Why sex evolved at all remains one of biology's great puzzles
Sexual reproduction looks like a bad bargain. Only half of a population, the females, bears young, and each parent passes on just half its genes. Biologists call this the two-fold cost of sex. Yet most animals and plants reproduce sexually, and one ant species manages entirely without males.
Reproduction is how organisms make offspring, and it comes in two forms. Asexual reproduction needs only one parent and yields identical or near-identical copies. Bacteria split by binary fission, hydras and yeasts bud, and viruses hijack host cells. Most plants can clone themselves, and the ant Mycocepurus smithii is thought to reproduce asexually alone. Parthenogenesis, development of an embryo without fertilisation, occurs in water fleas, aphids, some bees and wasps, certain reptiles and fish, and very rarely domestic birds. Many organisms can switch modes: hydras, yeasts and jellyfish reproduce both ways, and bacteria swap genes by conjugation.
Sexual reproduction fuses two gametes, each carrying half the usual chromosomes, produced by meiosis. Where the gametes differ, as in most species, the sperm or microspore producer is called male and the egg producer female. Some organisms have look-alike gametes: the alga Chlamydomonas reinhardtii has plus and minus types, while numerous fungi, along with Paramecium aurelia, sort into three or more mating types. Offspring inherit one version of each gene, an allele, from each parent. Mosses keep sex even though their dominant generation is haploid and gains nothing from hybrid vigour, which hints that genetic recombination itself pays off.
Meiosis turns one diploid cell into four haploid ones over two divisions, while ordinary mitosis produces two identical cells. Mammalian testes deploy special DNA repair systems during meiosis to protect the genomes passed to offspring. Researchers are exploring same-sex procreation: Japanese scientists produced mice from two eggs in 2004, and in 2010 American researchers bred viable mice with two fathers using modified stem cells.
Strategies vary. Humans and northern gannets mature slowly and have few young, a pattern called K-selection. Rabbits, mature at 8 months, can have 10 to 30 offspring a year, and fruit flies up to 900, the r-selection route. Semelparous organisms, such as grain crops, some salmon and bamboo, breed once and often die soon after.
Source: Reproduction
