Turbellarian taxonomic database

Abstract / Notes

"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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