Date of Award


Degree Name

Master of Arts


Biological Sciences

First Advisor

Dr. Dona J. Fowler

Second Advisor

Dr. Clarence Goodnight

Third Advisor

Dr. Gyula Ficsor

Access Setting

Masters Thesis-Open Access



Daphnia pulex Leydig (Crustacea, Cladocera) is a common component of the zooplankton of freshwater lakes and ponds. Its importance in the aquatic trophic structure as food for small fish has long been recognized.

Typical populations of daphnia consist almost exclusively of females. Under favorable conditions this situation is perpetuated by parthenogenetic reproduction. Eggs produced in this manner are referred to as parthenogenetic or summer eggs. When stressed by falling temperatures, drought, overpopulation, or starvation, populations shift to sexual.reproduction. Parthenogenetic eggs are produced which develop into males. Other eggs known as ephippial or winter eggs are also produced, which after fertilization survive the environmental stress inside cocoon-like ephippia. Upon return to favorable conditions, these eggs hatch into normal diploid females and help reestablish the population.

The cause of male induction is intriguing. Cytological (Mortimer, 1936; von Dehn, 1488, Ojilla., 1958) and genetic (Banta and Wood, 1928) studies have shown that both sexes in daphnids are diploid and possess identical genomes. Thus, some (Mortimer, 1936; von Dehn, 1955) believe that sex determination is purely phenotypic. Furthermore, it is their contention that phenotype is under environmental control. This position has been succinctly stated by von Dehn (1955): ''Die Geschlectsbestimmung kann...nur modifikatorisch erfolgen unter der Wirkung von Aussenfaktoren." Whether this environmental control is absolute is open to debate, but its dominant role in the process is generally accepted (Waterman, 1960).

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