid, which can reach up to 220 cm in total length (Greene & Jaksic, 1992), with a typical color pattern consisting of a broad dorsal brown colored band, flanked by two thinner white-yellowish bands that run longitudinally down the body (Donoso-Barros, 1962). In this study, we describe this pattern as the "typical morph" ( Figure  1). Demangel (2016) adds to description that the species has a whitish-gray ventral coloration and there are specimens with a tendency to melanism, stating that some individuals had black spots of different sizes distributed irregularly throughout the body, while others are almost totally black (in his book, he accompany this description with a photograph of a specimen with partial melanism from Chacayes, Machalí, O'Higgins Region, Chile). Here we describe this last pattern or morph as a phenotypic variant called "partial melanism".
A large variety of color patterns have been described in snake species worldwide, patterns that have been naturally selected to favor survival of different species by increasing thermoregulation and concealment among others (Bechtel, 1978;1991). The production of different color patterns in snakes depends on four types of chromatophores in the epithelium (melanophores, erythrophores, xanthophores and iridophores) (Laus & Buric, 2012). When the color pattern of an individual differs from other specimens of the same species, it is catalogued as an abnormality in the chroma-togenesis, wich has been documented in various Viperidae and Colubridae species (Bittner, 2000;Jadzík, 2004;Krecsak, 2008;Pernetta & Reading, 2009;Laus & Buric, 2012). Melanophores are the chromatophores that produce black pigments in the skin. Partial melanism has been described in a variety of snake species and subspecies presenting no taxonomic value (e.g., Natrix natrix, Thamnophis sirtalis) (Bittner, 2000;Jadzík, 2004). However, most of the studies are individually based and few evaluate the chromatogenesis abnormalities in the complete distribution of a species.
This article describes the percentage of occurrence of the phenotypic variant called partial melanism in P. chamissonis recorded in the O'Higgins Region, and compares it to the complete distribution of the species, postulating that this morph has a greater expression in this territory than in the rest of its range.

MATERIALS AND METHODS
We collected and evaluated records of P. chamissonis from the O'Higgins Region, sighted between the years 2006 and 2017, through the following routes: A Each activity had a duration varying from 1 to 5 days, with the participation of 1 to 4 people per survey, which generated a total sampling effort of 1080 hours/man. All P. chamissonis sightings were recorded and tabulated. C.-Review of photographic material of P. chamissonis from the O'Higgins Region, which was delivered to SAG by naturalists from this area, through the "Wildlife of O'Higgins Region Network" (https://www.facebook.com/ groups/777592725622613).
D.-Three records from photographic material taken in the O'Higgins Region, reported in the book "Reptiles en Chile" (Demangel, 2016).
At the same time, records from other regions of the country were evaluated throughout the complete distribution of this species, by means of personal collections, photographs from different Chilean naturalists, and through the digital herpetological network "Reptiles de Chile" (https://www.facebook. com/groups/208533175833879).
For each record, the data of date of observation, locality, georeference (if possible), state of development, morph and author of the observation were registered.

RESULTS
A total of 107 records of P. chamissonis from other Chilean regions (from Atacama to Araucanía), different to the O'Higgins Region, were obtained in this study (Table 1). All these records presented a typical morph pattern. However, after the period of bibliographic review and photographic evaluation carried out for this study (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017), recent photographs (2018-2020) of P. chamissonis specimens with partial melanism pattern have appeared in different naturalistic social networks, attributed to location to Maule region (at south of O'Higgins Region), which would indicate the presence of this morphotype at least in the O'Higgins and Maule Regions.   A total of 77 records of P. chamissonis were obtained in the O'Higgins Region (Table 2). Of those, 33 individuals (42.8%) presented the phenotypic pattern of partial melanism, in different degrees of pigment coverage, from small diffuse black spots along the body to dark individuals with black pigmentation over a large area of their body, generally over a soft pink or grey basal colour (Figures 2-3  In the O´Higgins region, records of specimens with partially melanic morph range altitudinally between 30 and 1139 m.a.s.l. (Puertecillo and R.N. Rio Cipreses, respectively), and latitudinally, from Codegua (34°03'S) in the north, to Chimbarongo (34°46'S) in the south, occupying a large diversity of habitats, such as xerophytic scrubland (Peumo), littoral (Puertecillo), urban green areas (Rancagua), cultivated fields (Paniahue, Chimbarongo), sclerophyll forest (Requínoa, Coltauco) and montane pre-Andean forest (Chacayes, Las Nieves, Popeta) ( Figure 4). Therefore, we do not identify any association between the occurrence of this morph and some particular altitudinal/latitudinal distribution or habitat. Thomas (1976) establishes the origin of the genus Philodryas (within the Tachymenoides complex) in the central Andes, extending to the south coastal line of Peru during the Miocene, when the mountain range had a considerably lower altitude than today. After the Miocene, the altitudinal growth of the Andes mountain range (Gregory-Wodzicki, 2000), and the formation of the Atacama Desert, isolated the population of Philodryas spp. in the southwest zone of the Andes, favoring the allopatric speciation of P. chamissonis (Thomas, 1976). Colonization of the Chilean territory by the species occurred from north to south, through different ecosystems and climatic variants, which in an ectothermic species like this, imposed significant selective forces, generating intraspecific divergence and reducing the gene flow between the different environments (Sallaberry-Pincheira et al., 2011). This evidence supports the development of haplogroups with different genetic expression potential, being the one that distributed to the south of Maipo River the most consistent of these. Maipo River was a determining barrier in the last Pleistocene glaciation, with a glacial coverage that even reached the central valley in the southern area of Santiago (Lamborot & Eaton, 1997), and a wide river flow, which generated allopatric distribution and corresponding genetic divergence to south (Sallaberry-Pincheira et al., 2011). Within this haplogroup, the first populations of P. chamissonis in the process of expansion towards the south correspond to those of the O'Higgins Region, which would conform the ancestral region of the southern haplogroup.

DISCUSSION
Two different records obtained in this study correspond to videotape and photograph of specimens with partial melanism in the process of mating ( Figure 5), which shows that there is a natural crossing of specimens with this morph, generating a greater possibility of transmitting this morphological trait, if it is genetically transmitted, to their offspring.
Partial melanism was only observed in adult specimens, not in juveniles, so further studies should address the possibility that it is a characteristic that is expressed only at a certain level of maturity. Dark color in snakes has been proved to result in better thermoregulation than in white individuals, and more melanin in skin protects the internal organs from deleterious solar radiation; therefore, this partial melanism might be naturally selected due to its advantageous effects on the species (Krecsák, 2008). More studies need to be conducted to evaluate why and in which individuals this characteristic is occurring and if a positive correlation is found with species fitness.