Evidence for strategic egg production in a hermaphroditic cestode

Abstract

The cestode Schistocephalus solidus is a simultaneous hermaphrodite that grows in 2 intermediate hosts and reproduces rapidly within a few days in the gut of a bird. Reproduction takes place by self-or cross-fertilization. Here, it was tested whether egg production differs between S. solidus that reproduce alone and those that are allowed to reproduce in pairs. Egg production in an in vitro system was found to depend on the cestodes' social situation. When kept alone, larger cestodes produced larger eggs. This was not so when kept in pairs -the difference between these 2 reproductive modes being highly significant in this respect. Further experiments revealed that, within the first 3 days, these hermaphrodites produced a larger total egg mass when kept alone than when kept in pairs. This was also reflected by the energy contents of the cestodes after this time-span : selfers had used up more energy than paired worms. Furthermore, S. solidus appeared to adjust its investment per egg depending on whether the offspring will be the result of self-or cross-fertilization. Selfers produced larger numbers of eggs, but these eggs were smaller and contained even smaller embryos per given egg size than eggs of potentially outbreeding cestodes. Selfed eggs reached lower hatching rates. Although this is to be expected from inbreeding depression it may also be an effect of the reduced maternal investment per egg. The observed phenotypic plasticity in the reproduction of S. solidus is discussed within 4 evolutionary frameworks : local mate competition adjusted for hermaphrodites, the hermaphrodite's dilemma, bet-hedging, and sib-competition.

Lifetime reproductive output in a hermaphrodite cestode when reproducing alone or in pairs: a time cost of pairing

“…We were further able to replicate two key ®ndings of an earlier study (Wedekind et al, 1998), namely that worms that were allowed to reproduce in pairs (a) produced larger eggs, and (b) produced less egg mass until three days after the start of the experiment. This again suggests a di erence in the reproductive modes employed by isolated and paired worms.…”

“…Fitting this growth function to the collected data allowed us to estimate parameters b, k, and a for each replicate of singles and pairs. Di erences in each of these parameters between singles and pairs could have produced the di erence in egg mass by day three observed by Wedekind et al (1998). A di erence in the starting point of reproduction would be indicated by a di erence in parameter b between the treatment groups, a lower rate of egg production in pairs would be indicated by a lower value in parameter k, and a lower magnitude of egg production in pairs would be indicated by a lower value of parameter a.…”

“…We analyzed about 20% of the total sample. Egg volume is expected to range between 50,000 to 100,000 lm 3 (Wedekind et al, 1998) and we used a 280 lm diameter ori®ce tube with the sampling range set between 30,700 and 131,600 lm 3 . We analyzed an average of 1487 (SE 85) eggs per sample.…”

“…Reproductive strategies employed in facultatively sel®ng simultaneous hermaphrodites are therefore expected to depend on the presence or absence of a mating partner, and the costs and bene®ts associated with self-vs. crossfertilisation. Wedekind et al (1998) investigated reproductive parameters between individuals of the simultaneous hermaphrodite tapeworm Schistocephalus solidus (MuÈ ller, 1776). Worms were allowed to reproduce in an in vitro system replacing the intestine of the ®nal host, while being kept isolated (singles, only self-fertilisation possible) or in groups of two (pairs, both self-and crossfertilisation possible).…”

“…Here we test for di erences in the starting point, the rate, and the magnitude of egg production between individuals allowed to reproduce alone (only self-fertilisation possible) or in pairs (both self-and cross-fertilisation possible). Speci®cally, we want to distinguish between alternative processes responsible for the lower egg production in paired individuals observed in an earlier study (Wedekind et al, 1998). We designed an improved in vitro system, replacing the bird ®nal host that allows us to measure, with high temporal resolution, the timing and magnitude of lifetime egg production of worms in these two social situations.…”