Size-dependent sex allocation in a simultaneous hermaphrodite parasite

Abstract

Most models of sex allocation distinguish between sequential and simultaneous hermaphrodites, although an intermediate sexual pattern, size‐dependent sex allocation, is widespread in plants. Here we investigated sex allocation in a simultaneous hermaphrodite animal, the tapeworm Schistocephalus solidus, in which adult size is highly variable. Sex allocation was determined using stereological techniques, which allow measuring somatic and reproductive tissues in a common currency, namely volume. We investigated the relationships between individual volume and allocation to different reproductive tissues using an allometric model. One measure of female allocation, yolk gland volume, increased more than proportionally with individual volume. This is in contrast to the measure of male allocation, testis volume, which showed a strong tendency to increase less than proportionally with individual volume. Together these patterns led to sex allocation being strongly related to individual volume, with large individuals being more biased towards female allocation. We discuss these findings in the light of current ideas about size‐dependent sex allocation in, primarily, plants and try to extend them to simultaneous hermaphrodite animals.

Stress promotes maleness in hermaphroditic modular animals

“…Thus, in cases like C. hyalina where maternal investment prolongs risk to mortality, the flurry of male activity induced by factors such as mechanical damage or desiccation may be regarded as an act of reproductive bailout. Another example of reproductive bailout may be found in the tape worm Schistocephalus solidus, if it is assumed that the observed inverse relationship between relative male allocation and body size is a response to the decreased life expectancy of individuals stunted by environmental conditions (57). We conclude that increased proportional allocation to the male function is a general response of hermaphroditic modular organisms to constrained productivity and increased risk of mortality associated with localized environmental stress.…”

Sexual Conflict in Hermaphrodites

“…In simultaneous hermaphrodites, male and female fitness is probably also often distributed differently over individuals in a population, and although individuals generally exhibit both sex functions simultaneously, the emphasis often shifts with size (or age), leading to size-dependent sex allocation (Klinkhamer and de Jong 1997;Schärer et al 2001;Schärer 2009). So here again we can expect linkage to arise between sexually antagonistic alleles and alleles that influence sex allocation (and note here that the latter can again be viewed as largely equivalent to the former).…”

How does breeding system variation modulate sexual antagonism?

“…Similarly, hermaphrodites may mitigate deleterious effects of SA mutations by adjusting sex allocation (proportion of reproductive resources invested in male versus female reproduction). For example, individuals carrying an allele enhancing female fitness benefit from biasing sex allocation towards their female function (Schärer et al 2001). The existence or subsequent evolution of linkage between the SA and the sex allocation loci provides one potential mechanism, among others (Lloyd 1975;Charlesworth & Charlesworth 1978;Pannell et al 2008) for transitions from hermaphroditism to dioecy.…”