. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 14 21 28 t I me (d) 35 42 Figure 1. Examples for the time course of switching from the vege- tative to the bisexual mode of reproduction induced by the photoperiod in Sty/aria lacusiris. Vegetatively reproducing worms from the field were exposed to short-day conditions of either LD = 12:12 or LD = 6: 18 at day 0 in the figure; A = Weddel 1985, B = Ilmenau 1986 popula- tion; the points for LD = 16:8 are not shown, for all of them would lie along the abscissa (= 0% sexual reproduction); N > 1000 for each population. calcula
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 14 21 28 t I me (d) 35 42 Figure 1. Examples for the time course of switching from the vege- tative to the bisexual mode of reproduction induced by the photoperiod in Sty/aria lacusiris. Vegetatively reproducing worms from the field were exposed to short-day conditions of either LD = 12:12 or LD = 6: 18 at day 0 in the figure; A = Weddel 1985, B = Ilmenau 1986 popula- tion; the points for LD = 16:8 are not shown, for all of them would lie along the abscissa (= 0% sexual reproduction); N > 1000 for each population. calculated from the initial population size (N 1), the final population size (N2) and the time (t) in days between the countings: mdt = log 2 t/(log N2 - log N1). Statistics. The non-parametric Mann-Whitney-U test (two-tailed) was used to compare the means of two inde- pendent samples, and the Jonckheere test was used to look for monotonous trends in three or more indepen- dent samples (Lienert, 1976). The number of statistical replicates ( number of cultures tested under the same experimental conditions) is given as n in the text, whereas N means the total number of worms checked for their reproductive mode during one experiment. The alternation of reproductive mode worked only in one direction, , only vegetative worms switched to sexual reproduction. Once this switch had occurred, it was cum grano salis irreversible. Ninety-one clitellate worms had been transferred back from LD = 6:18 to LD = 16:8 (T = 20°C) and watched for 42 continuous days. Only three worms reverted to vegetative asexual repro- duction by forming tomits; in these three worms the cli- tellum of the parent individual was retained, whereas the daughter individuals showed normal asexual shape. Bisexual reproduction excluded fission, , clitellate worms never formed tomits. Therefore, once a popula- tion became sexual, the number of worms per chamber became constant or even declined slightly, because some
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