The maintenance of sex is one of the hottest topics in evolutionary biology. The most obvious advantage of sexual reproduction is genetic recombination and the ability to evolve. However, there are a number of disadvantages associated with sexual reproduction, such as costs for the production of males, the breakdown of favourable gene combinations and the risk of remaining unmated. Nevertheless, broad-scale replacement of sexual species by asexual clones has not occurred.
Diploid species reproduce sexually and are normally self-incompatible.
Triploids are asexual and produce seeds through parthenogenic development of unreduced egg cells. Mixed populations of sexuals and asexuals occur in western and central Europe and offer the opportunity to study the mechanisms for the maintenance of sex. The application of allozyme markers in crosses between sexual diploids and asexual triploids showed that most of the progeny was due to selfing. Diploid hybrids were sexual, but triploid hybrids showed various degrees of asexual reproduction, ranging from purely sexual to allmost completely asexual. Tetraploids also were produced in the crosses and were allmost purely asexual.