Reproductive investment strategies across hermaphroditic freshwater snails

Research output: PhD ThesisPhD-Thesis - Research and graduation internal

Abstract

Studies on sexual selection and sexual conflict often focus on species with sexually dimorphic traits (e.g. differences in size, shape, colour, and or structures between the sexes), leaving research on species that lack such differences lagging, notably hermaphrodites. However, hermaphrodites experience strong selection on reproductive traits and experience sexual conflicts. Possessing both male and female reproductive organs provides a new dimension for sexual conflict and warrants attention. Hermaphrodites and separate-sexed organisms have gamete dimorphism (numerous tiny sperm vs few large eggs), resulting in different male and female functions and reproductive strategies. In hermaphrodites, conflicts over sex-role decisions and sex allocation further complicate the matter. By being flexible in allocating resources to either sex function, hermaphrodites can experience severe sexual conflict. The reproductive investment strategies employed by Lymnaea stagnalis, a hermaphroditic freshwater snail, are studied and expanded to include other species. In L. stagnalis, egg masses often have multiple paternity (which they can store for two months) together with a small portion of self-fertilised eggs. After insemination, individuals experience a short-term fitness loss in their female and male functions (suppression in egg-laying and sperm transfer). With such fitness reduction, we expect both partners to prefer being male, resulting in a conflict of interest. We show that younger and smaller individuals take on the male role first. After insemination, male mating motivation does not decrease in older and larger snails, even though they transfer fewer sperm. Lymnaea stagnalis semen carries accessory gland proteins (Acps) suppressing egg-laying and sperm transfer. Ovipostatin, the Acp reducing egg-laying in the recipient, is fully characterised in chapter three. After ejaculation, Ovipostatin expression increases in the prostate for both primary (only ejaculate) and secondary (ejaculate after being inseminated) donors. Ovipostatin likely plays a role in (the regulation of) reproduction. Chapter four is primarily a visual aid to sedate or euthanise a snail and remove the prostate and seminal vesicles. Intravaginally injecting sedated snails with different compounds (sperm, prostate fluid or its separate compounds) helps elucidate their effects on female reproduction by quantifying egg-laying and reproductive success (egg masses, eggs per mass, egg and hatchling size and hatchling success are measured). Using the above techniques helped identify the effect of seminal fluid proteins in L. stagnalis, with Ovipostatin affecting the female function (egg-laying) and LyAcp5 and LyAcp8b the male function (sperm transfer) of the partner. To determine if the receipt of an ejaculate commonly suppresses female fecundity in freshwater snails, we tested eight species in chapter five. We compared egg output and egg size of isolated non-virgin snails to paired snails to calculate changes in female reproductive success. Changes varied between the species in reduced female fecundity (e.g. fewer egg masses ad or eggs) as shifts in resource allocation in eggs (e.g. fewer but bigger eggs). Together with receiving Acps, changes in female fecundity may result from differing mating costs in either the male or female function. Hence, changes in the maternal environment, including receiving Acps, affect offspring investment in these species highlighting this is not exclusive to gonochorists. Receiving an ejaculate shows a similar reduction in the male function with sperm transfer near halved (via LyAcp5 and LyAcp8b). Cross-reactivity tests determine whether the prostate fluid of related species causes a comparable effect in L. stagnalis. Of the species used, only the lymnaeids reduced sperm transfer, while the planorbids had no effect. Although this implies an evolutionarily conserved function, more species are needed to confirm a phylogenetic signal. In sum, sexual conflicts include but are not limited to separate-sexed organisms and, simultaneous hermaphrodites are more complex and fascinating than many realise and deserve more attention from the scientific world.
Original languageEnglish
QualificationPhD
Awarding Institution
  • Vrije Universiteit Amsterdam
Supervisors/Advisors
  • Ellers, J, Supervisor
  • van Straalen, Nico, Supervisor
  • Koene, JM, Co-supervisor
Award date10 Dec 2021
Place of Publications.l.
Publisher
Print ISBNs9789464235425
Publication statusPublished - 10 Dec 2021

Keywords

  • Sexual conflict, hermaphrodites, sex allocation, sex peptide, mating history, pond snail, sperm competition, size-advantage model, reproductive strategy, Lymnaea stagnalis

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