It is the method of sexual reproduction involving the union of sperm and egg in green plants like bryophytes. The large, non-motile egg formed in heterogamy is full of stored food. After fertilization this stored food is used to nourish the early stages in the development of new offsprings.
The genetic variation in bacteria is achieved either by mutation or genetic recombination. In bacteria, sexual reproduction occurs by genetic recombination which is a primitive type of sexual reproduction. In genetic recombination. DNA from two different sources combine. The cells do not fuse, only a piece of DNA of donor cell is inserted in the recipient cell. In recipient cell, the DNA portion of the donor cell orients itself in such a way that the homologous genes come close to one another. A fragment of the DNA of the recipient is knocked off and DNA of donor is integrated into it. The recipient cell is now called a recombinant cell. There are three methods of bacterial recombination i.e. transformation, transduction and conjugation.
Conjugation is a recombination process in which living bacteria come into direct contact and the donor cell transfers DNA to the recipient cell. The DNA transfer is one way. The process was studied in 1946 by Lederberg and Tatum in Escherichia coli.
Normally Escherichia coli can synthesize all amino acids it requires. It was exposed to shortwave radiation and two mutants were isolated. One mutant was unable to synthesize biotin (a vitamin) and amino acid methionine. The other mutant could not synthesize amino acids threonine and leucine. The four chemicals are essential for the growth of bacteria. The two mutants were mixed and cultured in a common medium lacking in all the four compounds. None of the cells would have grown in the absence of essential chemicals, but to the great astonishment of researchers, hundreds of colonies of bacteria developed. This suggests that exchange of genes has occurred between two parental bacteria and new recombinants were formed which did not require the four essential compounds for growth. Later studies made by electron microscope confirmed the close contact and the formation of conjugation tube between die parental cells.