Asexual and sexual reproduction

In sexual reproduction the gametes of two parents (male and female sex cells) fuse to form a zygote; the genotype of the new organism differs from the parents’, so sexual reproduction is a source of combinative variation and is important in evolution. Forms: isogamy (identical motile gametes – Ulothrix), heterogamy (motile but the female larger – Chlamydomonas), oogamy (a large immobile egg cell and a small spermatozoon or sperm cell). In animals gametogenesis occurs in the gonads: zones of multiplication (mitosis), growth, maturation (meiosis) and shaping; in spermatogenesis one cell gives 4 spermatozoa, in oogenesis one large egg cell and 3 small cells (which later die). In flowering plants the pollen grain (with 2 sperm cells) develops in the anther through meiosis and mitosis, and the embryo sac (an egg cell and a central cell with 2 nuclei) in the ovule. In double fertilisation one sperm fuses with the egg to form the zygote (2n), the other with the central cell to form the endosperm; the ovule becomes a seed and the ovary a fruit. In ciliates conjugation (exchange of nuclei) renews hereditary information but does not increase the number of offspring. In parthenogenesis an egg develops without fertilisation (drones in honeybees).