Intraspecific morphological variation of the sperm storing organ in two hermaphroditic land snail species

Alexandra Staikou,Evripidis Koemtzopoulos

doi:10.1186/s40709-019-0093-y

Abstract

BackgroundPostcopulatory sexual selection is very important in species with reproductive strategies that involve multiple mating and prolonged sperm storage. The sperm storage organ has been hypothesized to evolve in response to different levels of sperm competition in several species while population density has been considered as a factor that approximates sperm competition risk and intensity in the field. Apart from population density, local microclimatic conditions may also play a role in determining sperm competition levels in natural populations of land snails by affecting their chances of encountering mates. The goal of this study was to investigate the variation of the structure of the sperm storage organ in the simultaneously hermaphroditic land snails Helix lucorum and Cepaea vindobonensis occurring sympatrically in two sites which differed in habitat humidity. The populations of both species from the two sites, also differed in density and in duration of reproductive period. Multiple samples were taken from each population in order to test for temporal variation.ResultsIn both species, the spermatheca consisted of a simple fertilization chamber and a variable number of lateral tubules. The length of the spermatheca showed no temporal or spatial differentiation nor did it show any correlation with snail size. The number of tubules in Helix lucorum ranged from five to sixteen and in Cepaea vindobonensis from one to eight and in both species a significant difference of this trait between the two study sites was detected. In Cepaea vindobonensis, the difference in tubule number led to difference of the total tubule length which reflects sperm storage capacity of the spermatheca but this was not the case with H. lucorum in which no increase in total tubule length was detected.ConclusionsIntraspecific variation in the spermatheca was observed in both snail species studied. The variation observed was independent of snail size, and reproduction status, while the two species responded differently to higher sperm competition levels.

Highlights

Postcopulatory sexual selection is very important in species with reproductive strategies that involve multiple mating and prolonged sperm storage
Spermatheca morphology The morphology of the spermatheca was studied in 276 snails (156 H. lucorum and 120 C. vindobonensis)
The ‘main tubule” was the longest one and in the vast majority of the snails examined it was longer than the fertilization chamber except for two snails from Axios

Summary

Introduction

Postcopulatory sexual selection is very important in species with reproductive strategies that involve multiple mating and prolonged sperm storage. Sperm storage is a common phenomenon in most animals with internal fertilization and, as it temporarily separates copulation from fertilization, it can be adaptive in ecological diverse habitats [1] In this context sperm storage may affect life history and mating system and provides increased opportunity for Females of most sperm storing animals, such as birds [2, 3], reptiles [e.g. 4] amphibians [5], insects [6, 7] and gastropods [8,9,10,11], possess highly specialized structures in their genital systems where sperm is kept alive and capable of fertilization for a variable time period. In several taxa the females may evolve adaptations that favor selective use of sperm from certain donors over others (cryptic female choice) [18]

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