Giant Sperm in nonmarine ostracods (Cypridoidea): male fillers or female control?

• Applicant: Dr. Renate Matzke-Karasz
• Subject Area: Evolutionary Cell and Developmental Biology (Zoology)
• Term: from 2011 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 195109972

Recent cypridoidean ostracods (Podocopa) are living mainly in fresh or brackish water environments. They reproduce with filamentous giant sperm, reaching up to ten times the body length of the animal. A common origin of their reproduction with giant sperm is well supported by fossil evidence from the lower Cretaceous, speaking for an evolutionary success of this reproductive mode. Considering the energetic costs related to production, transport and storage of giant sperm, the question about the advantages stands to reason. Hypotheses that have been put forward for other groups with giant sperm (e.g. copulatory plug, female cryptic choice or zygote nutrition) must be seen as approxima-tions, which are each applicable for at least a few of the regarding taxa. However, none of the existing hypotheses can be satisfactorily assigned to the situation on ostracods. The present project outline features two hypotheses, of which one, hypothesis B, has been developed in consideration of the special anatomy of ostracods. The hypotheses are: (A) male freshwater ostracods developed giant sperm in order to fill the female receptacles and thus to prevent other males inseminating the female (aim: optimization of fertilization success) and (B) giant sperm help female freshwater ostracods to reduce the sperm transfer rate per time unit (aim: diversification of paternities). A lower sperm transfer rate not only reduces the absolute sperm number per completed insemination; female ostracods potentially have the opportunity to interrupt copulation by closing their valves. A low sperm transfer rate thus allows for an improved control of the number of sperms transferred by actively determining the copulation duration. Evolution of extraordinary long female sperm ducts and a complex female reproductive system may have caused a selective pressure on males towards the evolution of giant sperm.

• DFG Programme: Research Grants