POLYPHYLETIC ORIGINS OF ASEXUALITY IN DAPHNIA PULEX. I. BREEDING-SYSTEM VARIATION AND LEVELS OF CLONAL DIVERSITY

Area of Science:

• Evolutionary Biology
• Genetics
• Reproductive Biology

Background:

• Transitions from sexual to asexual reproduction are increasingly linked to internal genetic factors.
• In Daphnia pulex, a shift to asexuality is associated with meiosis suppression.
• Cyclically parthenogenetic populations of Daphnia pulex exist in the southern part of its range.

Purpose of the Study:

• To investigate the genotypic structure of Daphnia pulex populations across a breeding system shift.
• To test the meiosis-suppressor model's predictions regarding the origins and coexistence of asexual and sexual reproduction.
• To examine microgeographical variation in breeding systems within Daphnia pulex.

Main Methods:

• Allozyme analysis of 77 Daphnia pulex populations.
• Genotypic characterization of obligately asexual and cyclically parthenogenetic clones.
• Microgeographical population surveys in Michigan and Ontario.

Main Results:

• Identified 50 obligately asexual clones, with two distinct origins: single species and interspecific hybridization.
• Confirmed the prediction of unstable coexistence between cyclically parthenogenetic and male-producing asexual clones.
• Discovered a low incidence of cyclically parthenogenetic populations in Ontario, exhibiting high genotypic diversity.

Conclusions:

• Asexuality in Daphnia pulex likely has polyphyletic origins, driven by meiosis suppression.
• The genetic diversity in rare sexual populations suggests resistance to asexual reproduction and adaptation.
• Internal genetic factors play a significant role in the evolution of reproductive strategies in Daphnia pulex.