pronuclei. Parthenogenesis Organisms that normally reproduce sexually can also reproduce via parthenogenesis, wherein an unfertilised female gamete produces viable offspring. These offspring may be clones of the mother, or in some cases genetically differ from her but inherit only part of her DNA. Parthenogenesis occurs in many plants and animals and may be induced in others through a chemical or electrical stimulus to the egg cell. In 2004, Japanese researchers led by Tomohiro Kono succeeded after 457 attempts to merge the ova of two mice by blocking certain proteins that would normally prevent the possibility; the resulting embryo normally developed into a mouse. Allogamy and autogamy Allogamy, which is also known as cross-fertilisation, refers to the fertilisation of an egg cell from one individual with the male gamete of another. Autogamy which is also known as self-fertilisation, occurs in such hermaphroditic organisms as plants and flatworms; therein, two gametes from one individual fuse. Other variants of bisexual reproduction Some relatively unusual forms of reproduction are: Gynogenesis: A sperm stimulates the egg to develop without fertilisation or syngamy. The sperm may enter the egg. Hybridogenesis: One genome is eliminated to produce haploid eggs. Canina meiosis: (sometimes called "permanent odd polyploidy") one genome is transmitted in the Mendelian fashion, others are transmitted clonally. Benefits of cross-fertilisation The major benefit of cross-fertilisation is generally thought to be the avoidance of inbreeding depression. Charles Darwin, in his 1876 book The Effects of Cross and Self Fertilisation in the Vegetable Kingdom (pages 466-467) summed up his findings in the following way. "It has been shown in the present volume that the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigour and fertility over the self-fertilised offspring from one of the same parents. And this fact is amply sufficient to account for the development of the sexual elements, that is, for the genesis of the two sexes." In addition, it is thought by some, that a long-term advantage of out-crossing in nature is increased genetic variability that promotes adaptation or avoidance of extinction (see Genetic variability). See also Cell fusion Conception cap Conception device Female sperm Fetal development In vitro fertilisation Kaguya (mouse) Parthenogenesis, a type of reproduction that does not involve fertilisation Pollination Pre-embryo Pronucleus Superfecundation Superfetation Symmetry breaking and cortical rotation Cortical reaction Polyspermy References