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Animal body coloration is a complex trait resulting from the interplay of multiple mechanisms. While many studies address the functions of animal coloration, the mechanisms of colour production still remain unknown in most taxa. Here we compare reflectance spectra, cellular, ultra- and nano-structure of colour-producing elements, and pigment types in two freshwater turtles with contrasting courtship behaviour, Trachemys scripta and Pseudemys concinna . The two species differ in the distribution of pigment cell-types and in pigment diversity. We found xanthophores, melanocytes, abundant iridophores and dermal collagen fibres in stripes of both species. The yellow chin and forelimb stripes of both P. concinna and T. scripta contain xanthophores and iridophores, but the post-orbital regions of the two species differ in cell-type distribution. The yellow post-orbital region of P. concinna contains both xanthophores and iridophores, while T. scripta has only xanthophores in the yellow-red postorbital/zygomatic regions. Moreover, in both species, the xanthophores colouring the yellow-red skin contain carotenoids, pterins and riboflavin, but T. scripta has a higher diversity of pigments than P. concinna . Trachemys s. elegans is sexually dichromatic. Differences in the distribution of pigment cell types across body regions in the two species may be related to visual signalling but do not match predictions based on courtship position. Our results demonstrate that archelosaurs share some colour production mechanisms with amphibians and lepidosaurs (i.e. vertical layering/stacking of different pigment cell types and interplay of carotenoids and pterins), but also employ novel mechanisms (i.e. nano-organization of dermal collagen) shared with mammals.

21Animal body coloration is a complex trait resulting from the interplay of multiple colour-producing mechanisms. 22Increasing knowledge of the functional role of animal coloration stresses the need to study the proximate causes of 23 colour production. Here we present a description of colour and colour producing mechanisms in two non-avian 24 archelosaurs, the freshwater turtles Trachemys scripta and Pseudemys concinna. We compare reflectance spectra; 25 cellular, ultra-, and nano-structure of colour-producing elements; and carotenoid/pteridine derivatives contents in 26 the two species. In addition to xanthophores and melanocytes, we found abundant iridophores which may play a 27 role in integumental colour production. We also found abundant dermal collagen fibres that may serve as 28 thermoprotection but possibly also play role in colour production. The colour of yellow-red skin patches results from 29 an interplay between carotenoids and pteridine derivatives. The two species differ in the distribution of pigment cell 30 types along the dorsoventral head axis, as well as in the diversity of pigments involved in colour production, which 31 may be related to visual signalling. Our results indicate that archelosaurs share some colour production mechanisms 32 with amphibians and lepidosaurs, but also employ novel mechanisms based on the nano-organization of the 33 extracellular protein matrix that they share with mammals. 34 35 *Jindřich Brejcha 2 36 65 Turtles are an early-diverging clade of Archelosauria, the evolutionary lineage of tetrapods leading to 66 crocodiles and birds [22]. Although many turtles have a uniform dull colour, conspicuous striped and spotted 67 patterns are common in all major lineages of turtles (for a comprehensive collection of photographs see [23-26]).68 These conspicuous colour patterns may be present in the hard-horny skin of shells, and/or in the soft skin of the 69 head, limbs or tail. The dark areas of the skin of turtles may have a threefold origin consisting either of dermal, 70 epidermal, or both epidermal and dermal melanocytes. Colourful bright regions are thought to be the result of the 71 presence of xanthophores in the dermis [27] and their interplay with dermal melanophores [28]. Iridophores have 72 never been shown to play role in coloration of turtles [27,29]. 73Pigment-bearing xanthophores were first described in the dermis of the Chinese softshell turtle (Pelodiscus 74 sinensis) [29]. Xanthophores have also been found sporadically in the dermis of the spiny softshell turtle Apalone 75 spinifera, the Murray river turtle (Emydura macquarii) and in the painted turtle (Chrysemys picta) [27]. Such scarcity 76 3 of carotenoid/pteridine derivatives-containing cells is in contrast with chemical analyses of the yellow and red 77 regions of the integument of the red-eared slider (Trachemys scripta elegans) and C. picta [30,31]. Two major 78 classes of carotenoids have been described in the integument of these turtles: short wavelength absorbing 79 apocarotenoids and longer wave...

The global exponential growth in the number of exotic turtle species exploited in the pet trade market facilitates hybridization events between distantly related species. The 1997 EU trade ban on red-eared slider turtles (Trachemys scripta elegans), followed by the Spanish ban on all T. scripta subspecies in 2011, resulted in the importation of other chelonians (e.g. Graptemys spp., Pseudemys spp., Mauremys spp.), mainly from America and Asia to Spain. The importation of the Chinese stripe-necked pond turtle, Mauremys sinensis, via the pet trade market resulted in its release into natural environments across Europe, including the Iberian Peninsula where it co-occurs with the Western Mediterranean pond turtle, Mauremys leprosa. We maintained three M. leprosa females and two M. sinensis males in captivity, and analysed the obtained offspring (two hatchlings) with mitochondrial (cytochrome b) and nuclear (R35) genetic markers. Both morphological (i.e. with intermediate morphological characters) and genetic (heterozygous hatchlings) results confirmed the hybridization between these two species and raise concern about the negative effects of acclimatised Mauremys sinensis across the Mauremys leprosa range. 

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