Other animals have different sex-determination systems, such as the ZW system in birds, the X0 system in insects, and various environmental systems, for example in crustaceans.
Fungi may also have more complex allelic mating systems, with sexes not accurately described as male, female, or hermaphroditic.
The defining characteristic of sexual reproduction in eukaryotes is the difference between the gametes and the binary nature of fertilization.
Multiplicity of gamete types within a species would still be considered a form of sexual reproduction.
The ZW sex-determination system is shared by birds, some fish and some crustaceans.
Sex comprises the arrangements that enable sexual reproduction, and has evolved alongside the reproduction system, starting with similar gametes (isogamy) and progressing to systems that have different gamete types, such as those involving a large female gamete (ovum) and a small male gamete (sperm).Typically, prior to an asexual division, a cell duplicates its genetic information content, and then divides. In sexual reproduction, there are special kinds of cells that divide without prior duplication of its genetic material, in a process named meiosis.The resulting cells are called gametes, and contain only half the genetic material of the parent cells.Gametes can be identical in form and function (known as isogamy), but, in many cases, an asymmetry has evolved such that two different types of gametes (heterogametes) exist (known as anisogamy).Physical differences are often associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience.