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Dr. Christian Josephs

Gastwissenschaftler / Guest Scientist

Research Intrest

My research interests aim for the occurrence of sexual processes in parthenogenetic reproducing daphnia-populations (Daphnia pulex, Daphnia magna, Moina

). Long since August Weismann (1876) and Banta (1939) this problem is a topic of many investigations (Peters and Bernhardi, 1987) and still not solved (Kaestner, 1993).

Since 1962 I deal with this problem using a flow-channel count-chamber for daphnia and the measurement of enviromental factors in 30l tanks, in which I start a self-purification process using duck dung. The results are quoted for the first time by Bick (1966). They show, that diploid parthenogenetic females of daphnia lay diploid eggs, from which emerge males, only under the condition of strong exponential population growth.

 

You can download the results in PDF-format below

by following the links behind the literature. After my retirement as a director of studies - I trained biology teachers - I was excited about an experimental access to the production of daphnia males. By isolating females of daphnia and investigating the sex ratio per clutch it is possible to calculate the body-size growth process for male determinants in the oocyte of the ovary.

Furthermore, concerning the enviromental factors around the determination point, I am interested in the diet of daphnia.

 

Attention: The pdf-files are results from a scan, the files are big and a high speed connection for download is recommended. The size of a file is written beside each download link.
 

Ausgewählte Publikationen

  • Josephs, C. (2021). Die Entschlüsselung des Phänomens der Bisexualität bei den Daphniden Daphnia pulex und Daphnia magna. (publication in prep.) pdf (0.7 Mbyte)
  • Raveh, S., Langen, K., Bakker, T. C. M., Josephs, C. & Frommen J. G. (2018). Oddity, predation risk and social decisions in aquatic invertebrates. Ethology (published online, DOI: 10.1111/eth.12835).
  • Josephs, C. (2013). Die Daphnienmännchenbildung bei Daphnia pulex und Daphnia magna im Populationenvergleich bei Selbstreinigungs-Sukzessionsversuchen als Schlüssel zum Verständnis der Fortpflanzungsstrategie bei den Daphniden – ein weiterer experimenteller Zugriff zur Lösung eines alten Problems des Wechsels von Parthenogenesis zu Bisexualität (Crustacea, Cladocera, Daphnidae).

PhD Theses

  • Josephs, C. (1965). Experimentelle Untersuchung der Populationsdynamik bei Daphnia pulex DE GEER mit und ohne Wechsel von Parthenogenesis zu Bisexualität.
    Dissertation, University of Bonn, Germany. pdf (33.8 MByte)

MSc Theses

  • Josephs, C. (1963). Kulturversuche über die Populationsdynamik bei Daphnia pulex. Examensarbeit, University of Bonn, Germany.

 

 

 

 

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