Selection in Cladoccra on the Basis of a Physiological Caracter

Selection in Cladoccra on the Basis of a Physiological Caracter

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Selection in Cladoccra on the Basis of a Physiological Caracter

Selection in Cladoccra on the Basis of a Physiological Caracter

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Excerpt

For more than 8 years ( January 1920) the writer has been rearing parthenogenetic "pure lines" of Cladocera. The original object of rearing this material was the conducting of experiments on selection within the pure line. These experiments were completed in May 1917. Data bearing on other problems have also been secured from the handling of this material. The selection experiments only will be treated in the present paper.The writer undertook the selection experiments in order to get additional data on the effects of selection within the pure line. The Cladocera material was chosen for several reasons:

1. It reproduces rapidly.

2. Under favorable conditions it is readily handled in the laboratory.

3. It reproduces parthenogenetically with (under carefully controlled conditions) no possible question as to the occurrence of sexual reproduction.

4. In the maturation of the parthenogenetic eggs of Cladocera there is a single division without reduction (Weismann, 1886; Kühn, 1908). Hence with this material there are presumably no complications of segregation during maturation or of fertilization as with sexually reproducing forms.

5. No selection experiments had been made with a purely physiological character as the basis for selection. Aside from the fact that a physiological character had probably not been used previously as a basis for studies on the effect of selection, a purely physiological character seemed desirable to use for two additional reasons: (a) Physiological differences are frequently readily measurable and measurable in a precise way. Numerical series of measurements provide very usable series of data free from the errors of estimation and personal equation applying to series not directly numerical. The light reactions of Cladocera afford a character perhaps as definitely measurable as any readily handled physiological characteristic. (b) In both the ontogenetic and phylogenetic history of organisms physiological modifications may occur without observed morphological changes, and in the modification of organisms physiological modifications may readily precede distinguishable or measurable morphological changes. This latter consideration (b) had the greatest weight in determining the use of a physiological character as the basis for these experiments.

The rapidly breeding, easily propagated, parthenogenetic organism possessing a readily measurable physiological character afforded just the material desired.

The experimental object was to attempt to isolate by selection, through a number of generations, a strain more responsive to light than a second strain from the same pure line1 selected for its reduced responsiveness to light.2

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