Abstract
Colour polymorphism is a widespread phenomenon in natural populations of several species. In particular, it is especially common on marine gastropod species from the genus Littorina. Recently, it has been argued that intrapopulation shell colour polymorphism in Littorina fabalis could be caused by negative frequency-dependent sexual selection via a mechanism of mate choice (indirectly estimated through negative assortative mating). Here we try to determine the existence of negative assortative mating in three species of the subgenus Neritrema (L. fabalis, L. obtusata, L. saxatilis) that share a similar shell colour polymorphism, to ascertain if this mechanism could represent an ancestral character in this subgenus that could be contributing to the maintenance of the colour polymorphism observed in each species. We collected or reanalysed from previous studies a sample of mating pairs of these three species from seven locations from NW Spain and NE Russia and estimated assortative mating using the IPSI index. Our results suggest that all species and populations show a systematic tendency towards negative assortative mating when shell colour is grouped in the broad categories ‘light’ and ‘dark’. Moreover, a more detailed analysis of each colour separately suggests that shell colour may not be the main target of assortative mating, but perhaps a physically-linked trait to the real target of selection. This hypothesis opens interesting new lines of research in Littorina snails.
Highlights
Colour is a key trait in evolution, usually contributing to individual survivorship (Olsson et al 2013; RolánAlvarez et al 2015a) and reproduction (Cordero 1989; Hugall and Stuart-Fox 2012), and potentially influencing fitness
We try to determine the existence of negative assortative mating in three species of the subgenus Neritrema (L. fabalis, L. obtusata, L. saxatilis) that share a similar shell colour polymorphism, in order to ascertain if this mechanism could represent an ancestral character in this subgenus that could be contributing to the maintenance of the colour polymorphism in each species
Given that previous results (e.g. Estévez et al 2020) strongly suggest a causal link between negative assortative mating and both mate choice and negative frequency-dependent selection, we suggest that if a negative assortative mating pattern is found in these two new species, an ancestral plesiomorphic behavioural trait for mate choice may exist within the subgenus Neritrema, which encompasses L. fabalis, L. obtusata and L
Summary
Colour is a key trait in evolution, usually contributing to individual survivorship (Olsson et al 2013; RolánAlvarez et al 2015a) and reproduction (Cordero 1989; Hugall and Stuart-Fox 2012), and potentially influencing fitness. Under certain circumstances selection can maintain a polymorphism in a population, and even genetic drift can do so, transitorily and lasting from a few to several generations depending on effective population size and gene flow (Wright 1977; Clarke, 1979; Kimura 1983; Hartl and Clark 2006). This makes rather difficult to identify the true causal factor responsible for the evolutionary maintenance of colour polymorphism within a population. One of the most effective types of selection able to maintain a polymorphism in wild populations is negative frequency-dependent selection (Ayala and Campbell 1974; Fitzpatrick et al 2007; Svensson and Connallon 2019). Fitness will peak as frequency falls below a certain threshold, protecting the polymorphism from being lost in the population
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