Señales químicas, bacterias y ectoparásitos en aves

Abstract

The study of ecological relationships between bacteria (i.e. microbiota) and eukaryotes is essential to understand the evolution of animals (McFall-Ngai et al., 2013). These microorganisms usually establish complex relationships with animals, paying a key role in pathogenic, symbiotic or mutualistic processes (Steinert et al., 2000), as well as in animal physiology, development, morphology, and even behaviour (Clemente et al., 2012; Douglas, 2009; LeBlanc et al., 2013; Rosenbaum et al., 2015; Sherwin et al., 2019). Among the ecological processes in which microorganisms are involved, an emerging line of research is directed to understand the importance of the microbiota in the chemical signalling of their hosts (Ezenwa & Williams, 2014). The metabolism of symbiotic bacteria generates chemical volatiles, which should influence the odour profile of individuals. Furthermore, since the microbiota is closely linked to phenotypic traits and physiological activity of their hosts (Leclaire et al., 2017; Theis et al., 2013;Whittaker et al., 2019), volatiles of bacterial origin will be part of the chemical signals and cues of animals that inform con- or hetero-specifics of individual and environmental characteristics. The importance of microorganisms in animal chemical communication was first described by the fermentation hypothesis in the 1970s (Albone et al., 1974, 1978). This hypothesis was originally proposed to explain the odours of mammalian gland secretions, suggesting that variation in the host bacterial community generates variations in the individual odour profile that would function as recognition cues between conspecifics. Currently, the hypothesis applies to any chemical profile (odour) that operate in scenarios of social communications and that may be a consequence of the metabolism of animals bacterial symbionts. Evidence for a bacterial role in the contribution of host odours has been described in a wide taxonomic range, including mammals (Leclaire et al., 2017; Theis et al., 2013), insects (Engl & Kaltenpoth, 2018; Schmidtberg et al., 2019; Sharon et al., 2010), amphibians (Brunetti et al., 2019) and birds (Law-Brown, 2001; Martín-Vivaldi et al., 2010; Whittaker et al., 2019). Therefore, these odours are public information that can be used by both conspecifics and heterospecifics including unwanted receptors such as predators and/or parasites. In addition, to bacteria that are closely associated with the host (microbiota), metabolism of those inhabiting the environment where the animals live and/or reproduce would also produce cues for parasites or predators looking for victims. Bacteria growing in bird nests can metabolize organic compounds from bird reproductive activity that accumulate within the nest environment and, thus, those bacteria have been suggested to be partially responsible of the nest-environment volatiles (i.e. nest odours). Since ectoparasites primarily use olfaction to detect their hosts (Bowen, 1991; Poldy, 2020; Reneerkens et al., 2005; Takken & Knols, 1999; Zwiebel & Takken, 2004). Consequently, the symbiont bacteria associated with bird nests and the volatiles from their metabolism should influence the probability parasites detecting these nests and, therefore, the strength of selection pressures of parasitism suffered by nesting birds. In this thesis, we have explored this hypothesis, which implies the existence of two types of associations: (A) that volatiles of the nest environment and bacteria from nests material, as well as volatiles and bacterial of the secretions of avian must be related to each other; and (B) that both volatiles and bacteria should be associated with the probability and/or intensity of ectoparasitism suffered by nesting individuals. Furthermore, this thesis we pay special attention to detecting particular bacteria and volatiles that were related to each other as possible predictors of probability and intensity of ectoparasitism suffered by nesting birds and/or of fledgling success. I first review literature on these matters and, then, explore experimentally and empirically some predictions of the hypothesis both at the intraspecific level (mainly in hoopoes (Upupa epops) and at the interspecific levels. In hoopoes, the uropygial gland of nesting females and nestlings harbour complex bacterial communities responsible of the emission of some volatile-chemical compounds (Martín-Vivaldi et al., 2010). Hoopoes do not build nests and prefer to reuse holes or nest boxes that contain remains and materials from previous reproduction (Hoffmann et al., 2015; Martín-Vivaldi et al., 1999). Moreover, the bacterial community of the nest materials from previous reproductions affects the microbiota of the uropygial gland of female hoopoes (Díaz-Lora et al., 2019). To test the hypothetical role of bacteria determining nest odours and parasitism, we manipulated the bacterial environment of the hoopoe nest. The experiment consisted of the installation of new nest boxes in which we included materials from old hoopoe nests. In half of the new nest boxes the added mater were (experimental nests) or were not (control nests) we previously autoclaved. Latter, in control and experimental boxes that hoopoes used to reproduce, we sampled and analysed the microbiota of the nest material and of gland secretions of females and nestlings, as well as the volatile profile of the nest and secretion. Furthermore, we explored these relationships at the beginning, when the nestlings have not developed the uropygial gland and the female still spends most of the time within the nest; and the end of the nestling period, when all the nestlings have fully developed the gland. Moreover, at these nestling stages, we estimated the intensity of parasitism of a hematophagous ectoparasite (Carnus hemapterus) in nestlings and of chewing lice (suborder Mallophaga) in females (only at the beginning of the nestling period). C. hemapterus is a generalist hematophagous fly that actively searches for avian nests (Calero-Torralbo et al., 2013; Martín-Vivaldi et al., 2006), and that likely eavesdrop on the volatiles from active nests to find and select their hosts. Chewing lice feed on soft areas of feathers and skin, and when they cause bleeding they also suck blood (Agarwal et al., 2011; Mester, 1977). In the case of hoopoes, these ectoparasites are only detected in females and are placed in the feathers of their crest where they are more protected from grooming. Nest material collected during hoopoe reproduction in experimental nest-boxes presented bacterial communities of lower density diversity, and volatile profiles that more divers than that of the control nests. Furthermore, the intensity of parasitism (i.e., number of bites) detected in nestlings that developed in experimental nest-boxes was lower than that found in nestlings hatched in control nest-boxes. Consistent with the first part of the hypothesis, we found that the microbial communities of the uropygial secretions and of nest materials covaried with their volatile profiles, while the volatile profile of the secretions of females and nestlings explained the volatile profile of the nest. Supporting the second part of the hypothesis, we found a positive association between the bacterial density of the nest material during the nestling stage and the intensity of ectoparasitism an effect that was only evident in the experimental nest boxes with autoclaved material. Finally, some of the bacteria and volatiles respectively detected in the nest material and in the environment, as well as those detected in secretions, were associated with the intensity of ectoparasitism of females and nestlings, and with fledging success. These results suggest a link between the community of microorganisms in the nest materials and the intensity of ectoparasitism suffered by nesting birds. Furthermore, they support the existence of a close relationship between microbial communities and odours of animals and nests. Taken together, those results suggest that, in hoopoes, the associations between bacteria and both ectoparasitism and reproductive success are mediated by volatiles of bacterial origin. We also tested our hypothesis by exploring interspecific variability in microbial environments and volatile profiles of nests of ten bird species. we collected information on bacteria, volatiles and ectoparasitism in nests, at the beginning and at the end of the nestlings stay. In accordance with the hypothesis, microbial alpha diversity (the diversity of the community) associated with alpha diversity of volatiles in interaction with bird species identity, but the results varied depending on the index and the sample (age of nestlings) used in the analyses. In addition, (ii) beta diversity (the differences of diversity between samples) of bacterial community based in PhILR distances explained the beta diversity of volatile profile but only at the late stage of nestlings. Moreover, (iii) alpha diversity of volatiles associated with intensity of parasitism at the early stage of nestling period, while alpha diversity of bacterial community and beta diversity of volatiles were related to intensity of parasitism at the end of the nestling period. (iv) Only alpha diversity of the microbiota of the nest material at the begging of the nestling period explained the fledging success. Finally, some key bacteria and volatiles that were related to each other also associated with intensity of parasitism and, at lower rate, with fledging success. Taking together, those results support the expected links between microbial environment and nest odours in different bird species, and between them and ectoparasitism intensity and fledging success. Future research should prioritize experimental approaches directed to determine the role of particular bacteria and volatiles in the outcomes of host-ectoparasite interactions. Results presented in this thesis in general support the working hypothesis. Statistical support however varied depending on the diversity indices used to characterize the volatile profiles and bacterial communities, the time of sampling (at the beginning or at the end of the nestlings stay in the nest), the species under study and the used elements (bacteria or volatiles). Therefore, we conclude that the bacterial communities associated with bird nests are partially responsible for their volatile profiles and the risk of parasitism and fledging success of the nestlings.