Sperm exchange in a simultaneous hermaphrodite |
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Authors: | Nicolaas K Michiels Andrea Streng |
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Institution: | (1) Max-Planck-Institut für Verhaltensphysiologie, Seewiesen, P.O. Box 1564, D-82305 Starnberg, Germany Tel.: +49-8157-932.233; Fax: +49-8157-932.209; e-mail: michiels@mpi-seewiesen.mpg.de, DE |
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Abstract: | Many simultaneously hermaphroditic animals cross-inseminate when copulating, even though unilateral sperm exchange is functionally
possible. The question is why reciprocity is the rule. This study addresses sperm exchange in a hermaphroditic planarian flatworm,
Dugesia polychroa, in which self-fertilisation does not occur. We investigated the availability of self sperm for inseminating a partner, how
self sperm affects the likelihood of sperm donation, and whether sperm donation depends on reciprocation by the partner. The
amount of self sperm depended on body size, duration of isolation and experimental date, and animals that had more self sperm
were more likely to mate. Depletion of allosperm (sperm received from partners) played only an indirect role, suggesting that
animals copulate more to donate sperm rather than to replenish allosperm reserves. Among 60 copulating pairs, reciprocal insemination
was more common (63.3%) than unilateral transfer (18.3%). A surprising 18.3% did not exchange sperm in either direction. Individuals
were more likely to behave like their partners, resulting in more symmetrical matings (either reciprocity or no sperm exchange)
than asymmetrical matings (unilateral sperm donation). The amount of self sperm donated during a copulation depended mainly
on the amount available before the copulation and not on the amount received from the partner. The results suggest that conditional
sperm exchange, or sperm trading, takes place in D. polychroa.
Received: 31 May 1997 / Accepted after revision: 1 December 1997 |
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Keywords: | Platyhelminthes Hermaphroditism Mating conflict Mate assessment Sperm trading |
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