Fig. 1.

Experimental design and predictions of the test for plasticity in testis size and spermatogenesis kinetics according to the social environment in Macrostomum lignano. The experiment comprised a treatment phase in which flatworms were allocated to and maintained in standardized group sizes of either two or eight worms (‘pairs’ and ‘octets’, respectively). They were then pulsed for 30 min with 5 mM BrdU in order to label a pool of S-phase cells, which was then visualized after a variable chase phase of 5, 6 or 7d. One experimental subject per replicate was then used for a morphological assay of testis area and a second for a spermatogenesis assay that meant scoring their testes for the presence of elongated BrdU-labelled spermatids. The two images are z-projections of the testis region of M. lignano derived from confocal laser scanning micrographs of worms fixed 4 and 5 days following BrdU pulse (left and right, respectively), to illustrate the morphological appearance of testes containing no BrdU-labelled elongating spermatids and testes containing BrdU-labelled elongating spermatids, respectively (images modified from Schärer et al. 2007). Anterior is to the top left. The inset on the right-hand image highlights an area towards the centre of the testis where elongating spermatids are clearly visible (indicated by the arrowhead). For full details of the experimental design in this study, see Methods. Specific predictions depicted in the bottom panels are explained in the Results