Phenotypic tradeoffs between egg number and egg size in three parasitic anisakid nematodes

Abstract

Phenotypic tradeoffs between number and size of eggs were tested in three component populations of three marine anisakid nematodes: Anisakis simplex , Pseudoterranova decipiens and Contracaecum osculatum . Body and uterine volumes (as proxies of female size), and egg number, mean egg volume and clutch volume (as descriptors of reproductive output) were measured in 50 females of each species. Evidence of a phenotypic tradeoff was detected only in A. simplex ; the first time that has been found in a parasite population. Comparison of feasible values inferred from the van Noordwijk and de Jong's model and current data showed that interindividual variation in egg size was narrower than expected in the three populations. Structural constrictions in response to optimal allocation rules might account for this pattern. In P. decipiens and C. osculatum variation in egg size was the lowest and independent of female size, suggesting that the tradeoff is absent, rather than being present but masked by unaccounted variables. Spatial constrictions imposed by uterine size seemed to play an important role determining the emergence of the tradeoffs. So factors accounting for the tradeoff in A. simplex are probably constructional rather than physiological. Individual variability in investment in clutch volume was similar to previous studies but variation in allocation between number and size of eggs was much smaller than that reported previously. Perhaps differences in life-history strategies might explain this because the nematodes studied are either semelparous or short-lived iteroparus organisms whereas previous data derive from long-lived iteroparous ones. Despite the perception that parasites live in resource-rich habitats, the present study indicates that patterns relating number and size of eggs might not differ much from those observed in free-living populations and thus the same range of factors would operate in both types of organisms.The reproductive output of any individual can be partitioned into two components, propagule number and propagule size. The relationship between them is still poorly known in parasites, where only two studies concerning Graphidioides subterraneus , a parasitic nematode of rodents (Rossin et al. 2005), and Lernanthropus cynoscicola, a copepod parasite on fish (Timi et al. 2005), have been carried out at population level. Because both components compete directly for the limited energy budget and/or for the limited space within a mother, a tradeoff between them is expected. Previous studies (including the two of parasites mentioned above) have used correlations between number and size of eggs in order to test for the occurrence of this phenotypic tradeoff. It has been argued that this approach is of limited value because (1) it is impossible to infer causation from correlation, (2) environmental variation may mask or accentuate the correlation, and (3) phenotypic correlations do not demonstrate genetic control of the tradeoffs (Reznick 1985, Lessells et al. 1989. However, when selection acts on...

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