Can neutral genetic differentiation explain geographical variation in body size of the natterjack toad, Epidalea calamita?
From Firenze University Press Journal: Acta Herpetologica
Federico Marangoni, Departamento de Biología, Facultad de Ciencias Exactas y Naturales y Agrimensura. Universidad Nacional del Nordeste (FACENA-UNNE) and Consejo Nacional de Investigaciones Científicas y Técnicas
Geographic variation in phenotypic and genetic characteristics among species’ populations is a phenom-enon that has been very well documented since the 1950s (Stebbins, 1950; Mayr, 1963; Harper, 1977). Adaptive explanations for the evolution and maintenance of geo-graphic variation in body size have been put forward, particularly considering macrogeographical patterns as a response to environmental gradients (Bergmann, 1847; Ray, 1960; Lindsey, 1966; Adams and Church, 2008; Ashton, 2002; Cvetkovic et al., 2009; Sinsch et al., 2010). Nevertheless, few studies have evaluated it at smaller geo-graphical scales with, in many cases, lack of genetic isola-tion between populations (e.g., Skelly, 2004; Gomez-Mes-tre and Tejedo, 2004; Lee et al., 2020; Albert and García-Navas, 2022), and thus, what causes and maintains those patterns is still not well understood. Therefore, to infer on the processes causing these patterns, we need to know the mechanisms underlying the observed phenotypic variation, how they are connected to genetic variation, and how they interact with other traits and the environment (Stearns, 1989). This will also help us to understand the evolutionary significance of the geographic variation, which ultimately can lead to the formation of new species (Endler, 1977; Foster and Endler, 1999).The agents that change the gene frequencies of pop-ulations, that is, the factors of evolution, are mutation, genetic drift, gene flow, and natural selection (Slatkin, 1987). While drift and selection tend to increase popu-lation differentiation, gene flow promotes homogeniza-tion among connected populations, either increasing or decreasing the genetic diversity of the system (Lenor-mand, 2002). Thus, gene flow is a major component of population structure because it determines the extent to which each local population of a species is an independ-ent evolutionary unit. If there is a high gene flow between local populations, then all the populations evolve togeth-er; whereas in the presence of low gene flow, each popu-lation evolves almost independently (Slatkin, 1985). The natterjack toad (Epidalea calamita) populations from southwestern Spain exhibit a steep variation in body size and reproductive traits associated with changes in the geological substrate (Marangoni et al., 2008). This implies a decrease of 70.9% of body mass and 28.5% in snout-vent length, on a micro-geographic scale of only 60 km (Fig. 1). Previous studies suggested that considerable genetic differentiation may be the mechanism underlying the observed geographic variation in metamorphic traits in E. calamita (Marangoni, 2006) and Pelobates cultripes(Marangoni and Tejedo, 2008), which exhibit the same geographic pattern of adult body size variation (Maran-goni et al., 2008; Lee et al., 2020). Moreover, the study of age structure and growth pattern across populations of E. calamita suggests that both environmental varia-tions in resources availability associated with the sandy substrate (Marangoni, 2023), but also different growth and maturity pathways, may happen in response to con-trasting selective pressures (Marangoni et al., 2021). Two hypotheses could be suggested to explain the evolu-tion and maintenance of the observed cline in body size and reproductive parameters in E. calamita (Marangoni et al., 2008). In the first place, it could be expected that in the presence of gene flow between populations, the alleles favored by a selection pressure of intensity s do not decrease their high frequency, because the rate of immigration m of other alleles is lower than the intensity of selection s, that is, m < s (Slatkin, 1985). The second hypothesis would be that the differentiation between the populations along the cline had occurred in a context of relative population isolation or in the presence of scarce gene flow. Little empirical evidence has been able to dem-onstrate the role that gene flow plays in maintaining dif-ferentiation in populations at short geographical scales. It has been suggested that when gene flow is not homoge-neous, evolutionary differentiation can be rapid and can occur on small spatial scales (Kennington et al., 2003; Garant et al., 2005; Postma and van Noordwijk, 2005). In previous studies on E. calamita that exhibited local adaptation to osmotically stressful environments, micro-satellite markers revealed little population differentiation, lack of an isolation-by-distance pattern, and moderate gene flow connecting the populations (Gomez-Mestre 2001; Gomez-Mestre and Tejedo, 2004). Present study assess whether neutral genetic differentiation can explain the geographical variation in the body size, age and reproduc-tive parameters observed in natterjack toad, from south-ern Spain (Marangoni et al., 2008; 2021). This comparison between patterns of population genetic differentiation in neutral markers and quantitative traits can provide valu-able insights into the mechanisms driving variation within populations (Leinonen et al., 2007). Thus, this comparison can help us understand evolutionary forces by discerning the relative influences of genetic drift and natural selection on the evolution of quantitative traits (Gomez-Mestre and Tejedo, 2004; Knopp et al., 2007; Páez-Vacas et al., 2001). Secondly, it aids in identifying adaptive traits, for example those traits which might are related to adaptation to differ-ent environmental conditions (Gomez-Mestre and Tejedo, 2003; Luque et al., 20015). In addition, it also allows us to understand the distribution of genetic effects within the architecture of quantitative traits (see Leinonen et al. 2007 and references therein).The main goals in the present study were: i) ana-lyze the level of genetic structuring and gene flow in E. calamita populations along the geographic variation in body size observed (Marangoni et al., 2008), ii) compare genetic diversity between and within populations, as well as between ecological environments and iii) test for the existence of an isolation-by-distance pattern of popula-tions differentiation. We expect that the observed geo-graphical variation in the body size of natterjack toad has been evolved in absence of isolation-by-distance, and with a gene flow connecting the populations.
DOI: https://doi.org/10.36253/a_h-13774
Read Full Text: https://oaj.fupress.net/index.php/ah/article/view/13774
