Can sexual selection and disassortative mating contribute to the maintenance of a shell color polymorphism in an intertidal marine snail?

Abstract

Littorina fabalis is an intertidal snail commonly living on the brown algae Fucus vesiculosus and showing frequent shell-color polymorphisms in the wild. The evolutionary mechanism underlying this polymorphism is currently unknown. Shell color variation was studied in mated and non-mated specimens of this species from different microareas in one locality from NW Spain, in order to estimate sexual selection and assortative mating that may (still) be operating in this population. The analyses across microareas allowed us to investigate frequency-dependent selection and assortative mating components, mechanisms that could maintain the polymorphism. The presence of shell scars caused by crab attacks, an environmental variable not related with sexual selection or assortative mating, was used as experimental control. This study provides new evidence of significant disas-sortative mating and some degree of sexual selection against some shell colors, supporting the results found 21 years ago in a similar study, i.e. in the same species and locality. The similarity of these estimates during the studied period suggests that this experimental approach is consistent and valid to be extended to other populations and organisms. In addition, sexual selection and assortative mating estimates did not change across microareas differing in shell color frequencies, suggesting than the polymorphism can not be maintained by a frequency-dependent (sexual selection-based) mechanism. Our main hypothesis is that negative assortative mating could contribute to the maintenance of the polymorphism, perhaps by males showing distinct female color preferences when searching for mates.

The scale-of-choice effect and how estimates of assortative mating in the wild can be biased due to heterogeneous samples

“…Two samples (1990 and 2011) were reanalyzed from previous studies (Rolán‐Alvarez and Ekendahl and Rolán‐Alvarez et al. ), while the remaining two samples are described here for the first time (2012 and 2013; see raw data displayed in Table S1). Overall, data on a total of 918 mated and 1506 unmated specimens was collected.…”

“…Further details on methods have been given elsewhere (Rolán‐Alvarez and Ekendahl ; Rolán‐Alvarez et al. ).…”

“…We then focused our analysis on the two most common colors: brown (Brw+Brw2) and yellow (Yel+Yel2), which represent more than 90% of the sampled population snails (Rolán‐Alvarez et al. ). Raw data are shown in Table S1.…”

Notes from a snail island: Littorinid evolution and adaptation

“…Phenotypic polymorphism in Littorinids is not just limited to L. saxatilis; Daniel Estévez (University of Vigo, Spain) gave a talk on L. fabalis, a snail species that occurs on brown algae along European shores and shows a clear polymorphism for shell colour with striking brown and yellow individuals (Rolán‐Alvarez et al., ). How this polymorphism is maintained is unclear; a previous study suggested it might be maintained by negative assortative mating, with males choosing females of different colours (Rolán‐Alvarez et al., ). Longitudinal data offer an opportunity to test the predictability of evolution, which is dependent on many factors, including the nature of selection, environmental variation and genomic interaction (Nosil et al., ).…”

The relationship between sexual selection and speciation

“…Rolán- Alvarez et al (2012) find that shell color in the marine intertidal snail Littorina fabalis is under sexual selection, but serves as the basis of consistent disassortative, not assortative, mating. In this instance, therefore, the sexually selected trait would tend to cause population cohesion, rather than divergence.…”