ABSTRACT
Dioecy is of recent origin and has arisen independently 875 – 5,000 times. Of 18,004 dioecious angiosperm species, cytological evidence for heterogamety is available only for 50 species in 18 families. Notably, only a single incidence is reported for the most speciose (24,700) families like Asteraceae, while the highest frequency of 8 among 1,200 species is known from the less speciose Polygonaceae. Surprisingly, 80% angiosperms and animals are male heterogametics. Whereas females generate only one egg/ovule per oogonium, males produce four sperms/pollens per spermatogonium/pollen. Thereby the males produce a greater number of new gene combinations – the raw material for evolution and speciation. This is why many biologists consider the evolution as a male-driven process.
