Specification of polymorphism and classification of shell coloration in gastropods by the example of Littorina obtusata (Gastropoda: Littorinidae)

Abstract

Understanding the patterns of microevolutionary processes involves a wide range of population genetic studies on different species. However, the number of genetically studied species is limited due to significant methodological difficulties in testing the genetic conditionality of various traits. Developing population phenetics may become an alternative, which allows considering a large number of new species subject to development of morphologically and genetically-based system to describe the variability and classification of phenotypic traits. Gastropods are a classic object for carrying out population genetic studies based on the analysis of polymorphism of shell coloration. The parametric classification system proposed by S. Sergievsky et al. (1995) for periwinkles of the genus Littorina may serve as a basis for the developing of a universal system for classifying color traits for that taxonomic group. Since a large amount of new data has been published in recent years, this system requires correction and revision. The study aims to revise the system, taking into account new material on the pigment composition, as well as on the peculiarities of the formation and inheritance of color traits, their joint occurrence, etc. A revised and modified classification system for shell coloration traits is presented by the example of the White Sea gastropods L. obtusata; this system considers the idea of the formation of a phenotype as a combination of several elementary traits. These are traits associated with the formation of: 1) a shell background color (the ability to include different pigments in the ostracum and the hypostracum color); 2) a pattern of spots (the presence of inclusions of white and/or brown pigment); and 3) wide longitudinal bands (brown, white, and orange). Elementary traits are highlighted taking into account the pigments involved, as well as the mechanisms of their formation and inheritance. When describing the shell coloration, elementary traits are first used to describe relatively simple traits (groups of phenes “Shell background color”, “Hypostracum color”, “Pattern of spots”, and “Wide longitudinal bands”), which are subsequently combined to describe the phenotype as a whole. Our study provides an overview of the available data on the qualitative composition of shell pigments in periwinkles and patterns of formation and inheritance of color traits; their possible combinations are described. The phenes isolated by us are described together with their occurrence, color specification in the RGB system, and the peculiarities of the formation, taking into account the distribution of pigments in the shell. Visible traits, that may be used to assess the distribution of pigments in the shell, are indicated. The ontogenetic changes in traits are described. Despite the fact that the proposed classification system is developed by the example of L. obtusata, it can be used for same purposes for other periwinkle species and, with some modifications, for a wide range of gastropod species.

Keywords

• polymorphism
• shell coloration
• classification system
• gastropods
• Littorina