Lorenzi MC et al. (2021), Egg-trading worms start reciprocation with caut...

ECB-ART-49172

Sci Rep 2021 May 18;111:10552. doi: 10.1038/s41598-021-89979-7.

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Egg-trading worms start reciprocation with caution, respond with confidence and care about partners' quality.

Lorenzi MC , Schleicherová D , Robles-Guerrero FG , Dumas M , Araguas A .

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Conditional reciprocity (help someone who helped you before) explains the evolution of cooperation among unrelated individuals who take turns helping each other. Reciprocity is vulnerable to exploitations, and players are expected to identify uncooperative partners who do not return the help they received. We tested this prediction in the simultaneously hermaphroditic worm, Ophryotrocha diadema, which engages in mutual egg donations by alternating sexual roles (one worm releases' eggs and the other fertilizes them). We set up dyads with different cooperativeness expectations; partners were either the same or a different body size (body size predicts clutch size). Large worms offered larger clutches and did so sooner when paired with large rather than small partners. They also released smaller egg clutches when they started egg donations than when they responded to a partners' donation, fulfilling the prediction that a players' first move will be prudent. Finally, behavioral bodily interactions were more frequent between more size-dissimilar worms, suggesting that worms engaged in low-cost behavioral exchanges before investing in such costly moves as egg donations. These results support the hypothesis that simultaneously hermaphroditic worms follow a conditional reciprocity paradigm and solve the conflict over sexual roles by sharing the costs of reproduction via the male and the female functions.

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	Figure 1. Schematic representation of the experimental design for Experiment 1—reciprocation in size-matched and size-unmatched dyads. The effect of the size of the focal worm (large/small) and that of the partner (matched/unmatched) were tested. The star indicates the focal worms (i.e., yellow-phenotype worms, see Materials and Methods) used to take measurements (number of replicates: 36 per condition). Worm silhouettes by Scott Hartman are available under Public Domain license at PhyloPic (http://phylopic.org/).
	Figure 2. The start of reciprocity. (a) The latency to start egg donation for large and small focal worms as a function of the size of their partner (size-matched or size-unmatched partners). Large focal worms donated their eggs sooner than small focal worms, and significantly sooner when paired with large (matched) rather than small (unmatched) partners. (b) The size of the first clutch (the number of eggs) depended on the size of the focal worm (large or small) and the partner’s behavior (whether the focal worm started cooperation or responded to the partner’s donation). Large focal worms offered smaller clutches when they were the first to donate eggs than when they were reciprocating eggs to partners who had previously laid eggs. Small focal worms offered smaller, but relatively similarly sized egg-clutches, irrespective of condition.
	Figure 3. Egg investment and body growth throughout the experiment. (a) Egg investment (predicted values of the total number of eggs produced throughout the experiment) varied between large and small worms in different ways depending on the size of their partners. (b) The increase in body size (predicted number of segments gained throughout the experiment) varied between large and small worms in different ways depending on the size of their partners.
	Figure 4. Behavioral interactions between partners. Both the proportion of behavioral interactions (a) and the frequency of reciprocal rubbing behaviors (b) increased with increasing size differences among partners (points jittered to prevent overlap).
	Figure 5. Reciprocal rubbing behavior when focal worms were given the choice between two partners of different quality: (a) frequency (events/hour) and (b) duration (proportion of time) of rubbing behavior as a function of the relative body size and egg maturation level of the two potential partners. When given such a choice, focal worms rubbed the larger and more mature worm more often and for longer.

References [+] :

André, Perfect reciprocity is the only evolutionarily stable strategy in the continuous iterated prisoner's dilemma. 2007, Pubmed