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Bigger testes do work more: experimental evidence that testis size reflects testicular cell proliferation activity in the marine invertebrate, the free-living flatworm Macrostomum sp.

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Abstract

Evolutionary theory predicts that more resources are allocated to sperm production when sperm competition is high. This prediction is supported by both comparative and experimental data on static measures of male allocation, such as testis and ejaculate size. However, resource allocation is a dynamic process, and it is therefore important to evaluate if the static measures reflect this. Such an evaluation has to our knowledge never been done. Immunocytochemical labelling with bromodeoxyuridine (BrdU) allows visualisation of cells in S-phase. BrdU becomes incorporated into cells if, and only if, they are actively undergoing DNA duplication, which is a dynamic process. The number of BrdU-positive cells in the testis can hence serve as a dynamic measure of male allocation, i.e. testicular activity. We evaluate the relationship between testis size and testicular activity in the marine flatworm Macrostomum sp. In a previous study, we showed that testis size is phenotypically plastic in this species, and that worms make larger testes when they are raised in larger groups. We use this plasticity to experimentally produce variation in testis size, and demonstrate that larger testes are associated with higher testicular activity. Moreover, testis size and testicular activity were related linearly. We have thus, for the first time, shown that testis size is a good measure of resource allocation to the male function. Moreover, increased testicular activity is probably one of the first steps in the upregulation of sperm production. It is thus expected that testicular activity is a more sensitive measure of short-term variation in male allocation than the commonly used static measures.

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Acknowledgements

We would like to thank Martina Hrouda, Monika Mahlknecht, Katharina Nimeth and Willi Salvenmoser for help in the laboratory, Bernd Pelster for access to the laser scanning microscope, and the NIH-Image mailing list community, particularly Norbert Vischer, for invaluable help regarding image analysis and for providing an import macro for Zeiss LSM 510 files. Jacob Koella, Nico Michiels, Peter Sandner, Solveig Schjørring, Dita Vizoso, and two anonymous referees provided helpful discussion and comments on the manuscript. During this study, L.S. was supported by an IHP-fellowship (SNF, Switzerland) and a Lise-Meitner-fellowship (FWF, Austria), and P.L. by an APART fellowship (Austrian Academy of Science). The study was further supported by grant P15204 (FWF, Austria) to R.M.R. Animal experimentation was carried out in accordance with Austrian legal and ethical standards.

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Schärer, L., Ladurner, P. & Rieger, R.M. Bigger testes do work more: experimental evidence that testis size reflects testicular cell proliferation activity in the marine invertebrate, the free-living flatworm Macrostomum sp.. Behav Ecol Sociobiol 56, 420–425 (2004). https://doi.org/10.1007/s00265-004-0802-9

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