Small sperm, uniparental inheritance and selfish cytoplasmic elements: a comparison of two models

It has previously been suggested that small sperm size may be an adaptation to achieve uniparental inheritance of organelles, and hence to prevent the spread of selfish cytoplasmic elements. Such an explanation for anisogamy implies a mechanism whereby the male gamete eliminates its own cytoplasm pr ior to fusion with the egg. A model has been presented demonstrating the in vasion and persistence of a modifier that acts gametically to kill its own organelles. Here we show, however, that this model is far from robust; inde ed, if any cost is associated with the modifier it cannot persist. We also show that despite an empirically demonstrated association between anisogamy and multicellularity, this result also applies if the analysis is applied in the multicellular case. This class of model contrasts with the majority of analyses in which the modifier kills off the incoming gamete's organelle s. We show that these models are highly robust, even if uniparental inherit ance is imperfect.