12.1Sexual and Asexual Reproduction
Reproduction is roughly classified into sexual and asexual types. Asexual reproduction is a mechanism by which individuals produce multiple equivalent progeny by division or other means, and is not accompanied by gene-level changes. This mechanism is commonly found in bacteria (Fig. 12-1A) and protista*1, but is also found in animals (jellyfish, hydras, etc.; Fig. 12-1B) and plants (regeneration through vegetative organs (as seen in potatoes) or by cuttage; Fig. 12-1C). Eukaryotic genomes are generally diploid, and have n pairs of homologous chromosomes (n: the ploidy for each cell), generally expressed as 2n. In sexual reproduction, a haploid gamete derived from a paternal source (in humans, a sperm) and one derived from a maternal source (in humans, an ovum) fuse together to form a diploid zygote (in humans, a fertilized egg). In multicellular organisms, this zygote performs repeated cell division to create new individual (Fig. 12-1D). Unicellular organisms such as yeast may also perform sexual reproduction depending on their environment, and in such cases, a zygote creates new progeny. Meiosis is the mechanism by which haploid cells are created from diploid cells.
Protista: The term “protista” is used to collectively describe unicellular eukaryotes, and includes flagellata and ciliates. Their reproduction methods include binary fission, multiple fission and budding.