In vitro double fertilization in Nicotiana tabacum (L.): the role of cell volume in cell fusion

The role of cell volume in regulating cell fusion was investigated by in vi tro fertilization to elucidate mechanisms involved in double fertilization. The results revealed that in our model gamete fusion was more efficient th an the fusion of somatic cells under the same conditions. The fusion of sel ected mesophyll protoplasts of different sizes and their fusion with chloro plasts demonstrated that cell volume ratio played a role in this process: a s the ratio increased, fusion was more efficient and faster. When one of th e cells was as small as a sperm, the formation of a round fusion product wa s faster. This might explain why gamete fusion was highly efficient in all in vitro germ cell fusion systems. This finding may also explain why sperm evolved as small cells. The results reported here will be useful for interp reting and evaluating data of in vitro fertilization experiments and for di stinguishing gamete-specific characters.